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Geological and geochemical characterization of lacustrine shale: A case study of the Jurassic Da'anzhai member shale in the central Sichuan Basin, southwest China
Accepted Manuscript Geological and geochemical characterization of lacustrine shale: A case study of the Jurassic Da'anzhai member shale in the central Sichuan Basin, southwest China Qilu Xu, Bo Liu, Yongsheng Ma, Xinmin Song, Yongjun Wang, Zhangxin Chen PII:
S1875-5100(17)30380-3
DOI:
10.1016/j.jngse.2017.09.008
Reference:
JNGSE 2314
To appear in:
Journal of Natural Gas Science and Engineering
Received Date: 21 June 2017 Revised Date:
15 September 2017
Accepted Date: 27 September 2017
Please cite this article as: Xu, Q., Liu, B., Ma, Y., Song, X., Wang, Y., Chen, Z., Geological and geochemical characterization of lacustrine shale: A case study of the Jurassic Da'anzhai member shale in the central Sichuan Basin, southwest China, Journal of Natural Gas Science & Engineering (2017), doi: 10.1016/j.jngse.2017.09.008. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Geological and geochemical characterization of lacustrine shale: a case study of the Jurassic Da'anzhai Member shale in the central Sichuan Basin, Southwest China
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Qilu Xua,b,c, Bo Liuc,*, Yongsheng Maa, Xinmin Songd, Yongjun Wangd, Zhangxin Chenb a
School of Earth Sciences and Resources, China University of Geosciences(Beijing), Beijing 100083, China
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b
Chemical and Petroleum Engineering, University of Calgary, Calgary, Alberta T2N1N4, Canada
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c
Oil and Gas Research Center, Peking University, Beijing 100871, China
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d
PetroChina Research Institute of Petroleum Exploration and Development, Beijing 100083, China
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Abstract： ：
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Although numerous progresses have been achieved in characterizing marine shales, studies that are associated with lacustrine shales are limited.
To study lacustrine shales, samples were collected from
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the Jurassic Da'anzhai Member in the central Sichuan Basin of China, and their mineralogical, reservoir,
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OM, and paleoenvironmental characteristics were determined, as well as the relationships between them.
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Analysis of trace elements reveals that the shales formed in paleoenvironments that were oxic to suboxic,
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dry to humid, had moderate to strong weathering, and were characterized by fresh to salt water
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conditions. These environments are more variable than those of marine shales. The paleoenvironmental
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conditions and mineralogy of the shales, particularly the oxic to suboxic paleo-redox conditions, resulted
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in the relatively low levels (0.11-2.18%, average 0.97%). However, based on evaluation criteria for
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continental source rocks, the OM is of high quality because of its high level of maturity (Ro: 0.95-1.43;
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Tmax: 428-500°C) and favorable kerogen type (II2). There are well-developed intraparticle pores,
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interparticle pores and microcracks. The SBET (5.42-10.69 m2/g, average 6.92 m2/g) and VBJH (1.19-4.10
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mL/100 g, average 2.97 mL/100 g) values also indicate good nanoscale storage space. Terrestrial
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minerals (i.e., quartz and clay) and authigenic carbonate minerals (i.e., calcite) are, respectively,
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positively and negatively correlated with the nanoscale storage space, .. Small pores (3-5 nm) dominate
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the nanoscale storage space. The isotherms and hysteresis loops are of Type П and Type H3, respectively,
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which indicates wedge-shaped pores. However, the hysteresis loops indicate that the lacustrine shale has
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more dead-end pores and larger pores with more complex microstructures than other lacustrine
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reservoirs. In general, the Da’anzhai lacustrine shale has the potential for unconventional oil and gas
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exploration. The lacustrine shale’s mineralogy, reservoirs, and OM are closely related to each other, and
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their differences are mainly caused by the paleoenvironmental conditions in which the shale formed.
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Keywords: Lacustrine shale; Paleoenvironment; Nanoscale reservoir; Organic matter; Minerals; Mechanisms.
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1. Introduction
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The high commercial production of shale oil and gas in North America has made shale a focus of
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exploration in many countries and regions. However, the most widely developed shales were formed in
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marine systems; few studies have focused on the characteristics of lacustrine shales. Additionally,
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lacustrine shales are widely distributed in many areas, such as Africa, South America, Southeast Asia
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and China (Huang et al., 1984; Bohacs et al., 2000; Curtis, 2002; Schmoker, 2002; Chen et al., 2015;
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Jiang et al., 2016a; Yang et al., 2015).
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The most obvious characteristic of lacustrine shales is that they are subject to more variable paleoenvironments than marine shales. The formation environments of lacustrine and marine shales are
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very different in terms of the water depth, energy, provenance, sedimentary type and biological effects
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(Huang et al., 1984; Bohacs et al., 2000; Curtis, 2002; Schmoker, 2002). These features lead to
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differences in their basic characteristics including mineralogy, organic matter (OM), and reservoir type
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(Table 1). Minerals are the material basis of shales, and they affect not only OM generation but also
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reservoir development (Huang et al., 1984; Bohacs et al., 2000; Curtis, 2002; Schmoker, 2002; Chen et
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al., 2015; Yang et al., 2015; Jiang et al., 2016a). A major breakthrough in marine shale oil and gas
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exploration was based on the recognition that shale acts not only as a source rock but also as a reservoir.
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Unlike studies of conventional reservoirs, research on unconventional reservoirs has primarily focused
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on nanoscale reservoirs. Previous studies of marine and lacustrine shales suggest that shale gas is mainly
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controlled by a nanoscale pore system that is closely related to the mineral characteristics (Zou et al.,
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2011; Morad et al., 2010; He et al., 2012; Luo et al., 2013; Dang et al., 2015; Chalmers et al., 2012;
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Curtis et al., 2012; Jiang et al., 2015; Li et al., 2016; Jiang et al., 2017; Zhang et al., 2017). OM is the
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basic material that generates hydrocarbons, and its type and maturity dominate the formation and
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enrichment of shale oil and gas. The preservation and enrichment of OM in sediments is controlled by
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many factors, including the primary productivity, paleo-redox conditions, nutrient availability, clastic
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influx and minerals (Huang et al., 1984; Bohacs et al., 2000; Curtis, 2002; Schmoker, 2002; Chen et al.,
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2015; Yang et al., 2015; Jiang et al., 2016a; Xu et al., 2017). In summary, the paleoenvironment,
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minerals, reservoir and OM, which are the basic characteristics of shales, have been the focuses of shale
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oil and gas studies, and these characteristics are often interrelated. Additionally, studies of the
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characteristics and mechanisms have mainly focused on marine shales; fewer studies have examined
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lacustrine shales.The lacustrine shale of the Jurassic Da’anzhai Member from the central Sichuan Basin
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of China has been interpreted as fractured reservoirs, low permeability fractured reservoirs and tight
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reservoirs. However, most studies have only focused on the conventional carbonate reservoirs, and the
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shales have only been considered as source rocks for the carbonate reservoirs (Lu et al., 2014; Chen et
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al., 2015). Encouragingly, the Da’anzhai shale of the Yuanba Block in the northwestern Sichuan Basin,
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which has similar geological conditions to those of the central Sichuan Basin, hosts high-yield wells (He
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et al. 2012; Zhu, et al. 2016). In addition, some lacustrine shales have yielded oil or gas, such as the
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Triassic Chang 7 and Chang 9 Member in the Ordos Basin, the Paleogene strata (Es3 and Es4) in the
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Bohai Bay Basin and the Cretaceous strata in the Songliao Basin (Qing-1) (Zou et al., 2011; Luo et al.,
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2013; Dang et al., 2015; Li et al., 2015; Yang et al., 2015; Jiang et al., 2016b). These successful
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developments strongly indicate that the Da’anzhai shale in the central Sichuan Basin is worth studying.
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In this study, we assessed the Da’anzhai lacustrine shale’s characteristics. We performed
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paleoenvironmental evaluation using TE, mineralogical evaluation usingXRD analysis, reservoir
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evaluation using FE-SEMand LTNA analyses, and OM evaluation using analyses of the TOC,
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chloroform bitumen ‘A’, Rock-Eval, carbon isotopes, kerogen macerals and organic elements. We also
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compared the Da’anzhai shale to other lacustrine shales which have been successfully developed and
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assessed the similarities and differences of lacustrine shales. In addition, we summarized the
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relationships between these characteristics and mechanisms affecting them. Finally, we judged the
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development potential of the Da’anzhai shale.
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Table 1 Comparisons between basic parameters in the Da’anzhai shale and other successfully developed shales
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82 83 84
lacustrine shale
Central Sichuan Basin, China
layer
Barnett
Ordos Basin,
Ordos Basin,
Bohai Bay
Bohai Bay
Songliao
Fort Worth Basin,
China
China
Basin, China
Basin, China
Basin, China
Texas, US
downfaulted
downfaulted
downfaulted
basin
basin
basin
Paleogene
Paleogene
Cretaceous
Carboniferous
shale with
shale with
shale with
siliceous and
siltstone
siltstone
argillaceous
carbonaceous
limestone
shale, limestone,
interlayers
dolomite
Ohio Appalachian Basin, Kentucky, US
polycyclic superimposed
basin
basin
Triassic
Triassic
shale with
shale with
siltstone
siltstone
interlayers
interlayers
interlayers
interlayers
1900~3200
1500~2500
1600~2600
1500~4200
1500~5200
1000~2500
1980~2590
510~1800
10~70
30~70
10~15
100~500
250~350
70~150
15~60
9~31
(20~80) /54
(37~53) /43
(29~56) /45
(25~80) /36
(30~80) /36
(30~80) /36
35~80
45~60
Jurassic
shells and
combination
carbonaceous interlayers
effective
(%) brittle minerals
Qing-1
polycyclic
lithological
thickness
Es4
superimposed
foreland basin
shale with
depth (m)
Es3
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basin type
Chang 9
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basin
marine shale
Chang 7
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Da'anzhai
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parameters
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(%)
6
foreland basin
piedmont depression Devonian interbed of black shale and gray siliceous shale
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0.20~5.92
1~12
10
2~8
1.3~9.3
6~12
4~5
4~7
permeability
0.002~0.9
0.01~0.3
(0.3~5.0) /2.97
<0.50
<0.50
<0.15
0.01
<0.1
TOC (%)
(0.1~2.2) /1.0
(0.3~36) /8.3
(0.3~11.3) /3.1
(0.5~13.8) /3.5
(0.8~16.7) /3.2
(0.4~4.5) /2.2
2.0~7.0
0.5~4.7
Ro (%)
0.9~1.5
0.8~1.2
0.9~1.3
0.4~1.2
0.4~2.0
0.5~1.5
1.1~2.2
0.3~1.3
pyrolysis gas
pyrolysis gas
pyrolysis gas
Ⅰ-Ⅱ
Ⅱ
Ⅱ
8.50~9.91
1.69~2.83
genetic type
biogenic and
pyrolysis gas
pyrolysis gas
pyrolysis gas
Ⅱ
Ⅰ-Ⅱ1
Ⅰ-Ⅱ1
Ⅰ-Ⅱ1
0.87~1.98
0.92~2.91
(1.68~4.25)
(0.60~3.70)
/2.92
/2.00
pyrolysis gas
pyrolysis gas
kerogen
2011; Luo et references
al., 2013; Lu et al., 2014; Chen et al., 2015;
2011; Luo et al., 2013; Yang et al., 2015; Jiang et al., 2016a.
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Zou et al.,
Zou et al.,
2011; Luo et
Zou et al., 2011;
Zou et al.,
al., 2013; Shi et
Luo et al., 2013;
2011; Luo et
al., 2013; Yang
Dang et al.,
al., 2013; Li,
et al., 2015;
2015; Jiang et
2014; Jiang et
Jiang et al.,
al., 2016a
al., 2016a
2016a.
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0.31~1.24
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(m3/t)
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gas content
Ⅰ-Ⅱ1
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porosity (%)
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Schmoker, 2002;
Schmoker, 2002;
Zou et al.,
Bowker et al.,
Bowker et al.,
2011; Luo et
2007; Jarvie et al.,
2007; Jarvie et al.,
al., 2013; Jiang
2007; Martini et
2007; Martini et
et al., 2016a
al., 2008; Lu et al.,
al., 2008; Lu et al.,
2014
2014.
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2. Geological setting
89 90 91
Fig. 1. Study location, geological map (Chen et al., 2015) and simplified stratigraphic column of the Jurassic Da’anzhai
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Member in the central Sichuan Basin.
The study area is located in the central Sichuan Basin, which includes the Nanchong and Suining
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areas. It is bounded by the Huafu Mountains to the east and the Longquanshan basement faults to the
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west, and it covers an area of 6×104 km2 (Fig. 1) (Lu et al., 2014; Chen et al., 2015). A lacustrine
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environment dominated the Sichuan Basin during the Jurassic Period, and the Da’anzhai Member 8
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represents the largest and deepest stage of this environment. The sedimentary facies of the Da’anzhai
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Member can be divided into shore-shallow lacustrine, shallow lacustrine and semi-deep lacustrine
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subfacies, which include shell beach, calcareous flat, mud flat, and semi-deep lake microfacies. These
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facies are distributed in concentric belts around the lake basin’s center in the Yilong-Pingchang areas
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(Fig. 1) (Lu et al., 2014; Chen et al., 2015). From top to bottom, the Da’anzhai Member in the central
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Sichuan Basin can be divided into main three sub-members: Da1, Da13 and Da3 (Fig. 1). The thickest
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Da13 sub-member (30-50 m) is a set of widely distributed dark mudstones with lamellated fractures and
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rich OM; this sub-member is considered the most important source rock for hydrocarbons. The Da3
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sub-member deposits (5-14 m), which are initial deposition products, are primarily (mud) shell
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limestones with dark gray mudstone interlayers. The Da1 sub-member (30-45 m thick) primarily
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consists of shales and (mud) shell limestones; this sub-member contains the sedimentary products of the
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period of basin uplift (Lu et al., 2014; Chen et al., 2015).
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3. Samples and experimental methods
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3.1. Samples
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Thirty-nine representative core samples were obtained from the Da’anzhai Member shale in the
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central Sichuan Basin based on drilling, core observations, stratigraphic data, and log responses (Fig. 1
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and Table 3).
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3.2. Methods
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All 39 shale samples were analyzed to determine their bulk mineralogy viaXRD
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using a X'Pert
Pro MPD device with a working voltage of 40 kV and a current of 40 mA in the Key Laboratory of
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Orogenic Belts and Crustal Evolution at Peking University. To investigate the clay mineral fraction in
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detail, textured mounts were measured four times at a goniometer rate of 0.5θ/minute with a registration
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range from 2 to 42°2θ at the Beijing Research Institute of Uranium Geology.
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The micromorphology and oiliness of the samples were determined at the Key Laboratory of Marine Reservoir Evolution and Hydrocarbon Accumulation of the China University of Geosciences
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(Beijing) using an Axio Scope A1 pol Fluorescence-Polarization Microscope. An INCA Synergy system
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at the Key Laboratory of Orogenic Belts and Crustal Evolution at Peking University was used to analyze
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the microstructure of each sample via FE-SEM and the mineral composition of each sample via EDS.
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The samples were subjected to argon-ion polishing to obtain higher resolution images.
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LTNA analysis is capable of characterizing nanoscale pores in porous media, and adsorption tests were performed with dried samples (60-80 mesh) on an ASAP2020M device at the College of
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Chemistry and Chemical Engineering of the China University of Petroleum (Beijing). N2 adsorption
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isotherms were obtained at -196.15°C within a relative pressure range of 0.005 to 0.998. The BET
128
method for the equivalent specific surface areas and theBJH method for the total pore volumes were
129
applied in the calculations (Brunauer et al., 1938; Barrett et al., 1951).
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The OM richness was characterized by measuring the TOC values of 25 samples using a GHM-02
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analyzer; 10% hydrochloric acid was used to remove carbonates, and the test sensitivity was 10-13 mg/g.
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Chloroform bitumen ‘A’ was extracted from the crushed shale powders (<0.09 mm), and the weight was
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calculated at the China University of Petroleum (Beijing). A Rock-Eval 6 instrument was used to
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perform a pyrolysis analysis, which yielded the temperature of the Tmax, S0, S1 and S2. The OM maturity
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was determined based on the Ro value, which was obtained using a reflected light microscope under oil
136
immersion and calculated as the mean of 10-30 measurements taken for a single sample. Additionally,
137
measurements were taken to determine the isolated kerogen content and identify the organic kerogen
138
macerals, carbon isotope content and organic element content according to the (GB/T) 19144-2010,
139
(SY/T) 5125-2014, (GB/T) 18340.2-2010, (GB/T) 19143-2003 standards, respectively. The kerogen
140
measurements and the rock pyrolysis analysis were performed at the Keyuan Engineering Technology
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Testing Center.
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The TE concentrations of 17 samples were determined using ICP-MS, which was performed at the Beijing Research Institute of Uranium Geology. The detailed sample processing procedure as well as the
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analytical precision and accuracy followed the methods described for the (GB/T) 14506.28-2010
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standard.
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4. Results and discussions
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4.1. Paleoenvironmental properties and differences
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Lacustrine shales are different from marine shales mainly due to their formation paleoenvironments.
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Lakes are complex and dynamic systems, and the development of lacustrine shales is controlled more
150
significantly by the sedimentary environment (Huang et al., 1984; Bohacs et al., 2000; Curtis, 2002;
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Table 2 Trace elements of the Da’anzhai lacustrine shale. G10-2
G4-3
G6-2
J45-1
J45-2
J53-1
J61-1
J61-2
J61-3
J61-4
Li-1
Li-2
P1-1
S2-1
S2-3
S2-4
X44-1
Th(ppm)
11.70
10.40
5.43
4.32
12.00
10.60
1.72
4.13
7.74
10.20
11.30
9.50
11.60
2.31
10.50
6.21
13.70
Ni/Co
2.68
2.98
3.26
4.10
2.87
2.72
5.49
3.87
3.50
2.51
2.65
2.97
3.07
3.22
2.32
3.37
2.78
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1.76
1.58
1.44
1.50
1.64
1.64
2.47
1.56
1.33
1.69
1.60
1.50
1.61
1.77
1.30
1.41
1.45
0.78
0.80
0.63
0.63
0.78
0.78
0.67
0.68
0.70
0.77
0.78
0.74
0.78
0.61
0.68
0.68
0.77
V/Sc
9.78
8.70
6.88
7.73
9.36
8.91
13.81
8.41
7.94
9.03
7.93
7.21
9.22
9.80
8.54
7.14
7.81
Th/U
2.77
4.24
4.45
2.79
4.07
2.95
1.28
3.53
4.63
3.81
4.25
4.63
4.20
2.74
5.25
4.60
4.72
Sr/Cu
3.75
4.60
42.98
77.10
3.50
3.62
58.70
35.54
53.09
8.46
7.84
19.01
3.35
7.47
24.69
6.05
Sr/Ba
0.29
0.26
3.66
3.08
0.23
0.24
4.77
2.78
3.45
0.52
0.46
1.28
0.18
3.77
0.44
1.84
0.36
118.7
115.3
109.8
131.7
110.3
187.5
141.2
128.9
194.3
112.9
60.7
130.6
112.6
153.9
3
6
4
1
7
7
6
9
0
9
6
7
2
3
7.79
9.03
6.39
6.65
9.34
9.58
8.46
8.67
8.49
8.04
9.44
4.19
9.93
9.86
8.88
7.88
10.03
(La/Yb)N
6.65
7.75
6.57
8.37
6.82
8.04
9.85
9.82
11.14
8.00
7.50
4.35
7.49
8.62
7.67
9.05
(La/Sm)N
4.66
4.74
3.04
3.32
5.58
5.42
3.19
3.06
2.37
3.88
5.71
2.59
5.90
3.07
4.20
3.90
5.01
ΣLREE/ ΣHREE
88.73
38.15
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130.97
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V/Cr V/(V+Ni)
80.5 4
10.8 8
0.99
1.04
1.52
1.97
0.77
0.97
2.04
2.01
2.92
1.45
0.84
1.41
0.79
2.10
1.35
1.38
1.17
(Ce/Yb)N
4.48
5.38
4.98
5.83
4.54
5.42
7.21
7.33
9.24
5.58
4.97
3.22
5.00
8.49
6.10
5.59
6.15
δCe
0.92
0.94
0.97
0.95
0.92
0.93
0.94
0.93
0.99
0.92
0.92
0.92
0.93
0.96
0.95
0.97
0.92
CSr(‰)
0.19
0.18
0.77
1.12
0.17
0.16
0.63
0.56
0.88
0.36
0.27
0.61
0.15
0.72
0.12
0.35
0.23
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(Gd/Yb)N
XN, where N refers to the chondrite-normalized value (Gromet et al., 1984); δCe=CeN/(LaN×PrN)1/2 (Wright et al.,1984; Murray et al.,1990).
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Schmoker, 2002). TEs can be used as effective quantitative indicators of the paleoenvironment by
2
comparing their type and abundance with standard values (Wright et al., 1984; Murray et al., 1990;
3
Manning, 1994; Ross et al., 1995; Kimura and Watanabe, 2001; Jarvis et al., 2001; Rimmer, 2004; Jones
4
and Roy et al., 2007).
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Fig. 2. Identification of paleoenvironments using TEs (a-c) and clay (d).
4.1.1. Paleo-redox conditions
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Due to their responses to changing redox conditions, the ratios of redox-sensitive TEs, such as
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Ni/Co, V/Cr, V/Sc, Th/U, and δCe, are usually used to evaluate redox conditions. According to Jones
10
and Manning (1994), V/Cr ratios <2 indicate oxic conditions, V/Cr ratios from 2 to 4.25 indicate dysoxic
11
conditions, and V/Cr ratios >4.25 indicate suboxic to anoxic conditions. Kimura and Watanabe (2001)
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found that the V/Sc ratio is positively related to oxic conditions. Jones and Manning (1994) showed that
13
Ni and Co are enriched in pyrite, and high Ni/Co ratios indicate anoxic conditions, Ni/Co ratios <5 13
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indicate oxic conditions, Ni/Co ratios between 5 and 7 indicate dysoxic conditions, and Ni/Co ratios >7
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indicate suboxic to anoxic conditions. Similarly, Th/U ratios <1.5 represent anoxic conditions, Th/U
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ratios between 1.5 and 3 represent dysoxic to suboxic conditions, and Th/U ratios >3 represent oxic
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conditions (Jones and Manning, 1994). In addition, δCe values >1 (positive anomaly) and <0.95
18
(negative anomaly) suggest anoxic and oxic to suboxic conditions, respectively (Wright et al., 1984;
19
Murray et al., 1990).
SC
For the Da’anzhai shale in the central Sichuan Basin (Table 2), the ranges of the Ni/Co, V/Cr, V/Sc,
M AN U
20
RI PT
14
and Th/U ratios are relatively wide, from 2.32-5.49, 1.30-2.47, 6.88-13.81, and 1.28-5.25, respectively.
22
The values of δCe vary from 0.92 to 0.99 (average 0.94), which indicate a weakly negative anomaly
23
(Wright et al., 1984; Murray et al., 1990).Collectively, these proxies reveal that the shale was deposited
24
under variable paleo-redox conditions that were mainly oxic to suboxic (Fig. 2).
TE D
21
4.1.2. Paleo-salinity
26
Sr/Ba values vary directly with the distance to a sea or lake coast; Sr/Ba ratios <1 suggest
EP
25
freshwater conditions, and Sr/Ba ratios >1 suggest salt water conditions (Jarvis et al., 2001; Kimura and
28
Watanabe, 2001). In addition, low Sr concentrations (0.1-0.3‰) indicate a freshwater setting, and high
29
values (0.8-1‰) represent a salt water setting (Jarvis et al., 2001; Kimura and Watanabe, 2001). Th
30
easily dissolves in acidic water and hydrolyzes to oxide or hydroxide sediments under alkaline
31
conditions; therefore, high Th values indicate a salt water environment (Jones and Manning, 1994).
32 33
AC C
27
In the Da’anzhai shale samples (Table 2), the Sr/Ba ratios vary widely (0.18-4.77) with an average of 1.62. The Th content also varies widely (1.72-13.70 ppm) and averages 8.43 ppm, and the Sr 14
ACCEPTED MANUSCRIPT
34
concentrations vary from 0.12 to 1.12‰ (average 0.44‰). Generally, these proxies indicate variable
35
paleo-salinity conditions including both fresh and salt water settings (Fig. 2).
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4.1.3. Paleo-weathering
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36
37
39
Fig. 3. Chondrite normalized REE distribution curves for REE minerals. Chondrite values are from Gromet (1984).
When rocks are subjected to strong weathering, heavy rare earth elements (HREEs) dissolve and
TE D
38
migrate much more easily than light rare earth elements (LREEs). Therefore, high ΣLREE/ΣHREE
41
values indicate strong paleo-weathering conditions (Kimoto et al., 2006; Stevens and Quinton, 2008). In
42
addition, the (La/Yb)N, (La/Sm)N, (Gd/Yb)N and (Ce/Yb)N ratios, which are positively related to LREE
43
enrichment, are also positively correlated with paleo-weathering (Table 2) (Wright et al., 1984; Murray
44
et al., 1990; Ross et al., 1995; Roy et al., 2007).
AC C
45
EP
40
The ΣLREE/ΣHREE rates are between 4.19 and 10.03 (average 8.39), which indicate LREE
46
enrichment. The (La/Yb)N ratio is represented by the slope of the chondrite-normalized REE distribution
47
curve (Fig. 3), which indicates the degree of the graph’s inclination (Ross et al., 1995; Roy et al., 2007).
48
The (La/Yb)N values vary from 4.35 to 11.14 (average 8.15), which indicate that the curves are tilted to 15
ACCEPTED MANUSCRIPT
the right and that the samples are rich in LREE-acidic rocks (Fig. 3). The (La/Sm)N ratio, which
50
represents the degree of LREE fractionation, is positively related to the ΣLREE value (Ross et al., 1995;
51
Roy et al., 2007). The (La/Sm)N ratios range from 2.37 to 5.90 (average 4.10), which indicates a P(E)
52
model with abundant LREEs. The (Gd/Yb)N ratio, which indicates the degree of HREE fractionation, is
53
negatively correlated with the ΣHREE value (Ross et al., 1995; Roy et al., 2007). The (Gd/Yb)N values
54
vary from 0.77 to 2.92 (average 1.46), and the (Ce/Yb)N ratios vary from 3.22 to 9.24 (average of 5.85),
55
which indicate that the Da’anzhai shale is depleted in HREEs (Fig. 3). In summary, the REE results
56
(Table 2) indicate that the shale was deposited under moderate weathering conditions with periods of
57
strong weathering.
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49
4.1.4. Paleoclimate
59
∑REE values are closely related to paleoclimate conditions; that is, ∑REE values are higher in
TE D
58
warm and humid climates and lower in cold and dry climates (Wright et al., 1984; Murray et al., 1990;
61
Ross et al., 1995; Roy et al., 2007). The ΣREE values of the Da’anzhai shale vary widely from 38.15 to
62
194.3 µg/g (average 121.59 µg/g) (Table 2). The Sr/Cu ratio is another useful indicator of lacustrine
63
paleoclimate, with high values reflecting humid climates (Reheis, 1990; Chen et al., 2009).The Sr/Cu
64
ratios in the Da’anzhai shale also vary widely from 3.35 to 80.54%. In addition, illite usually forms in
65
dry conditions by the weathering depotassication of feldspar, mica and other aluminosilicate minerals.
66
Conversely, kaolinite forms in humid climates by the eluviation of feldspar, mica and pyroxene.
67
Therefore, high illite/clay ratios indicate dry, saline waters with high K+ content, and high kaolinite/clay
68
ratios suggest humid climates (Vanderaveroet, 2000; Gingele et al., 2001). The illite/clay ratios of the
AC C
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60
16
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Da’anzhai shale are relatively high (between 26 and 86%, average 52%). However, the kaolinite/clay
70
ratios are relatively low (between 3 and 19%, average 11%) (Fig. 2d). These results indicate a variable
71
climate with both dry and humid periods as well as a drying trend from humid climates.
72
RI PT
69
In general, the TE contents and types t in lacustrine shales are more complex and varied than those in marine shales, which indicates that lacustrine shales tend to experience more variable
74
paleoenvironmental conditions (Huang et al., 1984; Bohacs et al., 2000; Curtis, 2002; Schmoker, 2002).
75
The lacustrine Da’anzhai shale was formed under relatively dry climatic conditions that included both
76
arid and humid stages. In addition, the shale experienced a moderate degree of paleo-weathering with
77
stages of strong weathering, variable oxic to suboxic paleo-redox conditions that were weakly reducing,
78
and a predominantly freshwater setting interspersed with periods of saltwater.
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4.2. Mineralogy properties and differences
80
As shown in Table 3, the Da’anzhai shale primarily contains clay (42.4%), quartz (27.7%) and
81
calcite (21.1%). The minerals in these lacustrine shales vary widely. Clay comprises the majority of the
82
shale and ranges from 16.3 to 68.4%, while the quartz content ranges from 5.6 to 53.4%. Carbonate
83
minerals, which mainly include calcite, dolomite and aragonite, range from 3.0 to 46.6% with an
84
average of 27.4%. The Da’anzhai shale contains more brittle minerals than the other lacustrine shales
85
and marine shales (Table 1 and Fig. 4). For example, the brittle mineral content of the Da’anzhai shale
86
ranges from 24.2 to 80.3%, with an average of 53.7%, indicating medium-high brittleness that is
87
favorable for natural fractures and hydraulic fracturing. Similar to other lacustrine shales, lacustrine
88
shales have more siliceous or calcareous interlayers than marine shales, which can cause the wider
AC C
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TE D
79
17
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ranges of carbonate and quartz minerals and improve the brittleness index and fracturing (Figs1 and
90
5I).Ternary diagrams were constructed to compare the mineral compositions of hot shales that have been
91
successfully exploited including lacustrine shales (Fig. 4a) and marine shales (Fig. 4b). Generally, the
92
comparison shows that the mineral composition of the lacustrine shales is more variable and complex
93
than in marine shales (Fig. 4). For example, the Da’anzhai shale has relatively wide ranges of carbonate
94
(1-70%), clay (15-70%) and quartz (5-50%) (Fig. 4a). In addition, the Da’anzhai shale has the wider
95
mineral ranges than other lacustrine shales, especially for the range of carbonate minerals.
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89
EP
97
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96
Fig. 4. Ternary diagrams of mineral compositions. (a) Lacustrine shales: 1=Da’anzhai shale, 2=Jurassic shale in Sichuan
99
Basin (Jiang et al., 2017), 3=Triassic shale in Sichuan Basin(Jiang et al., 2017), 4=ES3 shale (Wang et al., 2015a). (b) Marine
AC C
98
100
shales: 5 Woodford shale (Jarvie et al. 2007 and Han et al. 2013), 6=Barnett shale (Jarvie et al. 2007 and Han et al. 2013),
101
7= Wufeng-Longmaxi shale (Wu et al., 2014), 8=Ohio shale (Jarvie et al. 2007 and Han et al. 2013).
102
Table 3 Minerals and TOC results for the Da’anzhai lacustrine shale.
103
18
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G10-3 2700.7 G10-4 2646.6 G4-1 2385.3 G4-2 2395.8
subquartz feldspar calcite dolomit aragonite siderite pyrite hematite total clay others carbonate brittleness member (%) (%) (%) e (%) (%) (%) (%) (%) (%) (%) (%) minerals (%) 44.2 3.0 52.8 3.0 47.2 Da1 22.1 1.1 0.9 1.2 6.3 68.4 2.1 24.2 Da13 37.5 2.0 2.9 2.6 55.0 2.9 40.4 Da13 1
4.2 2.1
G4-3 2405.7
Da Da1 Da1 Da13
28.1 28.2 35.2 25.8
2.5
G6-1 2537.1
Da13
21.4
2.6
G6-2 2580.1
8.7
1.2
47.6
19.5
1.1
0.8
J45-1 2632.4 J45-2 2656.0
Da3 Da1 Da13
12.5 23.4
0.6 6.1
27.9 3.9
36.3
0.4 0.4
0.7 1.1
J53-1 2827.3
Da13
31.9
4.1
1
14.4 55.3 14.3 17.5 69.4
6.4
17.0 13.4 12.5
4.5
4.6
0.7
1.6
1.2
Da13
8.5
J61-6 Li-1 Li-2 Li-3
2665.9 1713.8 1726.1 1734.3
Da1 Da1 Da1 Da13
25.1 30.4 24.2 40.7
1.8 2.6 1.0 2.3
15.6 9.8 28.9 3.1
1.0
P1-1 3215.9
Da13
23.3
0.9
0.6
1.0
P1-2 3221.2
Da13
34.8
4.1
P1-3 3239.2
Da13
30.3
14.9
P1-4 3258.6 S2-1 2836.1 S2-2 2849.9
3
Da Da1 Da1 Da13
23.2 19.8 40.3
51.8 48.2 10.7
Da3 Da3
18.9
S2-4
2889.0
S2-5 2891.6 X20-1 1844.4 X28-1 1952.2 X44-1 2089.4 X44-2 2093.0
Da1 Da13 Da13 Da13 Da13 Da13 1
53.4 49.4
3.6
LQ-1 3548.1 X8-1 1780.7
W8-1 2065.7
Da Da1
1.3
0.2 0.8 0.0 4.2
3.2
6.3 0.0
5.3 2.2
69.2 41.6 54.1
0.40 0.98
5.3
31.1
1.43
4.2
7.4
28.8
1.18
20.1
1.0
67.1
75.8
20.5 64.0
1.1 1.1
64.2 3.9
76.7 27.3
55.5
5.4
22.6 26.7 27.7 33.1
10.3
1.1
0.81
41.1 13.4 18.9
1.0 1.1 0.9 2.3
20.0 46.2 53.3 38.5 53.9
0.7
31.9
2.18
59.3 64.8 57.0 24.3
76.2 70.4 71.4 59.1
1.49
69.4
77.9
0.30
26.9 9.8 35.2 3.1
52.0 40.2 59.4 43.8
1.17 1.07 0.69
6.4
67.8
1.6
24.9
1.59
0.9
60.2
4.1
38.9
1.51
3.6
51.2
14.9
45.2
1.55
1.5 0.2 0.9
16.3 29.6 42.5
57.1 50.4 10.7
80.3 70.2 51.0
0.61 0.11 1.18
1.9 2.0
47.7
1.3
33.4
49.0
66.6
0.33
48.9
1.5
30.7
50.4
69.3
0.18 0.76
8.1
24.9 35.9
1.4
27.9
3.0
4.0
23.8
3.0
43.2
AC C
X44-3 2116.7
Da13
17.6
1.0
2.1
TE D
2867.1
44.9 9.5 42.7 6.8
0.9
TOC (%) 0.82 1.33
60.9
SC
J61-5 2706.8
S2-3
3.5
M AN U
Da1 Da13
16.9 5.6 14.4 34.8
26.6 54.9 38.4 64.8
3.1
2662.6 2664.4 2667.6 2698.1
Da Da1
3.5
7.4
25.6 8.5
EP
J61-1 J61-2 J61-3 J61-4
22.9
RI PT
depth (m) G10-1 2650.8 G10-2 2668.4 code
46.6 38.7
2.3
3.1 3.8 3.7 9.5
1.5
8.1
53.4 57.5
44.3 51.8
0.7
28.7 8.5
53.6 44.4
0.53
60.8
0.6
4.0
31.9
1.15
52.7
76.5
20.5
44.1
2.9
18.7
40.5
2.6
2.5
34.3
28.2
2.2
18.5
7.8
43.3
23.9
1.9
35.2
11.4
26.6
2.5
50.8
1.1
104
W8-2 2069.5
105
4.3. Characterization and controlling factors of lacustrine shale reservoirs
106
4.3.1. Microscopic characteristics
19
1.0
18.7
62.8
2.5
43.0
26.3
54.5
46.6
70.5
0.84
ACCEPTED MANUSCRIPT
108
AC C
EP
TE D
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107
109
Fig.5. Typical reservoirs images of lacustrine shale A:
Shells were stained using alizarin red to highlight the carbonate
110
minerals; the shells are well developed and preserved in terrestrial inputs and matrices. B: Greenish-yellow fluorescence.
111
Matrices with large amounts of terrestrial inputs between the shell grains have strong fluorescence intensity, indicating
112
effective reservoirs with good oiliness in the matrices. C: Lacustrine shale core sample with well-developed calcareous
113
interlayers, indicating a changeable sedimentary environment. D: Quartz particles with well-developed interparticle pores
20
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filled with clay minerals; quartz and clay minerals have well-developed intraparticle pores. E: Quartz particles have
115
well-developed intraparticle pores and interparticle pores; calcite minerals fill the storage spaces between quartz particles
116
(calcite cementation). F: Intraparticle pores located along the cleavage planes of clay particles; interparticle pores between
117
clay and carbonate minerals. G: OM with well-developed pores is accompanied by clay minerals. H: Calcite minerals fill the
118
spaces between particles (calcite cementation); quartz has well-developed intraparticle pores. I: Brittle minerals with
119
well-developed micro-cracks that are mainly filled by clay minerals. J: Model of different types of pores. K: Model of
120
carbonate mineral (e.g., calcite) diagenesis that results in reduced storage spaces.
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114
According to the International Union of Pure and Applied Chemistry (IUPAC) classification scheme, the pore types in unconventional reservoirs are divided by pore size into micropores, mesopores,
123
and macropores (<2 nm, 2-50 nm, and >50 nm, respectively) (IUPAC, 1994; Thommes et al., 2015).
124
Thin section observations of the shale indicate that the matrices fluoresce intensely (Fig. 5B). Nanoscale
125
reservoirs were examined using FE-SEM with argon-ion milling technology, and the minerals were
126
identified using EDS. Similar to other lacustrine shales, the pores and fractures in the Da’anzhai shale
127
are well developed and fall into four categories.
EP
(1) Interparticle pores: these pores represent the largest number of pores, and they have good
AC C
128
TE D
122
129
connectivity and form effective pore networks. The diameters generally range between 5 nm and 90 µm.
130
Many types of interparticle pores were observed, including pores between grains (Fig. 5D,E,H,J),
131
crystals and clay platelets (Fig. 5D,F,G,J), and pores at the edges of rigid grains (Fig. 5E,J).
132 133
(2) Intraparticle pores: these pores primarily formed through diagenesis, and some are primary. The diameters mainly range from 2 nm to 10 µm. The primary sub-types in the Da’anzhai shale are
21
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134
intraplatelet pores within clay aggregates (Fig. 5D,F,J), intercrystalline pores within pyrite framboids,
135
pores within fossil bodies and moldic pores after fossils (Fig. 5D,J). (3) Fracture pores: these pores are not controlled by individual particles and can be regarded as a
RI PT
136 137
type of pore that formed by rock deformation because of tectonic movements, sedimentation, diagenesis,
138
OM hydrocarbon generation and other geological effects (Fig. 5I,J).
(4) OM pores: these pores are found within OM (Fig. 5L) and are usually irregular, bubble-like, and
SC
139
oval. They generally range in diameter from 2 to 500 nm. Although most OM pores are incorrectly
141
regarded as isolated pores in two-dimensional planes, they are interconnected in three-dimensional space
142
and facilitate the development of reservoirs.
143
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Similar to other lacustrine and marine shale reservoirs, the Da’anzhai shale commonly contains well-developed pores and fractures, particularly intraparticle and interparticle pores (Curtis et al., 2002;
145
Chalmers et al., 2012; Milliken et al., 2013; Nie et al., 2015; Wang et al., 2015a; Zhou and Kang, 2016;
146
Jiang et al., 2017; Zhang et al., 2017). Especially, due to their rich terrestrial clastic mineral content, the
147
interparticle pores of lacustrine shales are better developed than those of marine shales. Most of the
148
intraparticle pores are nanoscale pores (<200 nm) (Fig. 5D,E,F) and the OM also contains nanopores
149
(Fig.5G). Microcracks can develop due to the high brittle mineral
150
However, these cracks are often filled by calcite and clay minerals due to the high clay content (42.4%)
151
(Fig.5I,J).
AC C
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TE D
144
22
content for the Da’anzhai shale.
ACCEPTED MANUSCRIPT
4.3.2. Nanoscale reservoirs characterized by LTNA
153
The LTNA test is the most widely used method of characterizing several parameters of nanoscale
154
pores in porous media (micropores and mesopores), includingVBJH, SBET, DA, ad/desorption isotherms
155
and PSD (Sing et al., 1985; IUPAC, 1994; Thommes et al., 2015; Wang et al., 2015c; Zhou et al., 2016;
156
Zhang et al., 2017).
157
Table 4 Comparisons of primary pore structure parameters measured by LTNA between the Da’anzhai shale and other
158
lacustrine shales
159
3
4
5
6
7
160
SBET (m2/g)
Sichuan
Jurassic Da'anzhai
1.75-10.69
Basin
Member
(6.92)
Upper Triassic
0.25-4.39
Chang 7 Member Xujiahe
Ordos Basin Sichuan Basin
3
(cm /100g)
D (nm)
1.19-4.10
7.21-24.71
(2.97)
(14.11)
0.02-0.90
6.67-18.40
(2.62)
(0.49)
(11.23)
0.62-14.70
0.22-2.97
4.91-13.93
(7.76)
(1.41)
(6.97)
Upper Triassic
1.46-5.24
0.72-1.40
9.40-25.00
Chang 7 Member
(2.94)
(1.01)
(16.40)
Bohai Bay
Shahejie
0.40-12.45
0.08-1.60
4.58-41.36
Basin
Formation
(3.62)
(0.62)
(16.84)
0.39-34.06
0.13-4.18
3.16-16.48
(10.92)
(1.59)
(9.56)
Upper Triassic
1.10-1.90
0.69-1.09
15.4-23.4
Chang 7 Member
(1.57)
(0.83)
(19.27)
Ordos Basin
Songliao
Formation
VBJH
TE D
2
strata
EP
1
location
Upper Cretaceous Qingshankou
AC C
NO.
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152
Basin
Ordos Basin
Formation
161
23
hysteresis
loops types
references
H3
H3 H3 H3 H3,H4
H2,H3
H3,H4
Jiang et al., 2016b Peng et al., 2016 Fu et al., 2015 Zhang et al., 2016 Wang et al., 2015b Yang et al., 2017
RI PT
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162
164
Fig. 6. PSD curves obtained from the low temperature N2 adsorption isothermswith BJH pore sizes.
SC
163
The VBJH value of the Da’anzhai shale (2.97 cm3/100 g) is slightly higher than the values in other shales, and the SBET value (6.92 m2/g) is nearly equal to those of the other shales (Table 4). These values
166
indicate that the Da’anzhai shale can also have significant nanoscale storage space.
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165
To investigate the distribution and contributions of each pore type, the VBJH with respect to the PSD
168
was determined from the adsorption branches of the BJH model (Wang et al., 2015b; Jiang et al., 2016b;
169
Zhou et al., 2016; Zhang et al., 2017). The PSD plots show that pores with diameters between 3 and 5
170
nm make up the largest proportion of the total pore volume and that the proportions decrease with
171
increasing pore size, which is similar to
EP
some lacustrine shales (Fig. 6) (Jiang et al., 2016b).
AC C
172
TE D
167
24
173 174
TE D
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Fig. 7. Comparisons between the nanopore structures of the Da’anzhai shale and other lacustrine shales. A, B, C: the Da’anzhai shale. D: Lacustrine shale of the Chang 7 Member in the Ordos Basin, China (Fu et al., 2015). E: Lacustrine shale
176
of the Songliao Basin, China (Wang et al., 2015b). F: Lacustrine shale of the Yanchang Formation in the Ordos Basin, China
177
(Jiang et al., 2016b; Wang et al., 2015c). G: Lacustrine shale of the Shahejie Formation in the Dongying Sag, China (Liu et
178
al., 2017). H: Lacustrine shale of the Shahejie Formation in the Zhanhua Sag, China. I: Lacustrine shale of the 5
179
Member of the Xujiahe Formation of Upper Triassic in Sichuan Basin, China (Peng, 2016).
180
AC C
EP
175
th
LTNA isotherms and hysteresis patterns can be used to investigate the physisorption mechanisms
181
and structural characteristics of shale pores (Sing et al., 1985; IUPAC, 1994; Thommes et al., 2015;
182
Zhou et al., 2016; Zhang et al., 2017). Adsorption isotherms are divided into five types (Types I-VI), and 25
ACCEPTED MANUSCRIPT
hysteresis loops are divided into four types (Types H1-H4) (Sing et al., 1985; IUPAC, 1994; Thommes et
184
al., 2015). Following this classification, the isotherms of the Da’anzhai shale samples are similar and
185
belong to Type П, and the hysteresis loops are characterized by Type H3 hysteresis loops that indicate
186
wedge-shaped pores (Sing et al., 1985; IUPAC, 1994; Thommes et al., 2015). However, compared to
187
other lacustrine shales, the Da’anzhai shale has relatively weak hysteresis loops and wide overlapping
188
ranges of adsorption and desorption isotherms (Fig. 7), which indicate many dead-end pores with highly
189
complex structures (De Boer, 1958; Nie et al., 2015).
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183
190
4.3.3. Factors that influence pore structure
191
SBET and VBJH are the two most basic LTNA parameters, and they can directly indicate the storage spaces of nanoscale reservoirs. Fig. 8a shows that the TOC values are positively correlated with SBET
193
and VBJH. However, the correlation is not strong because of the relatively low TOC values (average
194
0.97%). Fig. 8b shows that the lacustrine clay content is positively correlated with SBET and VBJH; this
195
correlation is mainly caused by the high clay content (42.4%) and well-developed intraparticle pores
196
(e.g., intraplatelet pores within clay aggregates) and interparticle pores (e.g., pores at the edges of
197
particles, crystals and clay platelets) (Figs. 5F, L). The quartz content is also positively correlated with
198
SBET and VBJH (Fig. 8c). Quartz particles in lacustrine shales usually occur within mineral matrices and
199
have well-developed interparticle pores at the edges of other minerals (e.g., clay and carbonate minerals).
200
The quartz particles also have well-developed nanoscale intraparticle pores. Additionally, the quartz
201
particles have no obvious cementation (Fig.5D). Conversely, the carbonate content is negatively
202
correlated with SBET and VBJH (Fig. 8d). Carbonate minerals, particularly calcite, can significantly affect
AC C
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TE D
192
26
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reservoir quality and generally has adverse effects (Wang et al., 2016).
Carbonate minerals (e.g.,
204
calcite) are easily dissolved and redistributed and can be formed in primary pores by various
205
mechanisms during the diagenesis processes (Figs. 5E, H, K) (Huang et al., 1984; Wang et al., 2016).
206
Based on qualitative observations using SEM, polarized light microscopy and fluorescence microscopy,
207
lacustrine carbonate minerals can effectively reduce the available storage space by diagenesis processes
208
including cementation, compaction, pressure-solution, recrystallization and replacement (Figs. 5E, H, K)
209
(Huang et al., 1984; Wang et al.,
210
2016).
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Fig. 8. Correlation plots between nanopore storage space indicators (SBET and VBJH) and values of TOC and minerals.
27
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4.4. Characterization and controlling factors of OM in lacustrine shale
213
4.4.1. Characterization of the OM content
214
Water conditions play an important role in the formation of source rocks. Lacustrine shales
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primarily form in semi-deep to deep lakes with fresh to low salinity water, whereas marine shales
216
primarily form in reductive, salty, weakly alkaline and low energy water. These features give rise to
217
different source rock characteristics (Huang et al., 1984; Bohacs et al., 2000; Curtis, 2002; Schmoker,
218
2002; Chen et al., 2015; Yang et al., 2015; Jiang et al., 2016a). Due to these differences, we evaluate the
219
lacustrine OM using a terrestrial oil generation theory that was derived from studies of continental
220
petroliferous basins in China and plays a special role in global petroleum theories (Huang et al., 1984;
221
Lu and Zhang, 2008) (Table 5).
222
Table 5 Factors used to evaluate OM in lacustrine source rock (Huang et al., 1984; Lu and Zhang, 2008).
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lake types
TOC(%)
freash~brackish water salt water~high salt water
chloroform bitumen ‘A’ (%) S1 +S2 (mg/g)
non-source rock <0.4
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index
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SC
215
<0.2
<0.015
poor
source rock medium good
very good
0.4~0.6
>0.6~1.0
>1.0~2.0
>2.0
0.2~0.4
>0.4~0.6
>0.6~0.8
>0.8
0.015~0.050 <0.050~0.100 >0.100~0.200 <2
2~6
>6~20
>0.200 >20
The mean
224
values (and ranges) of the shale’s kerogen macerals consist of 32% exinites (29-35%), 27%
225
sapropelinites (23-32%) and 24% vitrinites (18-26%), which indicates that the OM is primarily derived
226
from plankton and bacteria. The δ13CPDB values (-29.1 to -29.5‰, average -29.3‰) support this finding
227
(Table 7) (Huang et al., 1984; Lu and Zhang, 2008). KI is an effective indictor of OM type. The OM
228
types (I, II1, II2, and III) can be identified using the KI ranges >>80-100, 80-40, 40-0 and <<0 to -100, 28
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respectively (Huang et al., 1984; Lu and Zhang, 2008). The KI values for the Da’anzhai shale vary from
230
2 to 15 with an average of 8.7, which indicates that the OM is chiefly sapropelic-humic type II2 and has
231
significant potential to generate hydrocarbons. The mean values (and ranges) of the organic elements C,
232
H, and O make up 46.59% (31.12-61.41%), 3.53% (2.09-4.66%) and 1.56% (0.35-1.88%) of the shale’s
233
OM, respectively. The high H/C ratios (0.79-1.05, average 0.90) and low O/C ratios (0.10-0.30, average
234
0.17) (Fig. 9a) indicate that the OM is type II and has relatively high hydrocarbon potential (Huang et al.,
235
1984; Lu and Zhang, 2008). In addition, the Tmax-HI index (Fig. 9b), which is an important evaluation
236
index for the OM type based on Rock-Eval pyrolysis, gives a similar result (Espitaliéet al., 1985; Lu and
237
Zhang, 2008; Misch et al., 2016). Generally, the Da’anzhai shale has similar OM types with other
238
lacustrine and marine shales and the dominant type is of type II (Table 1), which can support the
239
Da’anzhai shale hydrocarbon generation capacity.
240
The TOC values of the Da’anzhai shale vary from 0.11 to 2.18% (average 0.97%), these values are
241
generally lower than those measured in the world's major lacustrine and marine shales (Table 1 and Fig.
242
9). The Ro values of the Da’anzhai shale are similar or even higher than those of other lacustrine and
243
marine shales, which indicate that the OM is highly mature. This could also be deduced from the Tmax
244
values, which vary from 428 to 500 ℃ with an average of 449 ℃ (Table 6). Table 6 Analysis results for
245
Rock-Eval, Chloroform Bitumen ‘A’ and TOC.
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1.15
0.50 3.97 0.16 1.03 0.71 2.12 1.09 1.28 0.55 1.85 0.94 0.26 1.32 0.05 1.83
OPI 0.14 0.34 0.12 0.12 0.24 0.07 0.21 0.10 0.11 0.48 0.14 0.29 0.10 0.35 0.04
G10-2 G4-3 J53-1 Li-1 Li-2 P1-1
maceral (%) 13 exinite vitrinite inertinite δ CP DB Ro （%） KI （‰） sporo normal amorphinite resinite sporinite subtotal fusinite pollinite vitrinite 0.73-1.17 27 4 4 26 34 24 15 11 -29.2 (0.95) 1.13-1.65 27 3 2 30 35 18 20 11 -29.5 (1.43) 1.19-1.47 32 4 4 22 30 22 16 15 -29.2 (1.32) 0.78-1.08 28 6 6 19 31 26 15 9 -29.1 (0.95) 0.95-1.25 27 6 2 21 29 26 18 4 -29.5 (1.17) 0.88-1.28 23 5 6 22 33 26 18 2 -29.2 (1.02) sapropelinite
AC C 249 250
HI (mg/g) 60.60 298.77 40.65 117.53 49.69 142.48 93.29 119.80 79.33 116.50 61.96 43.18 112.14 97.82 159.28
PC (%) 0.05 0.50 0.02 0.10 0.08 0.19 0.12 0.12 0.05 0.30 0.09 0.03 0.12 0.01 0.16
D (%) 5.95 37.51 3.98 11.13 5.48 12.79 9.86 11.17 7.48 18.58 6.05 5.18 10.34 14.22 13.91
RI PT
0.08 2.03 0.02 0.14 0.22 0.17 0.29 0.15 0.07 1.70 0.16 0.11 0.14 0.03 0.09
S1 +S2 (mg/g) 0.58 6.00 0.18 1.18 0.94 2.29 1.38 1.43 0.62 3.55 1.09 0.38 1.46 0.09 1.92
SC
0.009 0.006 0.007 0.004 0.009 0.009 0.008 0.009 0.006 0.006 0.008 0.006 0.006 0.008 0.009
Tmax (℃) 456 444 461 439 428 443 441 446 445 452 453 448 444 500 439
HCI (mg/g) 11.04 153.11 7.30 16.61 16.33 11.67 25.49 14.82 10.77 107.39 10.88 19.28 12.41 73.45 8.27
Table 7 Results for Ro, macerals, carbon isotopes and organic elements.
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0.24 0.11 0.27 0.01 0.40
S0 (mg/g) S1 (mg/g) S2 (mg/g)
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0.02 0.25 0.42 0.40 0.35 0.31 0.16
TOC (%) 0.82 1.33 0.40 0.88 1.43 1.49 1.17 1.07 0.69 1.59 1.51 0.61 1.18
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G10-1 G10-2 G4-1 G4-2 G4-3 J61-4 Li-1 Li-2 Li-3 P1-1 P1-2 P1-4 S2-2 S2-5 XI44-2
chloroform bitumen A（%） 0.03
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Fig.9. Discrimination diagrams for the OM type.
30
organic elements C (%) H (%) O (%)
H/C
O/C
39.89
3.07
0.75
0.92
0.1
46.31
4.07
1.88
1.05
0.3
49.26
3.87
1.37
0.94
0.2
31.12
2.09
0.35
0.81
0.1
61.41
4.66
1.62
0.91
0.2
51.54
3.39
0.96
0.79
0.1
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251
Terrestrial oil-generation theory (Huang et al., 1984; Lu and Zhang, 2008) suggests the following conclusions regarding the lacustrine shale:
253
medium to very good, even though the values of S1+S2 (genetic potential) are poor to medium (Tables 5
254
and 6). (2) Because of the strong hydrocarbon expulsion of the high maturity OM (Ro>0.5-0.7%), the
255
evaluation criteria for high maturity OM should be reduced. The shale with high Ro values (0.95-1.43%)
256
is highly mature. (3) Lacustrine carbonate can also generate hydrocarbons, which is an important feature
257
but easy to overlook. The petroleum generation threshold of carbonate (TOC: 0.12-0.4%) is lower than
258
that of mudstone (TOC: 0.4-0.5%), which means that carbonates can produce hydrocarbons at lower
259
TOC values. Thus, the low TOC values of the high carbonate shale may not indicate a low hydrocarbon
260
generation capacity. Additionally, carbonate interlayers in shale can play a role similar to that of seal
261
rocks, which are conducive to the preservation of oil and gas. In summary, the results of this study
262
indicate that the Da’anzhai shale has some hydrocarbon generation capacity, which is also supported by
263
the clear fluorescence of the thin sections (Figs. 5B, C).
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Based on microscopic observations of the Da’anzhai and lacustrine shales, the OM occurrence
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(1) The TOC and chloroform bitumen ‘A’ values are
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forms are generally similar to other lacustrine shales; they are mainly dispergated in shale matrices
266
(Fig.5) (Morad et al., 2010; Zou et al., 2011; He et al., 2012; Luo et al., 2013; Tian et al., 2014; Dang et
267
al., 2015; Xu et al., 2017). However, these lacustrine shales have more amorphous forms than marine
268
shales because of their different water conditions and biodegradation (Huang et al., 1984; Lu and Zhang,
269
2008; Tian et al., 2014; Xu et al., 2017). In addition, lacustrine OM is distributed in concentric belts
270
around the lake basin’s center, and OM abundance increases with lake depth. Thus, OM is generally
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more abundant in moderately deep to deep lake facies than in shallow lake facies, which can be
272
supported by the wide TOC range of lacustrine shales (Table 1 and Fig. 9) (Huang et al., 1984; Lu and
273
Zhang, 2008; Tian et al., 2014; Xu et al., 2017).
4.4.2. Factors that influence the OM content
Fig. 10. Correlation plots between TOC and V/Cr, V/Sc, Ni/Co and V/(V+Ni) values.
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277 278
Fig. 11. Correlation plots between TOC values and minerals.
279
TOC is commonly used to represent OM. As discussed in Section 4.1, the reducibility is positively correlated with the V/Cr, V/(V+Ni), and V/Sc ratios and negatively with the Ni/Co ratio. In addition,
281
pyrite is an important mineral in organic-rich sediments and is useful for reconstructing
282
paleoenvironmental conditions. A high pyrite content reflects a stable, high-salinity environment with
283
strong reducibility (Leventhal, 1983). The TOC values of the Da’anzhai shale are positively correlated
284
with the V/Cr, V/(V+Ni) and V/Sc ratios (Figs. 10a, b, c) and the pyrite content (Fig. 11a) and correlated
285
negatively with the Ni/Co ratio (Fig. 10d). These values indicate that paleo-redox conditions had a
286
significant effect on the formation and preservation of OM and that the strongly reducing conditions
287
were conducive to the preservation of OM (Zeng et al., 2015; Chen et al., 2016; Xu et al., 2017).
288 289
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Similar to other lacustrine shales, the TOC values of the Da’anzhai shale are positively correlated with the clay mineral content (Fig. 11b), which is due to the OM’s strong sorption ability of clay and its 33
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relatively high content for the lacustrine shales (Huang et al., 1984; Gingele et al., 2001; Dang et al.,
291
2015; Xu et al. 2017). The lacustrine clay minerals are small and rich in intraparticle and interparticle
292
pores, which creates storage space for OM (Figs.5D, F, G and J). The OM observed within the clay
293
minerals primarily has complex, dispergated and residual biological shapes, which indicate that the OM
294
and clay minerals are in close contact (Figs. 5G and J) (Huang et al., 1984; Lu and Zhang, 2008; Tian et
295
al., 2014; Xu et al., 2017). Conversely, the carbonate content is negatively associated with OM (Fig.
296
11c), which is similar to some lacustrine shales (Huang et al., 1984; Jarvis et al., 2001; Tian et al., 2014;
297
Wang et al., 2016). Carbonate minerals fill the pores via cementation, compaction, pressure-solution,
298
recrystallization and replacement , which effectively reduces the storage space for OM (Figs. 5E, H, K)
299
(Huang et al., 1984; Wang et al., 2016).. Furthermore, the high carbonate mineral content in the
300
lacustrine facies reflects relatively shallow water environments, such as the shore-shallow lake face and
301
shallow lacustrine facies, which are not favorable for the generation or storage of OM (Huang et al.,
302
1984; Wang et al., 2016; Xu et al., 2017). There is a weak correlation between the quartz content and
303
TOC values (Fig. 11d). The correlations between quartz and OM are not uniform; differences in the
304
correlation between quartz and TOC contents is related to the depositional environment of shale
305
reservoirs (Zeng et al., 2014; Liu et al., 2015; Ross and Bustin, 2007; Chalmers et al., 2012b; Tian et al.,
306
2013). Negative correlations are caused by the terrestrial quartz inputs (Zeng et al., 2014; Liu et al.,
307
2015), while the positive correlations are related to biogenic quartz (Ross and Bustin, 2007; Chalmers et
308
al., 2012b; Tian et al., 2013). Therefore, the quartz in the lacustrine shales may have both related to
309
biogenic and terrestrial origins. This could lead to the weak correlations between quartz and OM.
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310 311
Based on the analyses in Figs. 10 and 11, the low TOC values of the Da’anzhai lacustrine shales were primarily caused by the oxygen paleoenvironment and carbonate minerals. The relatively shallow
313
and turbulent lake water formed a high oxygen paleoenvironment with greater inputs of terrestrial
314
nutrients, which was conducive to the breeding of shell organisms and the formation of carbonate
315
minerals but not to the preservation of OM. In summary, the oxygen paleoenvironment and high
316
carbonate mineral content are primarily related to the unique lacustrine facies, which was not conducive
317
to the formation of OM.
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4.5. Relationships between the mineralogy, reservoirs, OM and paleoenvironmental conditions of lacustrine shales
The mineral content of lacustrine shales is mainly related to the paleoenvironment. Strong
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paleo-weathering can lead to a greater number of weathering products, which is favorable to the
322
development of terrestrial minerals in lacustrine facies. Humid (rainy) paleoclimates with more water
323
can improve river transportation and reduce lake salinity, and low salinity paleoenvironments inhibit the
324
formation of authigenic carbonate. In addition, fine-grained terrestrial minerals, especially clay minerals,
325
inhibit the formation of authigenic carbonate (Leventhal, 1983; Huang et al., 1984; Bohacs et al., 2000;
326
Chi et al., 2003; Fu et al., 2015). High hydrodynamic conditions can effectively remove fine-grained
327
terrestrial minerals, which favors the formation of authigenic carbonate.
328 329
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The lacustrine sedimentary environment is complex and the lacustrine shale formation is easily influenced by the paleoenvironment. The two most important differences are as follows: first, the
35
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lacustrine paleoenvironments are complex and varied with the short detention time and strong variation
331
of water level (Huang et al., 1984; Chen et al., 2015). (Figs. 1 and 5C, Tables 2 and 6). Second, lakes are
332
close to the terrestrial sources with less water, which lead to rich terrestrial inputs with efficient impact
333
on shale (Figs. 1 and 5A,B, Tables 3 and 6) (Huang et al., 1984; Chen et al., 2015) (Huang et al., 1984;
334
Bohacs et al., 2000; Curtis, 2002; Schmoker, 2002). Lakes are generally located near paleo-uplift
335
environments with large topographical differences, and terrestrial debris can be transported directly into
336
a lake basin over short distances. These features result in rich, proximal and variable sources for
337
lacustrine facies. (Figs. 1, 5A,5B; Table 3). The above two characteristics can effectively lead to the
338
complex and changeable mineral contents of lacustrine shales (Table 3; Fig. 4). In addition, there are
339
some other common points of the lacustrine shales including relatively small thickness of monolayer,
340
more siliceous or calcareous intercalations, and poor comparability with strong heterogeneity (Table 1),
341
which are also the results of the two characteristics of lacustrine sedimentation environments (Tian et al.,
342
2014; Wang et al., 2015a; Zhu et al., 2016; Xu et al., 2017).
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Similar to other lacustrine shales, the storage space of the Da’anzhai lacustrine shale is directly
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330
controlled by minerals (Fig. 12). Because of the large amount of terrestrial clastic minerals, the
345
interparticle pores of lacustrine shales are well-developed. Developed interparticle pores and intraparicle
346
pores can guarantee a considerable amount of storage space. In addition, numerous siliceous or
347
calcareous interlayers in lacustrine shale formations can result from varying changeable
348
paleoenvironments with rich terrestrial inputs. These interlayers can increase the brittleness index and
349
facilitate the development of cracks.
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350
Compared to other shales, the Da’anzhai lacustrine shale has a relatively low OM content (Table 6), which is mainly the result of more variable paleoenvironments with high oxygen content (Fig. 12)
352
(Huang et al., 1984; Lu and Zhang, 2008; Tian et al., 2014; Xu et al., 2017). Different minerals have
353
different effects on storage spaces of OM, and minerals in lacustrine shales are mainly controlled by
354
paleoenvironments (Fig. 12). In addition, the relatively low OM content of the Da’anzhai lacustrine
355
shale is the reason for OM weak effects on storage spaces (Fig. 8a).
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In summary, minerals, reservoirs, and OM in lacustrine shales are closely associated with each
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other, and they were all controlled by the paleoenvironment which was the dominant factor and the link
358
between these parameters (Fig.
12).
359
Fig.12 Relationships between mineralogy, reservoirs, OM and paleoenvironment for lacustrine shale
362
4. Conclusions
363 364
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1. There are some similarities between the Da’anzhai lacustrine shale and other lacustrine shales. (1)
365
The two most important differences are that lacustrine shales formed in more complex and variable
366
paleoenvironments and have more abundant terrestrial inputs than marine shales. (2) The mineral
367
compositions of lacustrine shales are variable and complex, and they have abundant terrestrial minerals. 37
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In addition, lacustrine shales have numerous siliceous or calcareous interlayers. (3) The interparticle
369
pores and interlayer pores of lacustrine shales are well-developed. Unlike other lacustrine shales, the
370
Da’anzhai shale has relatively weak hysteresis loops and wide overlapping ranges of adsorption and
371
desorption isotherms, indicating a greater number of dead-end pores with more complex structures. In
372
addition, the Da’anzhai shale has lower TOC values with more amorphous OM forms.
2. The minerals, reservoirs, and OM for lacustrine shales are closely associated with each other;
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368
these parameters are all controlled by the paleoenvironmental conditions in which the shales formed. (1)
375
Highly variable lacustrine paleoenvironment lead to variable mineral contents and abundant terrestrial
376
inputs give rise to a high terrestrial mineral content in lacustrine shales, which can also create a higher
377
number of siliceous or calcareous intercalations relative to marine shales. (2) Terrestrial minerals (e.g.,
378
clay and quartz) can effectively improve the storage spaces, but the authigenic carbonate minerals (e.g.,
379
calcite) have the opposite effect. Well-developed interparticle pores in lacustrine shales are caused by
380
abundant particles. (3) Clay content has a positive effect on the formation and preservation of OM, while
381
higher oxygenation and carbonate contents have negative effects. Lakes generally have relatively
382
shallow water and a variable paleoenvironment, which leads to a higher oxygen content and reduces the
383
OM abundance of lacustrine shales. The weak influence of OM on nanoscale storage spaces is due to the
384
low TOC values of the lacustrine shales.
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385
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3. Compared to other lacustrine shales, the Da’anzhai shale also demonstrates the potential for
386
unconventional oil and gas exploration. (1) The lacustrine shale has well-developed storage spaces due
387
to the high number of interparticle pores, intraparticle pores, and fractures. Although the lacustrine shale
38
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has more dead-end pores and larger pores with more complex microstructures, the types of isotherms
389
(Type П) and hysteresis loops (Type H3) are generally similar to those of lacustrine shales. The values of
390
VBJH (1.19-4.10 m3/100 g, average 2.97 m3/100 g) and SBET (1.75-10.69 m2/g, average 6.92 m2/g) are
391
similar to other shales, indicating a high amount of storage space in nanoscale reservoirs. (2) Abundant
392
terrestrial minerals in lacustrine facies can support a greater number of interparticle pores and
393
intraparticle pores. Numerous siliceous or calcareous interlayers in lacustrine shales can also improve
394
the brittleness index, facilitating the development of fractures and creating a seal for oil and gas. (3)
395
Although the TOC values of the Da’anzhai shale are lower than other lacustrine shales, the OM still has
396
several advantages according to the evaluation criteria for continental source rocks, including moderate
397
values of chloroform bitumen ‘A’ and TOC, a high level of OM maturity and a favorable kerogen type
398
(II).
399
Acknowledgments
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Our study is supported by the Institute Program of PetroChina Research Institute of Petroleum
401
Exploration and Development (Grant 2016yj01), and the National Natural Science Foundation of China
402
(Grants 41272137 and 41572117). The authors also sincerely appreciate the support from the China
403
Scholarship Council (CSC).
404
Nomenclature
405
TOC
406
Ro
Vitrinite reflectance
407
Tmax
Pyrolysis Temperature at Maximum Hydrocarbon Generation
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400
Total organic carbon, %
39
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SBET
Brunauer-Emmett-Teller specific surface area
409
VBJH
Barrett-Joyner-Halenda total pore volume
410
DA
Average pore diameter
411
OM
Organic matter
412
TE
Trace element
413
XRD
X-ray diffraction
414
FE-SEM
Field emission scanning electron microscopy
415
LTNA
Low temperature N2 adsorption
416
EDS
Energy dispersive spectrometry
417
S0
Gaseous hydrocarbons, mg/g
418
S1
Free hydrocarbons, mg/g
419
S2
Pyrolysis of hydrocarbons, mg/g
420
ICP-MS
Inductively coupled plasma-mass spectrometry
421
REE
Rare earth element, ppm
422
ΣHREE
Total content of heavy rare earth elements
423
ΣLREE
Total content of light rare earth elements
424
PSD
Pore size distribution
425
OPI
426
HI
Hydrogen index; HI=(S2/TOC)×100%, mg/g
427
PC
Effective carbon content; PC=0.083×(S0+S1+S2), %
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Oil production index; OPI=S1/(S0+S1+S2)
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D
Degradation rate; D=(PC/TOC) ×100%, %
429
HCI
Hydrocarbon index; HCI=(S0+S1)/ TOC×100, mg/g
430
KI
Kerogen type index; KI=Sapropelinite (%)×1+Exinite (%)×0.5+Vitrinite
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(%)×(-0.75)+Inertinite (%)×(-1) (Huang et al., 1984; Lu and Zhang, 2008)
432
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433
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435
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441 442
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Geological and geochemical characterization of lacustrine shale: a case study of the Jurassic Da'anzhai Member shale in the central Sichuan Basin, Southwest China
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Qilu Xua,b,c, Bo Liuc,*, Yongsheng Maa, Xinmin Songd, Yongjun Wangd, Zhangxin Chenb a
School of Earth Sciences and Resources, China University of Geosciences(Beijing), Beijing 100083, China
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b
Chemical and Petroleum Engineering, University of Calgary, Calgary, Alberta T2N1N4, Canada
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c
Oil and Gas Research Center, Peking University, Beijing 100871, China
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d
PetroChina Research Institute of Petroleum Exploration and Development, Beijing 100083, China
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Abstract：
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Although numerous progresses have been achieved in characterizing marine shales, studies that are associated with lacustrine shales are limited.
To study lacustrine shales, samples were collected from
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the Jurassic Da'anzhai Member in the central Sichuan Basin of China, and their mineralogical, reservoir,
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OM, and paleoenvironmental characteristics were determined, as well as the relationships between them.
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Analysis of trace elements reveals that the shales formed in paleoenvironments that were oxic to suboxic,
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dry to humid, had moderate to strong weathering, and were characterized by fresh to salt water
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conditions. These environments are more variable than those of marine shales. The paleoenvironmental
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conditions and mineralogy of the shales, particularly the oxic to suboxic paleo-redox conditions, resulted
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in the relatively low levels (0.11-2.18%, average 0.97%). However, based on evaluation criteria for
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continental source rocks, the OM is of high quality because of its high level of maturity (Ro: 0.95-1.43;
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Tmax: 428-500°C) and favorable kerogen type (II2). There are well-developed intraparticle pores,
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interparticle pores, and microcracks. The SBET (5.42-10.69 m2/g, average 6.92 m2/g) and VBJH (1.19-4.10
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mL/100 g, average 2.97 mL/100 g) values also indicate good nanoscale storage space. Terrestrial
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minerals (i.e., quartz and clay) and authigenic carbonate minerals (i.e., calcite) are, respectively,
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positively and negatively correlated with the nanoscale storage space, .. Small pores (3-5 nm) dominate
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the nanoscale storage space. The isotherms and hysteresis loops are of Type П and Type H3, respectively,
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which indicates wedge-shaped pores. However, the hysteresis loops indicate that the lacustrine shale has
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more dead-end pores and larger pores with more complex microstructures than other lacustrine
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reservoirs. In general, the Da’anzhai lacustrine shale has the potential for unconventional oil and gas
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exploration. The lacustrine shale’s mineralogy, reservoirs, and OM are closely related to each other, and
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their differences are mainly caused by the paleoenvironmental conditions in which the shale formed.
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Keywords: Lacustrine shale; Paleoenvironment; Nanoscale reservoir; Organic matter; Minerals; Mechanisms.
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1. Introduction
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The high commercial production of shale oil and gas in North America has made shale a focus of
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exploration in many countries and regions. However, the most widely developed shales were formed in
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marine systems; few studies have focused on the characteristics of lacustrine shales. Additionally,
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lacustrine shales are widely distributed in many areas, such as Africa, South America, Southeast Asia
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and China (Huang et al., 1984; Bohacs et al., 2000; Curtis, 2002; Schmoker, 2002; Chen et al., 2015;
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Jiang et al., 2016a; Yang et al., 2015).
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The most obvious characteristic of lacustrine shales is that they are subject to more variable paleoenvironments than marine shales. The formation environments of lacustrine and marine shales are
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very different in terms of the water depth, energy, provenance, sedimentary type and biological effects
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(Huang et al., 1984; Bohacs et al., 2000; Curtis, 2002; Schmoker, 2002). These features lead to
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differences in their basic characteristics including mineralogy, organic matter (OM), and reservoir type
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(Table 1). Minerals are the material basis of shales, and they affect not only OM generation but also
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reservoir development (Huang et al., 1984; Bohacs et al., 2000; Curtis, 2002; Schmoker, 2002; Chen et
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al., 2015; Yang et al., 2015; Jiang et al., 2016a). A major breakthrough in marine shale oil and gas
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exploration was based on the recognition that shale acts not only as a source rock but also as a reservoir.
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Unlike studies of conventional reservoirs, research on unconventional reservoirs has primarily focused
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on nanoscale reservoirs. Previous studies of marine and lacustrine shales suggest that shale gas is mainly
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controlled by a nanoscale pore system that is closely related to the mineral characteristics (Zou et al.,
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2011; Morad et al., 2010; He et al., 2012; Luo et al., 2013; Dang et al., 2015; Chalmers et al., 2012;
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Curtis et al., 2012; Jiang et al., 2015; Li et al., 2016; Jiang et al., 2017; Zhang et al., 2017). OM is the
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basic material that generates hydrocarbons, and its type and maturity dominate the formation and
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enrichment of shale oil and gas. The preservation and enrichment of OM in sediments is controlled by
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many factors, including the primary productivity, paleo-redox conditions, nutrient availability, clastic
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influx and minerals (Huang et al., 1984; Bohacs et al., 2000; Curtis, 2002; Schmoker, 2002; Chen et al.,
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2015; Yang et al., 2015; Jiang et al., 2016a; Xu et al., 2017). In summary, the paleoenvironment,
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minerals, reservoir, and OM, which are the basic characteristics of shales, have been the focuses of shale
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oil and gas studies, and these characteristics are often interrelated. Additionally, studies of the
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characteristics and mechanisms have mainly focused on marine shales; fewer studies have examined
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lacustrine shales.The lacustrine shale of the Jurassic Da’anzhai Member from the central Sichuan Basin
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of China has been interpreted as fractured reservoirs, low permeability fractured reservoirs and tight
665
reservoirs. However, most studies have only focused on the conventional carbonate reservoirs, and the
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shales have only been considered as source rocks for the carbonate reservoirs (Lu et al., 2014; Chen et
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al., 2015). Encouragingly, the Da’anzhai shale of the Yuanba Block in the northwestern Sichuan Basin,
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which has similar geological conditions to those of the central Sichuan Basin, hosts high-yield wells (He
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et al. 2012; Zhu, et al. 2016). In addition, some lacustrine shales have yielded oil or gas, such as the
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Triassic Chang 7 and Chang 9 Member in the Ordos Basin, the Paleogene strata (Es3 and Es4) in the
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Bohai Bay Basin and the Cretaceous strata in the Songliao Basin (Qing-1) (Zou et al., 2011; Luo et al.,
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2013; Dang et al., 2015; Li et al., 2015; Yang et al., 2015; Jiang et al., 2016b). These successful
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developments strongly indicate that the Da’anzhai shale in the central Sichuan Basin is worth studying.
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In this study, we assessed the Da’anzhai lacustrine shale’s characteristics. We performed
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paleoenvironmental evaluation using TE, mineralogical evaluation using XRD analysis, reservoir
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evaluation using FE-SEMand LTNA analyses, and OM evaluation using analyses of the TOC,
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chloroform bitumen ‘A’, Rock-Eval, carbon isotopes, kerogen macerals and organic elements. We also
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compared the Da’anzhai shale to other lacustrine shales which have been successfully developed and
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assessed the similarities and differences of lacustrine shales. In addition, we summarized the
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relationships between these characteristics and mechanisms affecting them. Finally, we judged the
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development potential of the Da’anzhai shale.
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Table 1 Comparisons between basic parameters in the Da’anzhai shale and other successfully developed shales lacustrine shale
basin
Sichuan Basin, China
basin type
layer
foreland basin
Jurassic shale with
Chang 9
Es3
Es4
Qing-1
Barnett
Ordos Basin,
Ordos Basin,
Bohai Bay
Bohai Bay
China
China
Basin, China
Basin, China
polycyclic
polycyclic
superimposed
superimposed
downfaulted
downfaulted
basin
basin
basin
basin
basin
Triassic
Triassic
Paleogene
Paleogene
Cretaceous
Carboniferous
shale with
siliceous and
argillaceous
carbonaceous
limestone
shale, limestone,
interlayers
dolomite
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Central
Chang 7
Songliao
Fort Worth Basin,
Basin, China
Texas, US
downfaulted
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Da'anzhai
marine shale
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parameters
foreland basin
Ohio Appalachian Basin, Kentucky, US piedmont depression Devonian
shale with
shale with
shale with
shale with
siltstone
siltstone
siltstone
siltstone
interlayers
interlayers
interlayers
interlayers
1900~3200
1500~2500
1600~2600
1500~4200
1500~5200
1000~2500
1980~2590
510~1800
10~70
30~70
10~15
100~500
250~350
70~150
15~60
9~31
(20~80) /54
(37~53) /43
(29~56) /45
(25~80) /36
(30~80) /36
(30~80) /36
35~80
45~60
porosity (%)
0.20~5.92
1~12
permeability
0.002~0.9
TOC (%) Ro (%)
lithological
shells and
combination
carbonaceous interlayers
depth (m)
thickness (%)
minerals (%)
genetic type
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brittle
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effective
interbed of black shale and gray siliceous shale
10
2~8
1.3~9.3
6~12
4~5
4~7
0.01~0.3
(0.3~5.0) /2.97
<0.50
<0.50
<0.15
0.01
<0.1
(0.1~2.2) /1.0
(0.3~36) /8.3
(0.3~11.3) /3.1
(0.5~13.8) /3.5
(0.8~16.7) /3.2
(0.4~4.5) /2.2
2.0~7.0
0.5~4.7
0.9~1.5
0.8~1.2
0.9~1.3
0.4~1.2
0.4~2.0
0.5~1.5
1.1~2.2
0.3~1.3
pyrolysis gas
pyrolysis gas
pyrolysis gas
pyrolysis gas
Ⅰ-Ⅱ1
Ⅰ-Ⅱ
Ⅱ
Ⅱ
AC C
682
pyrolysis gas
pyrolysis gas
pyrolysis gas
Ⅱ
Ⅰ-Ⅱ1
Ⅰ-Ⅱ1
biogenic and pyrolysis gas
kerogen type
Ⅰ-Ⅱ1
55
ACCEPTED MANUSCRIPT
al., 2013; Lu et al., 2014; Chen et al., 2015;
/2.00
0.31~1.24
8.50~9.91
Zou et al.,
Zou et al., 2011; Luo et al., 2013; Yang et al., 2015; Jiang et al., 2016a.
/2.92
2011; Luo et
Zou et al., 2011;
Zou et al.,
al., 2013; Shi et
Luo et al., 2013;
2011; Luo et
al., 2013; Yang
Dang et al.,
al., 2013; Li,
et al., 2015;
2015; Jiang et
2014; Jiang et
Jiang et al.,
al., 2016a
al., 2016a
2016a.
Zou et al.,
56
1.69~2.83
Schmoker, 2002;
Schmoker, 2002;
Bowker et al.,
Bowker et al.,
RI PT
references
(0.60~3.70)
2011; Luo et
2007; Jarvie et al.,
2007; Jarvie et al.,
al., 2013; Jiang
2007; Martini et
2007; Martini et
et al., 2016a
al., 2008; Lu et al.,
al., 2008; Lu et al.,
2014
2014.
SC
2011; Luo et
(1.68~4.25)
M AN U
Zou et al.,
0.92~2.91
TE D
(m /t)
0.87~1.98
EP
3
AC C
gas content
ACCEPTED MANUSCRIPT
2. Geological setting
686 687
EP
685
Fig. 1. Study location, geological map (Chen et al., 2015) and simplified stratigraphic column of the Jurassic Da’anzhai
Member in the central Sichuan Basin.
AC C
684
TE D
M AN U
SC
RI PT
683
The study area is located in the central Sichuan Basin, which includes the Nanchong and Suining
688
areas. It is bounded by the Huafu Mountains to the east and the Longquanshan basement faults to the
689
west, and it covers an area of 6×104 km2 (Fig. 1) (Lu et al., 2014; Chen et al., 2015). A lacustrine
690
environment dominated the Sichuan Basin during the Jurassic Period, and the Da’anzhai Member
691
represents the largest and deepest stage of this environment. The sedimentary facies of the Da’anzhai 57
ACCEPTED MANUSCRIPT
Member can be divided into shore-shallow lacustrine, shallow lacustrine and semi-deep lacustrine
693
subfacies, which include shell beach, calcareous flat, mud flat, and semi-deep lake microfacies. These
694
facies are distributed in concentric belts around the lake basin’s center in the Yilong-Pingchang areas
695
(Fig. 1) (Lu et al., 2014; Chen et al., 2015). From top to bottom, the Da’anzhai Member in the central
696
Sichuan Basin can be divided into main three sub-members: Da1, Da13 and Da3 (Fig. 1). The thickest
697
Da13 sub-member (30-50 m) is a set of widely distributed dark mudstones with lamellated fractures and
698
rich OM; this sub-member is considered the most important source rock for hydrocarbons. The Da3
699
sub-member deposits (5-14 m), which are initial deposition products, are primarily (mud) shell
700
limestones with dark gray mudstone interlayers. The Da1 sub-member (30-45 m thick) primarily
701
consists of shales and (mud) shell limestones; this sub-member contains the sedimentary products of the
702
period of basin uplift (Lu et al., 2014; Chen et al., 2015).
TE D
M AN U
SC
RI PT
692
3. Samples and experimental methods
704
3.1. Samples
705
Thirty-nine representative core samples were obtained from the Da’anzhai Member shale in the
EP
703
central Sichuan Basin based on drilling, core observations, stratigraphic data, and log responses (Fig. 1
707
and Table 3).
AC C
706
708
3.2. Methods
709
All 39 shale samples were analyzed to determine their bulk mineralogy via XRD using a X'Pert Pro
710
MPD device with a working voltage of 40 kV and a current of 40 mA in the Key Laboratory of Orogenic
711
Belts and Crustal Evolution at Peking University. To investigate the clay mineral fraction in detail,
58
ACCEPTED MANUSCRIPT
712
textured mounts were measured four times at a goniometer rate of 0.5θ/minute with a registration range
713
from 2 to 42°2θ at the Beijing Research Institute of Uranium Geology. The micromorphology and oiliness of the samples were determined at the Key Laboratory of
RI PT
714
Marine Reservoir Evolution and Hydrocarbon Accumulation of the China University of Geosciences
716
(Beijing) using an Axio Scope A1 pol Fluorescence-Polarization Microscope. An INCA Synergy system
717
at the Key Laboratory of Orogenic Belts and Crustal Evolution at Peking University was used to analyze
718
the microstructure of each sample via FE-SEM and the mineral composition of each sample via EDS.
719
The samples were subjected to argon-ion polishing to obtain higher resolution images.
M AN U
720
SC
715
LTNA analysis is capable of characterizing nanoscale pores in porous media, and adsorption tests were performed with dried samples (60-80 mesh) on an ASAP2020M device at the College of
722
Chemistry and Chemical Engineering of the China University of Petroleum (Beijing). N2 adsorption
723
isotherms were obtained at -196.15°C within a relative pressure range of 0.005 to 0.998. The BET
724
method for the equivalent specific surface areas and the BJH method for the total pore volumes were
725
applied in the calculations (Brunauer et al., 1938; Barrett et al., 1951).
EP
TE D
721
The OM richness was characterized by measuring the TOC values of 25 samples using a GHM-02
727
analyzer; 10% hydrochloric acid was used to remove carbonates, and the test sensitivity was 10-13 mg/g.
728
Chloroform bitumen ‘A’ was extracted from the crushed shale powders (<0.09 mm), and the weight was
729
calculated at the China University of Petroleum (Beijing). A Rock-Eval 6 instrument was used to
730
perform a pyrolysis analysis, which yielded the temperature of the Tmax, S0, S1, and S2. The OM maturity
731
was determined based on the Ro value, which was obtained using a reflected light microscope under oil
AC C
726
59
ACCEPTED MANUSCRIPT
immersion and calculated as the mean of 10-30 measurements taken for a single sample. Additionally,
733
measurements were taken to determine the isolated kerogen content and identify the organic kerogen
734
macerals, carbon isotope content and organic element content according to the (GB/T) 19144-2010,
735
(SY/T) 5125-2014, (GB/T) 18340.2-2010, (GB/T) 19143-2003 standards, respectively. The kerogen
736
measurements and the rock pyrolysis analysis were performed at the Keyuan Engineering Technology
737
Testing Center.
SC
The TE concentrations of 17 samples were determined using ICP-MS, which was performed at the
M AN U
738
RI PT
732
739
Beijing Research Institute of Uranium Geology. The detailed sample processing procedure as well as the
740
analytical precision and accuracy followed the methods described for the (GB/T) 14506.28-2010
741
standard.
4. Results and discussions
743
4.1. Paleoenvironmental properties and differences
744
Lacustrine shales are different from marine shales mainly due to their formation paleoenvironments.
EP
TE D
742
Lakes are complex and dynamic systems, and the development of lacustrine shales is controlled more
746
significantly by the sedimentary environment (Huang et al., 1984; Bohacs et al., 2000; Curtis, 2002;
AC C
745
60
ACCEPTED MANUSCRIPT
Table 2 Trace elements of the Da’anzhai lacustrine shale. G10-2
G4-3
G6-2
J45-1
J45-2
J53-1
J61-1
J61-2
J61-3
J61-4
Li-1
Li-2
P1-1
S2-1
S2-3
S2-4
X44-1
Th(ppm)
11.70
10.40
5.43
4.32
12.00
10.60
1.72
4.13
7.74
10.20
11.30
9.50
11.60
2.31
10.50
6.21
13.70
Ni/Co
2.68
2.98
3.26
4.10
2.87
2.72
5.49
3.87
3.50
2.51
2.65
2.97
3.07
3.22
2.32
3.37
2.78
RI PT
code
1.76
1.58
1.44
1.50
1.64
1.64
2.47
1.56
1.33
1.69
1.60
1.50
1.61
1.77
1.30
1.41
1.45
0.78
0.80
0.63
0.63
0.78
0.78
0.67
0.68
0.70
0.77
0.78
0.74
0.78
0.61
0.68
0.68
0.77
V/Sc
9.78
8.70
6.88
7.73
9.36
8.91
13.81
8.41
7.94
9.03
7.93
7.21
9.22
9.80
8.54
7.14
7.81
Th/U
2.77
4.24
4.45
2.79
4.07
2.95
1.28
3.53
4.63
3.81
4.25
4.63
4.20
2.74
5.25
4.60
4.72
Sr/Cu
3.75
4.60
42.98
77.10
3.50
3.62
58.70
35.54
53.09
8.46
7.84
19.01
3.35
80.54
7.47
24.69
6.05
Sr/Ba
0.29
0.26
3.66
3.08
0.23
0.24
4.77
2.78
3.45
0.52
0.46
1.28
0.18
3.77
0.44
1.84
0.36
118.7
115.3
109.8
131.7
110.3
187.5
141.2
128.9
194.3
112.9
130.6
112.6
3
6
4
1
7
7
6
9
0
9
7
2
7.79
9.03
6.39
6.65
9.34
9.58
8.46
8.67
8.49
8.04
9.44
4.19
9.93
9.86
8.88
7.88
10.03
(La/Yb)N
6.65
7.75
6.57
8.37
6.82
8.04
9.85
9.82
11.14
8.00
7.50
4.35
7.49
10.88
8.62
7.67
9.05
(La/Sm)N
4.66
4.74
3.04
3.32
5.58
5.42
3.19
3.06
2.37
3.88
5.71
2.59
5.90
3.07
4.20
3.90
5.01
(Gd/Yb)N
0.99
1.04
1.52
1.97
0.77
0.97
2.04
2.01
2.92
1.45
0.84
1.41
0.79
2.10
1.35
1.38
1.17
(Ce/Yb)N
4.48
5.38
4.98
5.83
4.54
5.42
7.21
7.33
9.24
5.58
4.97
3.22
5.00
8.49
6.10
5.59
6.15
ΣLREE/ ΣHREE
88.73
M AN U
)
130.97
38.15
TE D
ΣREE(ppm
SC
V/Cr V/(V+Ni)
60.76
153.93
0.92
0.94
0.97
0.95
0.92
0.93
0.94
0.93
0.99
0.92
0.92
0.92
0.93
0.96
0.95
0.97
0.92
0.19
0.18
0.77
1.12
0.17
0.16
0.63
0.56
0.88
0.36
0.27
0.61
0.15
0.72
0.12
0.35
0.23
EP
δCe CSr(‰)
AC C
XN, where N refers to the chondrite-normalized value (Gromet et al., 1984); δCe=CeN/(LaN×PrN)1/2 (Wright et al.,1984; Murray et al.,1990).
61
ACCEPTED MANUSCRIPT
Schmoker, 2002). TEs can be used as effective quantitative indicators of the paleoenvironment by
2
comparing their type and abundance with standard values (Wright et al., 1984; Murray et al., 1990;
3
Manning, 1994; Ross et al., 1995; Kimura and Watanabe, 2001; Jarvis et al., 2001; Rimmer, 2004; Jones
4
and Roy et al., 2007).
5 6
TE D
M AN U
SC
RI PT
1
Fig. 2. Identification of paleoenvironments using TEs (a-c) and clay (d).
4.1.1. Paleo-redox conditions
8
Due to their responses to changing redox conditions, the ratios of redox-sensitive TEs, such as
AC C
9
EP
7
Ni/Co, V/Cr, V/Sc, Th/U, and δCe, are usually used to evaluate redox conditions. According to Jones
10
and Manning (1994), V/Cr ratios <2 indicate oxic conditions, V/Cr ratios from 2 to 4.25 indicate dysoxic
11
conditions, and V/Cr ratios >4.25 indicate suboxic to anoxic conditions. Kimura and Watanabe (2001)
12
found that the V/Sc ratio is positively related to oxic conditions. Jones and Manning (1994) showed that
13
Ni and Co are enriched in pyrite, and high Ni/Co ratios indicate anoxic conditions, Ni/Co ratios <5 62
ACCEPTED MANUSCRIPT
indicate oxic conditions, Ni/Co ratios between 5 and 7 indicate dysoxic conditions, and Ni/Co ratios >7
15
indicate suboxic to anoxic conditions. Similarly, Th/U ratios <1.5 represent anoxic conditions, Th/U
16
ratios between 1.5 and 3 represent dysoxic to suboxic conditions, and Th/U ratios >3 represent oxic
17
conditions (Jones and Manning, 1994). In addition, δCe values >1 (positive anomaly) and <0.95
18
(negative anomaly) suggest anoxic and oxic to suboxic conditions, respectively (Wright et al., 1984;
19
Murray et al., 1990).
SC
For the Da’anzhai shale in the central Sichuan Basin (Table 2), the ranges of the Ni/Co, V/Cr, V/Sc,
M AN U
20
RI PT
14
and Th/U ratios are relatively wide, from 2.32-5.49, 1.30-2.47, 6.88-13.81, and 1.28-5.25, respectively.
22
The values of δCe vary from 0.92 to 0.99 (average 0.94), which indicate a weakly negative anomaly
23
(Wright et al., 1984; Murray et al., 1990).Collectively, these proxies reveal that the shale was deposited
24
under variable paleo-redox conditions that were mainly oxic to suboxic (Fig. 2).
TE D
21
4.1.2. Paleo-salinity
26
Sr/Ba values vary directly with the distance to a sea or lake coast; Sr/Ba ratios <1 suggest
EP
25
freshwater conditions, and Sr/Ba ratios >1 suggest salt water conditions (Jarvis et al., 2001; Kimura and
28
Watanabe, 2001). In addition, low Sr concentrations (0.1-0.3‰) indicate a freshwater setting, and high
29
values (0.8-1‰) represent a salt water setting (Jarvis et al., 2001; Kimura and Watanabe, 2001). Th
30
easily dissolves in acidic water and hydrolyzes to oxide or hydroxide sediments under alkaline
31
conditions; therefore, high Th values indicate a salt water environment (Jones and Manning, 1994).
32 33
AC C
27
In the Da’anzhai shale samples (Table 2), the Sr/Ba ratios vary widely (0.18-4.77) with an average of 1.62. The Th content also varies widely (1.72-13.70 ppm) and averages 8.43 ppm, and the Sr 63
ACCEPTED MANUSCRIPT
34
concentrations vary from 0.12 to 1.12‰ (average 0.44‰). Generally, these proxies indicate variable
35
paleo-salinity conditions including both fresh and salt water settings (Fig. 2).
RI PT
4.1.3. Paleo-weathering
M AN U
SC
36
37
39
Fig. 3. Chondrite normalized REE distribution curves for REE minerals. Chondrite values are from Gromet (1984).
When rocks are subjected to strong weathering, heavy rare earth elements (HREEs) dissolve and
TE D
38
migrate much more easily than light rare earth elements (LREEs). Therefore, high ΣLREE/ΣHREE
41
values indicate strong paleo-weathering conditions (Kimoto et al., 2006; Stevens and Quinton, 2008). In
42
addition, the (La/Yb)N, (La/Sm)N, (Gd/Yb)N and (Ce/Yb)N ratios, which are positively related to LREE
43
enrichment, are also positively correlated with paleo-weathering (Table 2) (Wright et al., 1984; Murray
44
et al., 1990; Ross et al., 1995; Roy et al., 2007).
AC C
45
EP
40
The ΣLREE/ΣHREE rates are between 4.19 and 10.03 (average 8.39), which indicate LREE
46
enrichment. The (La/Yb)N ratio is represented by the slope of the chondrite-normalized REE distribution
47
curve (Fig. 3), which indicates the degree of the graph’s inclination (Ross et al., 1995; Roy et al., 2007).
48
The (La/Yb)N values vary from 4.35 to 11.14 (average 8.15), which indicate that the curves are tilted to 64
ACCEPTED MANUSCRIPT
the right and that the samples are rich in LREE-acidic rocks (Fig. 3). The (La/Sm)N ratio, which
50
represents the degree of LREE fractionation, is positively related to the ΣLREE value (Ross et al., 1995;
51
Roy et al., 2007). The (La/Sm)N ratios range from 2.37 to 5.90 (average 4.10), which indicates a P(E)
52
model with abundant LREEs. The (Gd/Yb)N ratio, which indicates the degree of HREE fractionation, is
53
negatively correlated with the ΣHREE value (Ross et al., 1995; Roy et al., 2007). The (Gd/Yb)N values
54
vary from 0.77 to 2.92 (average 1.46), and the (Ce/Yb)N ratios vary from 3.22 to 9.24 (average of 5.85),
55
which indicate that the Da’anzhai shale is depleted in HREEs (Fig. 3). In summary, the REE results
56
(Table 2) indicate that the shale was deposited under moderate weathering conditions with periods of
57
strong weathering.
M AN U
SC
RI PT
49
4.1.4. Paleoclimate
59
∑REE values are closely related to paleoclimate conditions; that is, ∑REE values are higher in
TE D
58
warm and humid climates and lower in cold and dry climates (Wright et al., 1984; Murray et al., 1990;
61
Ross et al., 1995; Roy et al., 2007). The ΣREE values of the Da’anzhai shale vary widely from 38.15 to
62
194.3 µg/g (average 121.59 µg/g) (Table 2). The Sr/Cu ratio is another useful indicator of lacustrine
63
paleoclimate, with high values reflecting humid climates (Reheis, 1990; Chen et al., 2009).The Sr/Cu
64
ratios in the Da’anzhai shale also vary widely from 3.35 to 80.54%. In addition, illite usually forms in
65
dry conditions by the weathering depotassication of feldspar, mica and other aluminosilicate minerals.
66
Conversely, kaolinite forms in humid climates by the eluviation of feldspar, mica, and pyroxene.
67
Therefore, high illite/clay ratios indicate dry, saline waters with high K+ content, and high kaolinite/clay
68
ratios suggest humid climates (Vanderaveroet, 2000; Gingele et al., 2001). The illite/clay ratios of the
AC C
EP
60
65
ACCEPTED MANUSCRIPT
Da’anzhai shale are relatively high (between 26 and 86%, average 52%). However, the kaolinite/clay
70
ratios are relatively low (between 3 and 19%, average 11%) (Fig. 2d). These results indicate a variable
71
climate with both dry and humid periods as well as a drying trend from humid climates.
72
RI PT
69
In general, the TE contents and types t in lacustrine shales are more complex and varied than those in marine shales, which indicates that lacustrine shales tend to experience more variable
74
paleoenvironmental conditions (Huang et al., 1984; Bohacs et al., 2000; Curtis, 2002; Schmoker, 2002).
75
The lacustrine Da’anzhai shale was formed under relatively dry climatic conditions that included both
76
arid and humid stages. In addition, the shale experienced a moderate degree of paleo-weathering with
77
stages of strong weathering, variable oxic to suboxic paleo-redox conditions that were weakly reducing,
78
and a predominantly freshwater setting interspersed with periods of saltwater.
M AN U
4.2. Mineralogy properties and differences
TE D
79
SC
73
As shown in Table 3, the Da’anzhai shale primarily contains clay (42.4%), quartz (27.7%) and calcite
81
(21.1%). The minerals in these lacustrine shales vary widely. Clay comprises the majority of the shale
82
and ranges from 16.3 to 68.4%, while the quartz content ranges from 5.6 to 53.4%. Carbonate minerals,
83
which mainly include calcite, dolomite, and aragonite, range from 3.0 to 46.6% with an average of
84
27.4%. The Da’anzhai shale contains more brittle minerals than the other lacustrine shales and marine
85
shales (Table 1 and Fig. 4). For example, the brittle mineral content of the Da’anzhai shale ranges from
86
24.2 to 80.3%, with an average of 53.7%, indicating medium-high brittleness that is favorable for natural
87
fractures and hydraulic fracturing. Similar to other lacustrine shales, lacustrine shales have more
88
siliceous or calcareous interlayers than marine shales, which can cause the wider ranges of carbonate and
AC C
EP
80
66
ACCEPTED MANUSCRIPT
quartz minerals and improve the brittleness index and fracturing (Figs1 and 5I).Ternary diagrams were
90
constructed to compare the mineral compositions of hot shales that have been successfully exploited
91
including lacustrine shales (Fig. 4a) and marine shales (Fig. 4b). Generally, the comparison shows that
92
the mineral composition of the lacustrine shales is more variable and complex than in marine shales (Fig.
93
4). For example, the Da’anzhai shale has relatively wide ranges of carbonate (1-70%), clay (15-70%)
94
and quartz (5-50%) (Fig. 4a). In addition, the Da’anzhai shale has the wider mineral ranges than other
95
lacustrine shales, especially for the range of carbonate minerals.
M AN U
SC
RI PT
89
EP
97
TE D
96
Fig. 4. Ternary diagrams of mineral compositions. (a) Lacustrine shales: 1=Da’anzhai shale, 2=Jurassic shale in Sichuan
99
Basin (Jiang et al., 2017), 3=Triassic shale in Sichuan Basin(Jiang et al., 2017), 4=ES3 shale (Wang et al., 2015a). (b) Marine
AC C
98
100
shales: 5 Woodford shale (Jarvie et al. 2007 and Han et al. 2013), 6=Barnett shale (Jarvie et al. 2007 and Han et al. 2013),
101
7= Wufeng-Longmaxi shale (Wu et al., 2014), 8=Ohio shale (Jarvie et al. 2007 and Han et al. 2013).
67
ACCEPTED MANUSCRIPT
Table 3 Minerals and TOC results for the Da’anzhai lacustrine shale. feldspar
calcite
dolomite
aragonite
siderite
pyrite
hematite
(%)
(%)
(%)
(%)
(%)
(%)
(%)
(%)
G10-1
2650.8
Da1
44.2
G10-2
2668.4
Da13
22.1
1.1
0.9
2700.7
Da13
37.5
2.0
2.9
2646.6
1
28.1
4.2
22.9
1
G10-3 G10-4
Da
3.0 1.2
6.3 2.6
1.2
17.0
2385.3
Da
28.2
G4-2
2395.8
Da1
35.2
2.1
6.4
12.5
G4-3
2405.7
Da13
25.8
2.5
4.6
0.7
2537.1
Da13
21.4
2.6
2580.1
3
G6-2
Da
7.4
0.82
24.2
1.33
55.0
2.9
40.4
0.81
26.6
41.1
69.2
54.9
4.5
38.4
1.6
64.8
3.5
60.9
13.4
41.6
0.40
18.9
54.1
0.98
5.3
31.1
1.43
4.2
7.4
28.8
1.18
0.9
36.3
0.4
0.7
20.5
1.1
64.2
76.7
0.4
1.1
64.0
1.1
3.9
27.3
3.1
55.5
5.4
0.2
22.6
1.0
59.3
76.2
1.0
0.8
26.7
1.1
64.8
70.4
0.0
27.7
0.9
57.0
71.4
1.3
4.2
33.1
2.3
24.3
59.1
1.49
2.1
20.0
69.4
77.9
0.30
46.2
26.9
52.0
9.8
40.2
1.17
38.5
35.2
59.4
1.07
53.9
3.1
43.8
0.69
6.4
67.8
1.6
24.9
1.59
4.1
0.9
60.2
4.1
38.9
1.51
3.6
51.2
14.9
45.2
1.55
1.5
16.3
57.1
80.3
0.61
3.9
2827.3
Da13
31.9
4.1
2662.6
1
16.9
14.4
1
Da Da
5.6
55.3
1
44.9 9.5
J61-3
2667.6
Da
14.4
14.3
J61-4
2698.1
Da13
34.8
17.5
2706.8
Da13
8.5
69.4
2665.9
1
25.1
1.8
15.6
1
30.4
2.6
9.8
1.0
28.9
6.3
2.3
3.1
0.0
0.9
0.6
Da
1
1726.1
Da
24.2
Li-3
1734.3
Da13
40.7
3215.9
Da13
23.3
3221.2
Da13
34.8
3239.2
Da13
30.3
14.9
3258.6
3
23.2
51.8
Da
42.7 6.8
EP
Da
Li-2
P1-4
47.2
2.1
75.8
27.9
P1-3
3.0
68.4
67.1
6.1
P1-2
52.8
1.0
0.6
P1-1
(%)
20.1
12.5
1713.8
minerals
0.8
23.4
Li-1
(%)
1.1
Da
J61-6
(%)
(%)
19.5
Da13
J61-5
TOC
47.6
2656.0
2664.4
brittleness
1.2
2632.4
J61-2
carbonate
8.7
J45-2
J61-1
others
clay
1
J45-1
J53-1
3.5
TE D
G6-1
13.4
M AN U
G4-1
total
RI PT
(m)
sub-member
quartz
SC
code
depth
AC C
102
1.0
10.3 3.2
53.3 1.1
1.0
5.3
68
0.7
1.9
31.9
2.18
ACCEPTED MANUSCRIPT
2836.1
Da1
19.8
2849.9
1
Da
40.3
2867.1
Da13
17.6
47.7
1.3
S2-4
2889.0
3
Da
18.9
48.9
1.5
S2-5
2891.6
Da3
53.4
X20-1
1844.4
Da13
49.4
1952.2
1
Da
24.9
2089.4
Da13
35.9
X44-2
2093.0
Da13
27.9
3.0
4.0
X44-3
2116.7
Da13
23.8
3.0
43.2
LQ-1
3548.1
Da13
44.1
2.9
18.7
1780.7
Da13
40.5
2.6
2.5
2065.7
1
28.2
2.2
18.5
7.8
43.3
1
23.9
1.9
35.2
11.4
26.6
X8-1 W8-1 W8-2
2069.5
Da Da
29.6
0.9
42.5
50.4
70.2
0.11
10.7
51.0
1.18
33.4
49.0
66.6
0.33
30.7
50.4
69.3
0.18
2.0
8.1 1.4
2.3
25.6
3.1
8.5
RI PT
46.6
3.8
3.7
SC
X44-1
0.2
9.5
2.5
M AN U
X28-1
10.7
TE D
S2-3
3.6
2.2
EP
S2-2
48.2
AC C
S2-1
69
53.4
38.7
1.5
8.1
57.5
44.3
0.7
28.7
53.6
8.5
44.4
0.53 1.15
51.8
60.8
4.0
31.9
20.5
52.7
76.5
34.3
18.7
62.8
2.5
43.0
26.3
54.5
46.6
70.5
50.8
0.6
1.1
1.0
0.76
0.84
ACCEPTED MANUSCRIPT
4.3. Characterization and controlling factors of lacustrine shale reservoirs
104
4.3.1. Microscopic characteristics
105
AC C
EP
TE D
M AN U
SC
RI PT
103
106
Fig.5. Typical reservoirs images of lacustrine shale. A: Shells were stained using alizarin red to highlight the carbonate
107
minerals; the shells are well developed and preserved in terrestrial inputs and matrices. B: Greenish-yellow fluorescence.
108
Matrices with large amounts of terrestrial inputs between the shell grains have strong fluorescence intensity, indicating
70
ACCEPTED MANUSCRIPT
effective reservoirs with good oiliness in the matrices. C: Lacustrine shale core sample with well-developed calcareous
110
interlayers, indicating a changeable sedimentary environment. D: Quartz particles with well-developed interparticle pores
111
filled with clay minerals; quartz and clay minerals have well-developed intraparticle pores. E: Quartz particles have
112
well-developed intraparticle pores and interparticle pores; calcite minerals fill the storage spaces between quartz particles
113
(calcite cementation). F: Intraparticle pores located along the cleavage planes of clay particles; interparticle pores between
114
clay and carbonate minerals. G: OM with well-developed pores is accompanied by clay minerals. H: Calcite minerals fill the
115
spaces between particles (calcite cementation); quartz has well-developed intraparticle pores. I: Brittle minerals with
116
well-developed micro-cracks that are mainly filled by clay minerals. J: Model of different types of pores. K: Model of
117
carbonate mineral (e.g., calcite) diagenesis that results in reduced storage spaces.
SC
M AN U
118
RI PT
109
According to the International Union of Pure and Applied Chemistry (IUPAC) classification scheme, the pore types in unconventional reservoirs are divided by pore size into micropores, mesopores,
120
and macropores (<2 nm, 2-50 nm, and >50 nm, respectively) (IUPAC, 1994; Thommes et al., 2015).
121
Thin section observations of the shale indicate that the matrices fluoresce intensely (Fig. 5B). Nanoscale
122
reservoirs were examined using FE-SEM with argon-ion milling technology, and the minerals were
123
identified using EDS. Similar to other lacustrine shales, the pores and fractures in the Da’anzhai shale
124
are well developed and fall into four categories.
EP
AC C
125
TE D
119
(1) Interparticle pores: these pores represent the largest number of pores, and they have good
126
connectivity and form effective pore networks. The diameters generally range between 5 nm and 90 µm.
127
Many types of interparticle pores were observed, including pores between grains (Figs. 5D,E,H,J),
128
crystals and clay platelets (Figs. 5D,F,G,J), and pores at the edges of rigid grains (Figs. 5E,J).
71
ACCEPTED MANUSCRIPT
129
(2) Intraparticle pores: these pores primarily formed through diagenesis, and some are primary. The diameters mainly range from 2 nm to 10 µm. The primary sub-types in the Da’anzhai shale are
131
intraplatelet pores within clay aggregates (Figs. 5D,F,J), intercrystalline pores within pyrite framboids,
132
pores within fossil bodies and moldic pores after fossils (Figs. 5D,J).
133
RI PT
130
(3) Fracture pores: these pores are not controlled by individual particles and can be regarded as a type of pore that formed by rock deformation because of tectonic movements, sedimentation, diagenesis,
135
OM hydrocarbon generation and other geological effects (Figs. 5I,J).
M AN U
136
SC
134
(4) OM pores: these pores are found within OM (Fig. 5L) and are usually irregular, bubble-like, and oval. They generally range in diameter from 2 to 500 nm. Although most OM pores are incorrectly
138
regarded as isolated pores in two-dimensional planes, they are interconnected in three-dimensional space
139
and facilitate the development of reservoirs.
140
TE D
137
Similar to other lacustrine and marine shale reservoirs, the Da’anzhai shale commonly contains well-developed pores and fractures, particularly intraparticle and interparticle pores (Curtis et al., 2002;
142
Chalmers et al., 2012; Milliken et al., 2013; Nie et al., 2015; Wang et al., 2015a; Zhou and Kang, 2016;
143
Jiang et al., 2017; Zhang et al., 2017). Especially, due to their rich terrestrial clastic mineral content, the
144
interparticle pores of lacustrine shales are better developed than those of marine shales. Most of the
145
intraparticle pores are nanoscale pores (<200 nm) (Figs. 5D,E,F) and the OM also contains nanopores
146
(Fig.5G). Microcracks can develop due to the high brittle mineral content for the Da’anzhai shale.
147
However, these cracks are often filled by calcite and clay minerals due to the high clay content (42.4%)
148
(Figs. 5I,J).
AC C
EP
141
72
ACCEPTED MANUSCRIPT
4.3.2. Nanoscale reservoirs characterized by LTNA
150
The LTNA test is the most widely used method of characterizing several parameters of nanoscale
151
pores in porous media (micropores and mesopores), including VBJH, SBET, DA, ad/desorption isotherms
152
and PSD (Sing et al., 1985; IUPAC, 1994; Thommes et al., 2015; Wang et al., 2015c; Zhou et al., 2016;
153
Zhang et al., 2017).
154
Table 4 Comparisons of primary pore structure parameters measured by LTNA between the Da’anzhai shale and other
155
lacustrine shales
4
5
6
7
156 157
SC
Sichuan
Jurassic Da'anzhai
1.75-10.69
Basin
Member
(6.92)
M AN U
3
SBET (m2/g)
VBJH 3
(cm /100g)
D (nm)
1.19-4.10
7.21-24.71
(2.97)
(14.11)
0.02-0.90
6.67-18.40
(0.49)
(11.23)
0.22-2.97
4.91-13.93
Upper Triassic
0.25-4.39
Chang 7 Member
(2.62)
Sichuan
Xujiahe
0.62-14.70
Basin
Formation
(7.76)
(1.41)
(6.97)
1.46-5.24
0.72-1.40
9.40-25.00
Ordos Basin
Ordos Basin
TE D
2
strata
Upper Triassic
Chang 7 Member
(2.94)
(1.01)
(16.40)
Bohai Bay
Shahejie
0.40-12.45
0.08-1.60
4.58-41.36
Basin
Formation
(3.62)
(0.62)
(16.84)
0.39-34.06
0.13-4.18
3.16-16.48
(10.92)
(1.59)
(9.56)
Upper Triassic
1.10-1.90
0.69-1.09
15.4-23.4
Chang 7 Member
(1.57)
(0.83)
(19.27)
Songliao Basin
Upper Cretaceous
EP
1
location
Qingshankou Formation
AC C
NO.
RI PT
149
Ordos Basin
73
hysteresis
loops types
references
H3
H3 H3 H3 H3,H4
H2,H3
H3,H4
Jiang et al., 2016b Peng et al., 2016 Fu et al., 2015 Zhang et al., 2016 Wang et al., 2015b Yang et al., 2017
RI PT
ACCEPTED MANUSCRIPT
158
160
Fig. 6. PSD curves obtained from the low temperature N2 adsorption isotherms with BJH pore sizes.
SC
159
The VBJH value of the Da’anzhai shale (2.97 cm3/100 g) is slightly higher than the values in other shales, and the SBET value (6.92 m2/g) is nearly equal to those of the other shales (Table 4). These values
162
indicate that the Da’anzhai shale can also have significant nanoscale storage space.
M AN U
161
To investigate the distribution and contributions of each pore type, the VBJH with respect to the PSD
164
was determined from the adsorption branches of the BJH model (Wang et al., 2015b; Jiang et al., 2016b;
165
Zhou et al., 2016; Zhang et al., 2017). The PSD plots show that pores with diameters between 3 and 5
166
nm make up the largest proportion of the total pore volume and that the proportions decrease with
167
increasing pore size, which is similar to some lacustrine shales (Fig. 6) (Jiang et al., 2016b).
EP AC C
168
TE D
163
74
169 170
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 7. Comparisons between the nanopore structures of the Da’anzhai shale and other lacustrine shales. A, B, C: the
Da’anzhai shale. D: Lacustrine shale of the Chang 7 Member in the Ordos Basin, China (Fu et al., 2015). E: Lacustrine shale
172
of the Songliao Basin, China (Wang et al., 2015b). F: Lacustrine shale of the Yanchang Formation in the Ordos Basin, China
173
(Jiang et al., 2016b; Wang et al., 2015c). G: Lacustrine shale of the Shahejie Formation in the Dongying Sag, China (Liu et
174
al., 2017). H: Lacustrine shale of the Shahejie Formation in the Zhanhua Sag, China. I: Lacustrine shale of the 5
175
Member of the Xujiahe Formation of Upper Triassic in Sichuan Basin, China (Peng, 2016).
176
AC C
EP
171
th
LTNA isotherms and hysteresis patterns can be used to investigate the physisorption mechanisms
177
and structural characteristics of shale pores (Sing et al., 1985; IUPAC, 1994; Thommes et al., 2015;
178
Zhou et al., 2016; Zhang et al., 2017). Adsorption isotherms are divided into five types (Types I-VI), and 75
ACCEPTED MANUSCRIPT
hysteresis loops are divided into four types (Types H1-H4) (Sing et al., 1985; IUPAC, 1994; Thommes et
180
al., 2015). Following this classification, the isotherms of the Da’anzhai shale samples are similar and
181
belong to Type П, and the hysteresis loops are characterized by Type H3 hysteresis loops that indicate
182
wedge-shaped pores (Sing et al., 1985; IUPAC, 1994; Thommes et al., 2015). However, compared to
183
other lacustrine shales, the Da’anzhai shale has relatively weak hysteresis loops and wide overlapping
184
ranges of adsorption and desorption isotherms (Fig. 7), which indicate many dead-end pores with highly
185
complex structures (De Boer, 1958; Nie et al., 2015).
M AN U
SC
RI PT
179
186
4.3.3. Factors that influence pore structure
187
SBET and VBJH are the two most basic LTNA parameters, and they can directly indicate the storage spaces of nanoscale reservoirs. Fig. 8a shows that the TOC values are positively correlated with SBET
189
and VBJH. However, the correlation is not strong because of the relatively low TOC values (average
190
0.97%). Fig. 8b shows that the lacustrine clay content is positively correlated with SBET and VBJH; this
191
correlation is mainly caused by the high clay content (42.4%) and well-developed intraparticle pores
192
(e.g., intraplatelet pores within clay aggregates) and interparticle pores (e.g., pores at the edges of
193
particles, crystals and clay platelets) (Figs. 5F, L). The quartz content is also positively correlated with
194
SBET and VBJH (Fig. 8c). Quartz particles in lacustrine shales usually occur within mineral matrices and
195
have well-developed interparticle pores at the edges of other minerals (e.g., clay and carbonate minerals).
196
The quartz particles also have well-developed nanoscale intraparticle pores. Additionally, the quartz
197
particles have no obvious cementation (Fig.5D). Conversely, the carbonate content is negatively
198
correlated with SBET and VBJH (Fig. 8d). Carbonate minerals, particularly calcite, can significantly affect
AC C
EP
TE D
188
76
ACCEPTED MANUSCRIPT
reservoir quality and generally has adverse effects (Wang et al., 2016). Carbonate minerals (e.g., calcite)
200
are easily dissolved and redistributed and can be formed in primary pores by various mechanisms during
201
the diagenesis processes (Figs. 5E, H, K) (Huang et al., 1984; Wang et al., 2016). Based on qualitative
202
observations using SEM, polarized light microscopy and fluorescence microscopy, lacustrine carbonate
203
minerals can effectively reduce the available storage space by diagenesis processes including
204
cementation, compaction, pressure-solution, recrystallization and replacement (Figs. 5E, H, K) (Huang
205
et al., 1984; Wang et al., 2016).
207
AC C
206
EP
TE D
M AN U
SC
RI PT
199
Fig. 8. Correlation plots between nanopore storage space indicators (SBET and VBJH) and values of TOC and minerals.
77
ACCEPTED MANUSCRIPT
4.4. Characterization and controlling factors of OM in lacustrine shale
209
4.4.1. Characterization of the OM content
210
Water conditions play an important role in the formation of source rocks. Lacustrine shales
RI PT
208
primarily form in semi-deep to deep lakes with fresh to low salinity water, whereas marine shales
212
primarily form in reductive, salty, weakly alkaline and low energy water. These features give rise to
213
different source rock characteristics (Huang et al., 1984; Bohacs et al., 2000; Curtis, 2002; Schmoker,
214
2002; Chen et al., 2015; Yang et al., 2015; Jiang et al., 2016a). Due to these differences, we evaluate the
215
lacustrine OM using a terrestrial oil generation theory that was derived from studies of continental
216
petroliferous basins in China and plays a special role in global petroleum theories (Huang et al., 1984;
217
Lu and Zhang, 2008) (Table 5).
218
Table 5 Factors used to evaluate OM in lacustrine source rock (Huang et al., 1984; Lu and Zhang, 2008). lake types freash~brackish
salt water~high salt water
AC C
chloroform bitumen ‘A’ (%)
non-source rock
source rock poor
medium
good
very good
0.4~0.6
>0.6~1.0
>1.0~2.0
>2.0
<0.2
0.2~0.4
>0.4~0.6
>0.6~0.8
>0.8
<0.015
0.015~0.050
<0.050~0.100
>0.100~0.200
>0.200
<2
2~6
>6~20
>20
<0.4
EP
TOC(%)
water
TE D
index
M AN U
SC
211
S1+S2(mg/g)
219
The mean values (and ranges) of the shale’s kerogen macerals consist of 32% exinites (29-35%),
220
27% sapropelinites (23-32%) and 24% vitrinites (18-26%), which indicates that the OM is primarily
221
derived from plankton and bacteria. The δ13CPDB values (-29.1 to -29.5‰, average -29.3‰) support this
222
finding (Table 7) (Huang et al., 1984; Lu and Zhang, 2008). KI is an effective indictor of OM type. The 78
ACCEPTED MANUSCRIPT
OM types (I, II1, II2, and III) can be identified using the KI ranges >>80-100, 80-40, 40-0 and <<0 to
224
-100, respectively (Huang et al., 1984; Lu and Zhang, 2008). The KI values for the Da’anzhai shale vary
225
from 2 to 15 with an average of 8.7, which indicates that the OM is chiefly sapropelic-humic type II2
226
and has significant potential to generate hydrocarbons. The mean values (and ranges) of the organic
227
elements C, H, and O make up 46.59% (31.12-61.41%), 3.53% (2.09-4.66%) and 1.56% (0.35-1.88%)
228
of the shale’s OM, respectively. The high H/C ratios (0.79-1.05, average 0.90) and low O/C ratios
229
(0.10-0.30, average 0.17) (Fig. 9a) indicate that the OM is type II and has relatively high hydrocarbon
230
potential (Huang et al., 1984; Lu and Zhang, 2008). In addition, the Tmax-HI index (Fig. 9b), which is an
231
important evaluation index for the OM type based on Rock-Eval pyrolysis, gives a similar result
232
(Espitaliéet al., 1985; Lu and Zhang, 2008; Misch et al., 2016). Generally, the Da’anzhai shale has
233
similar OM types with other lacustrine and marine shales and the dominant type is of type II (Table 1),
234
which can support the Da’anzhai shale hydrocarbon generation capacity.
TE D
M AN U
SC
RI PT
223
The TOC values of the Da’anzhai shale vary from 0.11 to 2.18% (average 0.97%), these values are
236
generally lower than those measured in the world's major lacustrine and marine shales (Table 1 and Fig.
237
9). The Ro values of the Da’anzhai shale are similar or even higher than those of other lacustrine and
238
marine shales, which indicate that the OM is highly mature. This could also be deduced from the Tmax
239
values, which vary from 428 to 500 ℃ with an average of 449 ℃ (Table 6).
AC C
EP
235
79
ACCEPTED MANUSCRIPT
Table 6 Analysis results for Rock-Eval, Chloroform Bitumen ‘A’ and TOC.
code
bitumen A
TOC
Tmax
S1+S2
(℃)
(mg/g)
S0 (mg/g)
S1 (mg/g)
S2 (mg/g)
0.82
0.009
0.08
0.50
456
1.33
0.006
2.03
3.97
444
(%)
（%） G10-1
0.03
G10-2 G4-1
0.02
0.40
0.007
0.02
0.16
461
G4-2
0.25
0.88
0.004
0.14
1.03
439
0.42
1.43
0.009
0.22
0.71
428
0.40
1.49
0.009
0.17
2.12
443
Li-1
0.35
1.17
0.008
0.29
1.09
441
Li-2
0.31
1.07
0.009
0.15
1.28
Li-3
0.16
0.69
0.006
0.07
D
HCI
(mg/g)
(%)
(%)
(mg/g)
0.58
0.14
60.60
0.05
5.95
11.04
6.00
0.34
298.77
0.50
37.51
153.11
0.18
0.12
40.65
0.02
3.98
7.30
1.18
0.12
117.53
0.10
11.13
16.61
0.24
49.69
0.08
5.48
16.33
0.07
142.48
0.19
12.79
11.67
1.38
0.21
93.29
0.12
9.86
25.49
446
1.43
0.10
119.80
0.12
11.17
14.82
0.55
445
0.62
0.11
79.33
0.05
7.48
10.77
0.006
1.70
1.85
452
3.55
0.48
116.50
0.30
18.58
107.39
0.24
1.51
0.008
0.16
0.94
453
1.09
0.14
61.96
0.09
6.05
10.88
P1-4
0.11
0.61
0.006
0.11
0.26
448
0.38
0.29
43.18
0.03
5.18
19.28
S2-2
0.27
1.18
0.006
0.14
1.32
444
1.46
0.10
112.14
0.12
10.34
12.41
S2-5
0.01
0.008
0.03
0.05
500
0.09
0.35
97.82
0.01
14.22
73.45
XI44-2
0.40
0.009
0.09
1.83
439
1.92
0.04
159.28
0.16
13.91
8.27
1.15
TE D
1.59
P1-2
EP
241 242
PC
2.29
AC C
P1-1
HI
0.94
M AN U
G4-3 J61-4
OPI
RI PT
chloroform
SC
240
80
ACCEPTED MANUSCRIPT
Table 7 Results for Ro, macerals, carbon isotopes and organic elements. maceral (%)
Li-1
Li-2
P1-1
(1.43) 1.19-1.47 (1.32) 0.78-1.08 (0.95) 0.95-1.25 (1.17) 0.88-1.28 (1.02)
27
4
4
26
34
24
27
3
2
30
35
18
32
4
4
22
30
22
28
6
6
19
31
27
6
2
21
29
23
5
6
22
33
vitrinite
KI
fusinite
15
RI PT
1.13-1.65
sporinite
pollinite
subtotal
normal
resinite
（‰）
11
SC
J53-1
(0.95)
sporo
amorphinite
organic elements
δ13CPDB
inertinite
C (%)
H (%)
O (%)
H/C
O/C
-29.2
39.89
3.07
0.75
0.92
0.1
20
11
-29.5
46.31
4.07
1.88
1.05
0.3
16
15
-29.2
49.26
3.87
1.37
0.94
0.2
26
15
9
-29.1
31.12
2.09
0.35
0.81
0.1
26
18
4
-29.5
61.41
4.66
1.62
0.91
0.2
26
18
2
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51.54
3.39
0.96
0.79
0.1
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Fig.9. Discrimination diagrams for the OM type.
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Terrestrial oil-generation theory (Huang et al., 1984; Lu and Zhang, 2008) suggests the following
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conclusions regarding the lacustrine shale: (1) The TOC and chloroform bitumen ‘A’ values are medium
250
to very good, even though the values of S1+S2 (genetic potential) are poor to medium (Tables 5 and 6).
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(2) Because of the strong hydrocarbon expulsion of the high maturity OM (Ro>0.5-0.7%), the
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evaluation criteria for high maturity OM should be reduced. The shale with high Ro values (0.95-1.43%)
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is highly mature. (3) Lacustrine carbonate can also generate hydrocarbons, which is an important feature
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but easy to overlook. The petroleum generation threshold of carbonate (TOC: 0.12-0.4%) is lower than
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that of mudstone (TOC: 0.4-0.5%), which means that carbonates can produce hydrocarbons at lower
256
TOC values. Thus, the low TOC values of the high carbonate shale may not indicate a low hydrocarbon
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generation capacity. Additionally, carbonate interlayers in shale can play a role similar to that of seal
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rocks, which are conducive to the preservation of oil and gas. In summary, the results of this study
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indicate that the Da’anzhai shale has some hydrocarbon generation capacity, which is also supported by
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the clear fluorescence of the thin sections (Figs. 5B, C).
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Based on microscopic observations of the Da’anzhai and lacustrine shales, the OM occurrence forms are generally similar to other lacustrine shales; they are mainly dispergated in shale matrices
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(Fig.5) (Morad et al., 2010; Zou et al., 2011; He et al., 2012; Luo et al., 2013; Tian et al., 2014; Dang et
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al., 2015; Xu et al., 2017). However, these lacustrine shales have more amorphous forms than marine
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shales because of their different water conditions and biodegradation (Huang et al., 1984; Lu and Zhang,
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2008; Tian et al., 2014; Xu et al., 2017). In addition, lacustrine OM is distributed in concentric belts
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around the lake basin’s center, and OM abundance increases with lake depth. Thus, OM is generally
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more abundant in moderately deep to deep lake facies than in shallow lake facies, which can be
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supported by the wide TOC range of lacustrine shales (Table 1 and Fig. 9) (Huang et al., 1984; Lu and
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Zhang, 2008; Tian et al., 2014; Xu et al., 2017).
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Fig. 10. Correlation plots between TOC and V/Cr, V/Sc, Ni/Co and V/(V+Ni) values.
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Fig. 11. Correlation plots between TOC values and minerals.
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TOC is commonly used to represent OM. As discussed in Section 4.1, the reducibility is positively
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correlated with the V/Cr, V/(V+Ni), and V/Sc ratios and negatively with the Ni/Co ratio. In addition,
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pyrite is an important mineral in organic-rich sediments and is useful for reconstructing
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paleoenvironmental conditions. A high pyrite content reflects a stable, high-salinity environment with
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strong reducibility (Leventhal, 1983). The TOC values of the Da’anzhai shale are positively correlated
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with the V/Cr, V/(V+Ni) and V/Sc ratios (Figs. 10a, b, c) and the pyrite content (Fig. 11a) and correlated
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negatively with the Ni/Co ratio (Fig. 10d). These values indicate that paleo-redox conditions had a
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significant effect on the formation and preservation of OM and that the strongly reducing conditions
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were conducive to the preservation of OM (Zeng et al., 2015; Chen et al., 2016; Xu et al., 2017).
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Similar to other lacustrine shales, the TOC values of the Da’anzhai shale are positively correlated with the clay mineral content (Fig. 11b), which is due to the OM’s strong sorption ability of clay and its
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relatively high content for the lacustrine shales (Huang et al., 1984; Gingele et al., 2001; Dang et al.,
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2015; Xu et al. 2017). The lacustrine clay minerals are small and rich in intraparticle and interparticle
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pores, which creates storage space for OM (Figs. 5D,F,G,J). The OM observed within the clay minerals
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primarily has complex, dispergated and residual biological shapes, which indicate that the OM and clay
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minerals are in close contact (Figs. 5G and J) (Huang et al., 1984; Lu and Zhang, 2008; Tian et al., 2014;
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Xu et al., 2017). Conversely, the carbonate content is negatively associated with OM (Fig. 11c), which is
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similar to some lacustrine shales (Huang et al., 1984; Jarvis et al., 2001; Tian et al., 2014; Wang et al.,
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2016). Carbonate minerals fill the pores via cementation, compaction, pressure-solution,
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recrystallization and replacement, which effectively reduces the storage space for OM (Figs. 5E,H,K)
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(Huang et al., 1984; Wang et al., 2016). Furthermore, the high carbonate mineral content in the
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lacustrine facies reflects relatively shallow water environments, such as the shore-shallow lake face and
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shallow lacustrine facies, which are not favorable for the generation or storage of OM (Huang et al.,
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1984; Wang et al., 2016; Xu et al., 2017). There is a weak correlation between the quartz content and
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TOC values (Fig. 11d). The correlations between quartz and OM are not uniform; differences in the
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correlation between quartz and TOC contents are related to the depositional environment of shale
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reservoirs (Zeng et al., 2014; Liu et al., 2015; Ross and Bustin, 2007; Chalmers et al., 2012b; Tian et al.,
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2013). Negative correlations are caused by the terrestrial quartz inputs (Zeng et al., 2014; Liu et al.,
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2015), while the positive correlations are related to biogenic quartz (Ross and Bustin, 2007; Chalmers et
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al., 2012b; Tian et al., 2013). Therefore, the quartz in the lacustrine shales may have both related to
306
biogenic and terrestrial origins. This could lead to the weak correlations between quartz and OM. Based on the analyses in Figs. 10 and 11, the low TOC values of the Da’anzhai lacustrine shales
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were primarily caused by the oxygen paleoenvironment and carbonate minerals. The relatively shallow
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and turbulent lake water formed a high oxygen paleoenvironment with greater inputs of terrestrial
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nutrients, which was conducive to the breeding of shell organisms and the formation of carbonate
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minerals but not to the preservation of OM. In summary, the oxygen paleoenvironment and high
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carbonate mineral content are primarily related to the unique lacustrine facies, which was not conducive
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to the formation of OM.
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4.5. Relationships between the mineralogy, reservoirs, OM and paleoenvironmental conditions of lacustrine shales
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The mineral content of lacustrine shales is mainly related to the paleoenvironment. Strong paleo-weathering can lead to a greater number of weathering products, which is favorable to the
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development of terrestrial minerals in lacustrine facies. Humid (rainy) paleoclimates with more water
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can improve river transportation and reduce lake salinity, and low salinity paleoenvironments inhibit the
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formation of authigenic carbonate. In addition, fine-grained terrestrial minerals, especially clay minerals,
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inhibit the formation of authigenic carbonate (Leventhal, 1983; Huang et al., 1984; Bohacs et al., 2000;
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Chi et al., 2003; Fu et al., 2015). High hydrodynamic conditions can effectively remove fine-grained
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terrestrial minerals, which favors the formation of authigenic carbonate.
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The lacustrine sedimentary environment is complex and the lacustrine shale formation is easily influenced by the paleoenvironment. The two most important differences are as follows: first, the
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lacustrine paleoenvironments are complex and varied with the short detention time and strong variation
327
of water level (Huang et al., 1984; Chen et al., 2015) (Figs. 1 and 5C, Tables 2 and 6). Second, lakes are
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close to the terrestrial sources with less water, which lead to rich terrestrial inputs with efficient impact
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on shale (Figs. 1 and 5A,B, Tables 3 and 6) (Huang et al., 1984; Chen et al., 2015) (Huang et al., 1984;
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Bohacs et al., 2000; Curtis, 2002; Schmoker, 2002). Lakes are generally located near paleo-uplift
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environments with large topographical differences, and terrestrial debris can be transported directly into
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a lake basin over short distances. These features result in rich, proximal and variable sources for
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lacustrine facies. (Figs. 1, 5A, 5B; Table 3). The above two characteristics can effectively lead to the
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complex and changeable mineral contents of lacustrine shales (Table 3; Fig. 4). In addition, there are
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some other common points of the lacustrine shales including relatively small thickness of monolayer,
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more siliceous or calcareous intercalations, and poor comparability with strong heterogeneity (Table 1),
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which are also the results of the two characteristics of lacustrine sedimentation environments (Tian et al.,
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2014; Wang et al., 2015a; Zhu et al., 2016; Xu et al., 2017).
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Similar to other lacustrine shales, the storage space of the Da’anzhai lacustrine shale is directly
340
controlled by minerals (Fig. 12). Because of the large amount of terrestrial clastic minerals, the
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interparticle pores of lacustrine shales are well-developed. Developed interparticle pores and intraparicle
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pores can guarantee a considerable amount of storage space. In addition, numerous siliceous or
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calcareous interlayers in lacustrine shale formations can result from varying changeable
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paleoenvironments with rich terrestrial inputs. These interlayers can increase the brittleness index and
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facilitate the development of cracks. Compared to other shales, the Da’anzhai lacustrine shale has a relatively low OM content (Table 6),
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which is mainly the result of more variable paleoenvironments with high oxygen content (Fig. 12)
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(Huang et al., 1984; Lu and Zhang, 2008; Tian et al., 2014; Xu et al., 2017). Different minerals have
349
different effects on storage spaces of OM, and minerals in lacustrine shales are mainly controlled by
350
paleoenvironments (Fig. 12). In addition, the relatively low OM content of the Da’anzhai lacustrine
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shale is the reason for OM weak effects on storage spaces (Fig. 8a).
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In summary, minerals, reservoirs, and OM in lacustrine shales are closely associated with each
353
other, and they were all controlled by the paleoenvironment which was the dominant factor and the link
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between these parameters (Fig. 12).
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Fig.12 Relationships between mineralogy, reservoirs, OM and paleoenvironment for lacustrine shale
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4. Conclusions
1. There are some similarities between the Da’anzhai lacustrine shale and other lacustrine shales. (1)
359
The two most important differences are that lacustrine shales formed in more complex and variable
360
paleoenvironments and have more abundant terrestrial inputs than marine shales. (2) The mineral
361
compositions of lacustrine shales are variable and complex, and they have abundant terrestrial minerals. 88
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In addition, lacustrine shales have numerous siliceous or calcareous interlayers. (3) The interparticle
363
pores and interlayer pores of lacustrine shales are well-developed. Unlike other lacustrine shales, the
364
Da’anzhai shale has relatively weak hysteresis loops and wide overlapping ranges of adsorption and
365
desorption isotherms, indicating a greater number of dead-end pores with more complex structures. In
366
addition, the Da’anzhai shale has lower TOC values with more amorphous OM forms.
2. The minerals, reservoirs, and OM for lacustrine shales are closely associated with each other;
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these parameters are all controlled by the paleoenvironmental conditions in which the shales formed. (1)
369
Highly variable lacustrine paleoenvironment lead to variable mineral contents and abundant terrestrial
370
inputs give rise to a high terrestrial mineral content in lacustrine shales, which can also create a higher
371
number of siliceous or calcareous intercalations relative to marine shales. (2) Terrestrial minerals (e.g.,
372
clay and quartz) can effectively improve the storage spaces, but the authigenic carbonate minerals (e.g.,
373
calcite) have the opposite effect. Well-developed interparticle pores in lacustrine shales are caused by
374
abundant particles. (3) Clay content has a positive effect on the formation and preservation of OM, while
375
higher oxygenation and carbonate contents have negative effects. Lakes generally have relatively
376
shallow water and a variable paleoenvironment, which leads to a higher oxygen content and reduces the
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OM abundance of lacustrine shales. The weak influence of OM on nanoscale storage spaces is due to the
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low TOC values of the lacustrine shales.
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3. Compared to other lacustrine shales, the Da’anzhai shale also demonstrates the potential for
380
unconventional oil and gas exploration. (1) The lacustrine shale has well-developed storage spaces due
381
to the high number of interparticle pores, intraparticle pores, and fractures. Although the lacustrine shale
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has more dead-end pores and larger pores with more complex microstructures, the types of isotherms
383
(Type П) and hysteresis loops (Type H3) are generally similar to those of lacustrine shales. The values of
384
VBJH (1.19-4.10 m3/100 g, average 2.97 m3/100 g) and SBET (1.75-10.69 m2/g, average 6.92 m2/g) are
385
similar to other shales, indicating a high amount of storage space in nanoscale reservoirs. (2) Abundant
386
terrestrial minerals in lacustrine facies can support a greater number of interparticle pores and
387
intraparticle pores. Numerous siliceous or calcareous interlayers in lacustrine shales can also improve
388
the brittleness index, facilitating the development of fractures and creating a seal for oil and gas. (3)
389
Although the TOC values of the Da’anzhai shale are lower than other lacustrine shales, the OM still has
390
several advantages according to the evaluation criteria for continental source rocks, including moderate
391
values of chloroform bitumen ‘A’ and TOC, a high level of OM maturity and a favorable kerogen type
392
(II).
393
Acknowledgments
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Our study is supported by the Institute Program of PetroChina Research Institute of Petroleum
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Exploration and Development (Grant 2016yj01), and the National Natural Science Foundation of China
396
(Grants 41272137 and 41572117). The authors also sincerely appreciate the support from the China
397
Scholarship Council (CSC).
398
Nomenclature
399
TOC
400
Ro
Vitrinite reflectance
401
Tmax
Pyrolysis Temperature at Maximum Hydrocarbon Generation
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Total organic carbon, %
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SBET
Brunauer-Emmett-Teller specific surface area
403
VBJH
Barrett-Joyner-Halenda total pore volume
404
DA
Average pore diameter
405
OM
Organic matter
406
TE
Trace element
407
XRD
X-ray diffraction
408
FE-SEM
Field emission scanning electron microscopy
409
LTNA
Low temperature N2 adsorption
410
EDS
Energy dispersive spectrometry
411
S0
Gaseous hydrocarbons, mg/g
412
S1
Free hydrocarbons, mg/g
413
S2
Pyrolysis of hydrocarbons, mg/g
414
ICP-MS
Inductively coupled plasma-mass spectrometry
415
REE
Rare earth element, ppm
416
ΣHREE
Total content of heavy rare earth elements
417
ΣLREE
Total content of light rare earth elements
418
PSD
Pore size distribution
419
OPI
420
HI
Hydrogen index; HI=(S2/TOC)×100%, mg/g
421
PC
Effective carbon content; PC=0.083×(S0+S1+S2), %
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Oil production index; OPI=S1/(S0+S1+S2)
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D
Degradation rate; D=(PC/TOC) ×100%, %
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HCI
Hydrocarbon index; HCI=(S0+S1)/ TOC×100, mg/g
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KI
Kerogen type index; KI=Sapropelinite (%)×1+Exinite (%)×0.5+Vitrinite
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(%)×(-0.75)+Inertinite (%)×(-1) (Huang et al., 1984; Lu and Zhang, 2008)
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ACCEPTED MANUSCRIPT Highlights
1. The Da'anzhai Member is systematically studied as a shale system for the first time.
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2. The mechanisms of lacustrine shale paleoenvironment and mineral are discussed.
3. The mechanisms of lacustrine shale reservoirs and OM are discussed.
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4. The relationships between paleoenvironment, mineral, reservoir and OM are
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discussed.
S1875-5100(17)30380-3
DOI:
10.1016/j.jngse.2017.09.008
Reference:
JNGSE 2314
To appear in:
Journal of Natural Gas Science and Engineering
Received Date: 21 June 2017 Revised Date:
15 September 2017
Accepted Date: 27 September 2017
Please cite this article as: Xu, Q., Liu, B., Ma, Y., Song, X., Wang, Y., Chen, Z., Geological and geochemical characterization of lacustrine shale: A case study of the Jurassic Da'anzhai member shale in the central Sichuan Basin, southwest China, Journal of Natural Gas Science & Engineering (2017), doi: 10.1016/j.jngse.2017.09.008. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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First: Manuscript with track changes
Geological and geochemical characterization of lacustrine shale: a case study of the Jurassic Da'anzhai Member shale in the central Sichuan Basin, Southwest China
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Qilu Xua,b,c, Bo Liuc,*, Yongsheng Maa, Xinmin Songd, Yongjun Wangd, Zhangxin Chenb a
School of Earth Sciences and Resources, China University of Geosciences(Beijing), Beijing 100083, China
6
b
Chemical and Petroleum Engineering, University of Calgary, Calgary, Alberta T2N1N4, Canada
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c
Oil and Gas Research Center, Peking University, Beijing 100871, China
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d
PetroChina Research Institute of Petroleum Exploration and Development, Beijing 100083, China
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Abstract： ：
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Although numerous progresses have been achieved in characterizing marine shales, studies that are associated with lacustrine shales are limited.
To study lacustrine shales, samples were collected from
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the Jurassic Da'anzhai Member in the central Sichuan Basin of China, and their mineralogical, reservoir,
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OM, and paleoenvironmental characteristics were determined, as well as the relationships between them.
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Analysis of trace elements reveals that the shales formed in paleoenvironments that were oxic to suboxic,
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dry to humid, had moderate to strong weathering, and were characterized by fresh to salt water
16
conditions. These environments are more variable than those of marine shales. The paleoenvironmental
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conditions and mineralogy of the shales, particularly the oxic to suboxic paleo-redox conditions, resulted
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in the relatively low levels (0.11-2.18%, average 0.97%). However, based on evaluation criteria for
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continental source rocks, the OM is of high quality because of its high level of maturity (Ro: 0.95-1.43;
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Tmax: 428-500°C) and favorable kerogen type (II2). There are well-developed intraparticle pores,
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interparticle pores and microcracks. The SBET (5.42-10.69 m2/g, average 6.92 m2/g) and VBJH (1.19-4.10
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mL/100 g, average 2.97 mL/100 g) values also indicate good nanoscale storage space. Terrestrial
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minerals (i.e., quartz and clay) and authigenic carbonate minerals (i.e., calcite) are, respectively,
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positively and negatively correlated with the nanoscale storage space, .. Small pores (3-5 nm) dominate
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the nanoscale storage space. The isotherms and hysteresis loops are of Type П and Type H3, respectively,
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which indicates wedge-shaped pores. However, the hysteresis loops indicate that the lacustrine shale has
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more dead-end pores and larger pores with more complex microstructures than other lacustrine
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reservoirs. In general, the Da’anzhai lacustrine shale has the potential for unconventional oil and gas
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exploration. The lacustrine shale’s mineralogy, reservoirs, and OM are closely related to each other, and
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their differences are mainly caused by the paleoenvironmental conditions in which the shale formed.
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Keywords: Lacustrine shale; Paleoenvironment; Nanoscale reservoir; Organic matter; Minerals; Mechanisms.
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1. Introduction
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The high commercial production of shale oil and gas in North America has made shale a focus of
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exploration in many countries and regions. However, the most widely developed shales were formed in
36
marine systems; few studies have focused on the characteristics of lacustrine shales. Additionally,
37
lacustrine shales are widely distributed in many areas, such as Africa, South America, Southeast Asia
38
and China (Huang et al., 1984; Bohacs et al., 2000; Curtis, 2002; Schmoker, 2002; Chen et al., 2015;
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Jiang et al., 2016a; Yang et al., 2015).
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The most obvious characteristic of lacustrine shales is that they are subject to more variable paleoenvironments than marine shales. The formation environments of lacustrine and marine shales are
42
very different in terms of the water depth, energy, provenance, sedimentary type and biological effects
43
(Huang et al., 1984; Bohacs et al., 2000; Curtis, 2002; Schmoker, 2002). These features lead to
44
differences in their basic characteristics including mineralogy, organic matter (OM), and reservoir type
45
(Table 1). Minerals are the material basis of shales, and they affect not only OM generation but also
46
reservoir development (Huang et al., 1984; Bohacs et al., 2000; Curtis, 2002; Schmoker, 2002; Chen et
47
al., 2015; Yang et al., 2015; Jiang et al., 2016a). A major breakthrough in marine shale oil and gas
48
exploration was based on the recognition that shale acts not only as a source rock but also as a reservoir.
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Unlike studies of conventional reservoirs, research on unconventional reservoirs has primarily focused
50
on nanoscale reservoirs. Previous studies of marine and lacustrine shales suggest that shale gas is mainly
51
controlled by a nanoscale pore system that is closely related to the mineral characteristics (Zou et al.,
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2011; Morad et al., 2010; He et al., 2012; Luo et al., 2013; Dang et al., 2015; Chalmers et al., 2012;
53
Curtis et al., 2012; Jiang et al., 2015; Li et al., 2016; Jiang et al., 2017; Zhang et al., 2017). OM is the
54
basic material that generates hydrocarbons, and its type and maturity dominate the formation and
55
enrichment of shale oil and gas. The preservation and enrichment of OM in sediments is controlled by
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many factors, including the primary productivity, paleo-redox conditions, nutrient availability, clastic
57
influx and minerals (Huang et al., 1984; Bohacs et al., 2000; Curtis, 2002; Schmoker, 2002; Chen et al.,
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2015; Yang et al., 2015; Jiang et al., 2016a; Xu et al., 2017). In summary, the paleoenvironment,
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minerals, reservoir and OM, which are the basic characteristics of shales, have been the focuses of shale
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oil and gas studies, and these characteristics are often interrelated. Additionally, studies of the
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characteristics and mechanisms have mainly focused on marine shales; fewer studies have examined
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lacustrine shales.The lacustrine shale of the Jurassic Da’anzhai Member from the central Sichuan Basin
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of China has been interpreted as fractured reservoirs, low permeability fractured reservoirs and tight
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reservoirs. However, most studies have only focused on the conventional carbonate reservoirs, and the
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shales have only been considered as source rocks for the carbonate reservoirs (Lu et al., 2014; Chen et
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al., 2015). Encouragingly, the Da’anzhai shale of the Yuanba Block in the northwestern Sichuan Basin,
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which has similar geological conditions to those of the central Sichuan Basin, hosts high-yield wells (He
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et al. 2012; Zhu, et al. 2016). In addition, some lacustrine shales have yielded oil or gas, such as the
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Triassic Chang 7 and Chang 9 Member in the Ordos Basin, the Paleogene strata (Es3 and Es4) in the
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Bohai Bay Basin and the Cretaceous strata in the Songliao Basin (Qing-1) (Zou et al., 2011; Luo et al.,
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2013; Dang et al., 2015; Li et al., 2015; Yang et al., 2015; Jiang et al., 2016b). These successful
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developments strongly indicate that the Da’anzhai shale in the central Sichuan Basin is worth studying.
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In this study, we assessed the Da’anzhai lacustrine shale’s characteristics. We performed
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paleoenvironmental evaluation using TE, mineralogical evaluation usingXRD analysis, reservoir
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evaluation using FE-SEMand LTNA analyses, and OM evaluation using analyses of the TOC,
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chloroform bitumen ‘A’, Rock-Eval, carbon isotopes, kerogen macerals and organic elements. We also
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compared the Da’anzhai shale to other lacustrine shales which have been successfully developed and
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assessed the similarities and differences of lacustrine shales. In addition, we summarized the
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relationships between these characteristics and mechanisms affecting them. Finally, we judged the
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development potential of the Da’anzhai shale.
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Table 1 Comparisons between basic parameters in the Da’anzhai shale and other successfully developed shales
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82 83 84
lacustrine shale
Central Sichuan Basin, China
layer
Barnett
Ordos Basin,
Ordos Basin,
Bohai Bay
Bohai Bay
Songliao
Fort Worth Basin,
China
China
Basin, China
Basin, China
Basin, China
Texas, US
downfaulted
downfaulted
downfaulted
basin
basin
basin
Paleogene
Paleogene
Cretaceous
Carboniferous
shale with
shale with
shale with
siliceous and
siltstone
siltstone
argillaceous
carbonaceous
limestone
shale, limestone,
interlayers
dolomite
Ohio Appalachian Basin, Kentucky, US
polycyclic superimposed
basin
basin
Triassic
Triassic
shale with
shale with
siltstone
siltstone
interlayers
interlayers
interlayers
interlayers
1900~3200
1500~2500
1600~2600
1500~4200
1500~5200
1000~2500
1980~2590
510~1800
10~70
30~70
10~15
100~500
250~350
70~150
15~60
9~31
(20~80) /54
(37~53) /43
(29~56) /45
(25~80) /36
(30~80) /36
(30~80) /36
35~80
45~60
Jurassic
shells and
combination
carbonaceous interlayers
effective
(%) brittle minerals
Qing-1
polycyclic
lithological
thickness
Es4
superimposed
foreland basin
shale with
depth (m)
Es3
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basin type
Chang 9
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basin
marine shale
Chang 7
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Da'anzhai
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parameters
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(%)
6
foreland basin
piedmont depression Devonian interbed of black shale and gray siliceous shale
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0.20~5.92
1~12
10
2~8
1.3~9.3
6~12
4~5
4~7
permeability
0.002~0.9
0.01~0.3
(0.3~5.0) /2.97
<0.50
<0.50
<0.15
0.01
<0.1
TOC (%)
(0.1~2.2) /1.0
(0.3~36) /8.3
(0.3~11.3) /3.1
(0.5~13.8) /3.5
(0.8~16.7) /3.2
(0.4~4.5) /2.2
2.0~7.0
0.5~4.7
Ro (%)
0.9~1.5
0.8~1.2
0.9~1.3
0.4~1.2
0.4~2.0
0.5~1.5
1.1~2.2
0.3~1.3
pyrolysis gas
pyrolysis gas
pyrolysis gas
Ⅰ-Ⅱ
Ⅱ
Ⅱ
8.50~9.91
1.69~2.83
genetic type
biogenic and
pyrolysis gas
pyrolysis gas
pyrolysis gas
Ⅱ
Ⅰ-Ⅱ1
Ⅰ-Ⅱ1
Ⅰ-Ⅱ1
0.87~1.98
0.92~2.91
(1.68~4.25)
(0.60~3.70)
/2.92
/2.00
pyrolysis gas
pyrolysis gas
kerogen
2011; Luo et references
al., 2013; Lu et al., 2014; Chen et al., 2015;
2011; Luo et al., 2013; Yang et al., 2015; Jiang et al., 2016a.
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Zou et al.,
Zou et al.,
2011; Luo et
Zou et al., 2011;
Zou et al.,
al., 2013; Shi et
Luo et al., 2013;
2011; Luo et
al., 2013; Yang
Dang et al.,
al., 2013; Li,
et al., 2015;
2015; Jiang et
2014; Jiang et
Jiang et al.,
al., 2016a
al., 2016a
2016a.
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0.31~1.24
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Ⅰ-Ⅱ1
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porosity (%)
7
Schmoker, 2002;
Schmoker, 2002;
Zou et al.,
Bowker et al.,
Bowker et al.,
2011; Luo et
2007; Jarvie et al.,
2007; Jarvie et al.,
al., 2013; Jiang
2007; Martini et
2007; Martini et
et al., 2016a
al., 2008; Lu et al.,
al., 2008; Lu et al.,
2014
2014.
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2. Geological setting
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Fig. 1. Study location, geological map (Chen et al., 2015) and simplified stratigraphic column of the Jurassic Da’anzhai
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Member in the central Sichuan Basin.
The study area is located in the central Sichuan Basin, which includes the Nanchong and Suining
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areas. It is bounded by the Huafu Mountains to the east and the Longquanshan basement faults to the
93
west, and it covers an area of 6×104 km2 (Fig. 1) (Lu et al., 2014; Chen et al., 2015). A lacustrine
94
environment dominated the Sichuan Basin during the Jurassic Period, and the Da’anzhai Member 8
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represents the largest and deepest stage of this environment. The sedimentary facies of the Da’anzhai
96
Member can be divided into shore-shallow lacustrine, shallow lacustrine and semi-deep lacustrine
97
subfacies, which include shell beach, calcareous flat, mud flat, and semi-deep lake microfacies. These
98
facies are distributed in concentric belts around the lake basin’s center in the Yilong-Pingchang areas
99
(Fig. 1) (Lu et al., 2014; Chen et al., 2015). From top to bottom, the Da’anzhai Member in the central
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Sichuan Basin can be divided into main three sub-members: Da1, Da13 and Da3 (Fig. 1). The thickest
101
Da13 sub-member (30-50 m) is a set of widely distributed dark mudstones with lamellated fractures and
102
rich OM; this sub-member is considered the most important source rock for hydrocarbons. The Da3
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sub-member deposits (5-14 m), which are initial deposition products, are primarily (mud) shell
104
limestones with dark gray mudstone interlayers. The Da1 sub-member (30-45 m thick) primarily
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consists of shales and (mud) shell limestones; this sub-member contains the sedimentary products of the
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period of basin uplift (Lu et al., 2014; Chen et al., 2015).
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3. Samples and experimental methods
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3.1. Samples
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Thirty-nine representative core samples were obtained from the Da’anzhai Member shale in the
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central Sichuan Basin based on drilling, core observations, stratigraphic data, and log responses (Fig. 1
111
and Table 3).
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3.2. Methods
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All 39 shale samples were analyzed to determine their bulk mineralogy viaXRD
114
using a X'Pert
Pro MPD device with a working voltage of 40 kV and a current of 40 mA in the Key Laboratory of
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Orogenic Belts and Crustal Evolution at Peking University. To investigate the clay mineral fraction in
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detail, textured mounts were measured four times at a goniometer rate of 0.5θ/minute with a registration
117
range from 2 to 42°2θ at the Beijing Research Institute of Uranium Geology.
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The micromorphology and oiliness of the samples were determined at the Key Laboratory of Marine Reservoir Evolution and Hydrocarbon Accumulation of the China University of Geosciences
120
(Beijing) using an Axio Scope A1 pol Fluorescence-Polarization Microscope. An INCA Synergy system
121
at the Key Laboratory of Orogenic Belts and Crustal Evolution at Peking University was used to analyze
122
the microstructure of each sample via FE-SEM and the mineral composition of each sample via EDS.
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The samples were subjected to argon-ion polishing to obtain higher resolution images.
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LTNA analysis is capable of characterizing nanoscale pores in porous media, and adsorption tests were performed with dried samples (60-80 mesh) on an ASAP2020M device at the College of
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Chemistry and Chemical Engineering of the China University of Petroleum (Beijing). N2 adsorption
127
isotherms were obtained at -196.15°C within a relative pressure range of 0.005 to 0.998. The BET
128
method for the equivalent specific surface areas and theBJH method for the total pore volumes were
129
applied in the calculations (Brunauer et al., 1938; Barrett et al., 1951).
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The OM richness was characterized by measuring the TOC values of 25 samples using a GHM-02
131
analyzer; 10% hydrochloric acid was used to remove carbonates, and the test sensitivity was 10-13 mg/g.
132
Chloroform bitumen ‘A’ was extracted from the crushed shale powders (<0.09 mm), and the weight was
133
calculated at the China University of Petroleum (Beijing). A Rock-Eval 6 instrument was used to
134
perform a pyrolysis analysis, which yielded the temperature of the Tmax, S0, S1 and S2. The OM maturity
10
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was determined based on the Ro value, which was obtained using a reflected light microscope under oil
136
immersion and calculated as the mean of 10-30 measurements taken for a single sample. Additionally,
137
measurements were taken to determine the isolated kerogen content and identify the organic kerogen
138
macerals, carbon isotope content and organic element content according to the (GB/T) 19144-2010,
139
(SY/T) 5125-2014, (GB/T) 18340.2-2010, (GB/T) 19143-2003 standards, respectively. The kerogen
140
measurements and the rock pyrolysis analysis were performed at the Keyuan Engineering Technology
141
Testing Center.
SC
M AN U
142
RI PT
135
The TE concentrations of 17 samples were determined using ICP-MS, which was performed at the Beijing Research Institute of Uranium Geology. The detailed sample processing procedure as well as the
144
analytical precision and accuracy followed the methods described for the (GB/T) 14506.28-2010
145
standard.
TE D
143
4. Results and discussions
147
4.1. Paleoenvironmental properties and differences
148
Lacustrine shales are different from marine shales mainly due to their formation paleoenvironments.
EP
146
Lakes are complex and dynamic systems, and the development of lacustrine shales is controlled more
150
significantly by the sedimentary environment (Huang et al., 1984; Bohacs et al., 2000; Curtis, 2002;
AC C
149
11
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Table 2 Trace elements of the Da’anzhai lacustrine shale. G10-2
G4-3
G6-2
J45-1
J45-2
J53-1
J61-1
J61-2
J61-3
J61-4
Li-1
Li-2
P1-1
S2-1
S2-3
S2-4
X44-1
Th(ppm)
11.70
10.40
5.43
4.32
12.00
10.60
1.72
4.13
7.74
10.20
11.30
9.50
11.60
2.31
10.50
6.21
13.70
Ni/Co
2.68
2.98
3.26
4.10
2.87
2.72
5.49
3.87
3.50
2.51
2.65
2.97
3.07
3.22
2.32
3.37
2.78
RI PT
code
1.76
1.58
1.44
1.50
1.64
1.64
2.47
1.56
1.33
1.69
1.60
1.50
1.61
1.77
1.30
1.41
1.45
0.78
0.80
0.63
0.63
0.78
0.78
0.67
0.68
0.70
0.77
0.78
0.74
0.78
0.61
0.68
0.68
0.77
V/Sc
9.78
8.70
6.88
7.73
9.36
8.91
13.81
8.41
7.94
9.03
7.93
7.21
9.22
9.80
8.54
7.14
7.81
Th/U
2.77
4.24
4.45
2.79
4.07
2.95
1.28
3.53
4.63
3.81
4.25
4.63
4.20
2.74
5.25
4.60
4.72
Sr/Cu
3.75
4.60
42.98
77.10
3.50
3.62
58.70
35.54
53.09
8.46
7.84
19.01
3.35
7.47
24.69
6.05
Sr/Ba
0.29
0.26
3.66
3.08
0.23
0.24
4.77
2.78
3.45
0.52
0.46
1.28
0.18
3.77
0.44
1.84
0.36
118.7
115.3
109.8
131.7
110.3
187.5
141.2
128.9
194.3
112.9
60.7
130.6
112.6
153.9
3
6
4
1
7
7
6
9
0
9
6
7
2
3
7.79
9.03
6.39
6.65
9.34
9.58
8.46
8.67
8.49
8.04
9.44
4.19
9.93
9.86
8.88
7.88
10.03
(La/Yb)N
6.65
7.75
6.57
8.37
6.82
8.04
9.85
9.82
11.14
8.00
7.50
4.35
7.49
8.62
7.67
9.05
(La/Sm)N
4.66
4.74
3.04
3.32
5.58
5.42
3.19
3.06
2.37
3.88
5.71
2.59
5.90
3.07
4.20
3.90
5.01
ΣLREE/ ΣHREE
88.73
38.15
M AN U
m)
130.97
TE D
ΣREE(pp
SC
V/Cr V/(V+Ni)
80.5 4
10.8 8
0.99
1.04
1.52
1.97
0.77
0.97
2.04
2.01
2.92
1.45
0.84
1.41
0.79
2.10
1.35
1.38
1.17
(Ce/Yb)N
4.48
5.38
4.98
5.83
4.54
5.42
7.21
7.33
9.24
5.58
4.97
3.22
5.00
8.49
6.10
5.59
6.15
δCe
0.92
0.94
0.97
0.95
0.92
0.93
0.94
0.93
0.99
0.92
0.92
0.92
0.93
0.96
0.95
0.97
0.92
CSr(‰)
0.19
0.18
0.77
1.12
0.17
0.16
0.63
0.56
0.88
0.36
0.27
0.61
0.15
0.72
0.12
0.35
0.23
AC C
EP
(Gd/Yb)N
XN, where N refers to the chondrite-normalized value (Gromet et al., 1984); δCe=CeN/(LaN×PrN)1/2 (Wright et al.,1984; Murray et al.,1990).
12
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Schmoker, 2002). TEs can be used as effective quantitative indicators of the paleoenvironment by
2
comparing their type and abundance with standard values (Wright et al., 1984; Murray et al., 1990;
3
Manning, 1994; Ross et al., 1995; Kimura and Watanabe, 2001; Jarvis et al., 2001; Rimmer, 2004; Jones
4
and Roy et al., 2007).
5 6
TE D
M AN U
SC
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1
Fig. 2. Identification of paleoenvironments using TEs (a-c) and clay (d).
4.1.1. Paleo-redox conditions
8
Due to their responses to changing redox conditions, the ratios of redox-sensitive TEs, such as
AC C
9
EP
7
Ni/Co, V/Cr, V/Sc, Th/U, and δCe, are usually used to evaluate redox conditions. According to Jones
10
and Manning (1994), V/Cr ratios <2 indicate oxic conditions, V/Cr ratios from 2 to 4.25 indicate dysoxic
11
conditions, and V/Cr ratios >4.25 indicate suboxic to anoxic conditions. Kimura and Watanabe (2001)
12
found that the V/Sc ratio is positively related to oxic conditions. Jones and Manning (1994) showed that
13
Ni and Co are enriched in pyrite, and high Ni/Co ratios indicate anoxic conditions, Ni/Co ratios <5 13
ACCEPTED MANUSCRIPT
indicate oxic conditions, Ni/Co ratios between 5 and 7 indicate dysoxic conditions, and Ni/Co ratios >7
15
indicate suboxic to anoxic conditions. Similarly, Th/U ratios <1.5 represent anoxic conditions, Th/U
16
ratios between 1.5 and 3 represent dysoxic to suboxic conditions, and Th/U ratios >3 represent oxic
17
conditions (Jones and Manning, 1994). In addition, δCe values >1 (positive anomaly) and <0.95
18
(negative anomaly) suggest anoxic and oxic to suboxic conditions, respectively (Wright et al., 1984;
19
Murray et al., 1990).
SC
For the Da’anzhai shale in the central Sichuan Basin (Table 2), the ranges of the Ni/Co, V/Cr, V/Sc,
M AN U
20
RI PT
14
and Th/U ratios are relatively wide, from 2.32-5.49, 1.30-2.47, 6.88-13.81, and 1.28-5.25, respectively.
22
The values of δCe vary from 0.92 to 0.99 (average 0.94), which indicate a weakly negative anomaly
23
(Wright et al., 1984; Murray et al., 1990).Collectively, these proxies reveal that the shale was deposited
24
under variable paleo-redox conditions that were mainly oxic to suboxic (Fig. 2).
TE D
21
4.1.2. Paleo-salinity
26
Sr/Ba values vary directly with the distance to a sea or lake coast; Sr/Ba ratios <1 suggest
EP
25
freshwater conditions, and Sr/Ba ratios >1 suggest salt water conditions (Jarvis et al., 2001; Kimura and
28
Watanabe, 2001). In addition, low Sr concentrations (0.1-0.3‰) indicate a freshwater setting, and high
29
values (0.8-1‰) represent a salt water setting (Jarvis et al., 2001; Kimura and Watanabe, 2001). Th
30
easily dissolves in acidic water and hydrolyzes to oxide or hydroxide sediments under alkaline
31
conditions; therefore, high Th values indicate a salt water environment (Jones and Manning, 1994).
32 33
AC C
27
In the Da’anzhai shale samples (Table 2), the Sr/Ba ratios vary widely (0.18-4.77) with an average of 1.62. The Th content also varies widely (1.72-13.70 ppm) and averages 8.43 ppm, and the Sr 14
ACCEPTED MANUSCRIPT
34
concentrations vary from 0.12 to 1.12‰ (average 0.44‰). Generally, these proxies indicate variable
35
paleo-salinity conditions including both fresh and salt water settings (Fig. 2).
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4.1.3. Paleo-weathering
M AN U
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36
37
39
Fig. 3. Chondrite normalized REE distribution curves for REE minerals. Chondrite values are from Gromet (1984).
When rocks are subjected to strong weathering, heavy rare earth elements (HREEs) dissolve and
TE D
38
migrate much more easily than light rare earth elements (LREEs). Therefore, high ΣLREE/ΣHREE
41
values indicate strong paleo-weathering conditions (Kimoto et al., 2006; Stevens and Quinton, 2008). In
42
addition, the (La/Yb)N, (La/Sm)N, (Gd/Yb)N and (Ce/Yb)N ratios, which are positively related to LREE
43
enrichment, are also positively correlated with paleo-weathering (Table 2) (Wright et al., 1984; Murray
44
et al., 1990; Ross et al., 1995; Roy et al., 2007).
AC C
45
EP
40
The ΣLREE/ΣHREE rates are between 4.19 and 10.03 (average 8.39), which indicate LREE
46
enrichment. The (La/Yb)N ratio is represented by the slope of the chondrite-normalized REE distribution
47
curve (Fig. 3), which indicates the degree of the graph’s inclination (Ross et al., 1995; Roy et al., 2007).
48
The (La/Yb)N values vary from 4.35 to 11.14 (average 8.15), which indicate that the curves are tilted to 15
ACCEPTED MANUSCRIPT
the right and that the samples are rich in LREE-acidic rocks (Fig. 3). The (La/Sm)N ratio, which
50
represents the degree of LREE fractionation, is positively related to the ΣLREE value (Ross et al., 1995;
51
Roy et al., 2007). The (La/Sm)N ratios range from 2.37 to 5.90 (average 4.10), which indicates a P(E)
52
model with abundant LREEs. The (Gd/Yb)N ratio, which indicates the degree of HREE fractionation, is
53
negatively correlated with the ΣHREE value (Ross et al., 1995; Roy et al., 2007). The (Gd/Yb)N values
54
vary from 0.77 to 2.92 (average 1.46), and the (Ce/Yb)N ratios vary from 3.22 to 9.24 (average of 5.85),
55
which indicate that the Da’anzhai shale is depleted in HREEs (Fig. 3). In summary, the REE results
56
(Table 2) indicate that the shale was deposited under moderate weathering conditions with periods of
57
strong weathering.
M AN U
SC
RI PT
49
4.1.4. Paleoclimate
59
∑REE values are closely related to paleoclimate conditions; that is, ∑REE values are higher in
TE D
58
warm and humid climates and lower in cold and dry climates (Wright et al., 1984; Murray et al., 1990;
61
Ross et al., 1995; Roy et al., 2007). The ΣREE values of the Da’anzhai shale vary widely from 38.15 to
62
194.3 µg/g (average 121.59 µg/g) (Table 2). The Sr/Cu ratio is another useful indicator of lacustrine
63
paleoclimate, with high values reflecting humid climates (Reheis, 1990; Chen et al., 2009).The Sr/Cu
64
ratios in the Da’anzhai shale also vary widely from 3.35 to 80.54%. In addition, illite usually forms in
65
dry conditions by the weathering depotassication of feldspar, mica and other aluminosilicate minerals.
66
Conversely, kaolinite forms in humid climates by the eluviation of feldspar, mica and pyroxene.
67
Therefore, high illite/clay ratios indicate dry, saline waters with high K+ content, and high kaolinite/clay
68
ratios suggest humid climates (Vanderaveroet, 2000; Gingele et al., 2001). The illite/clay ratios of the
AC C
EP
60
16
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Da’anzhai shale are relatively high (between 26 and 86%, average 52%). However, the kaolinite/clay
70
ratios are relatively low (between 3 and 19%, average 11%) (Fig. 2d). These results indicate a variable
71
climate with both dry and humid periods as well as a drying trend from humid climates.
72
RI PT
69
In general, the TE contents and types t in lacustrine shales are more complex and varied than those in marine shales, which indicates that lacustrine shales tend to experience more variable
74
paleoenvironmental conditions (Huang et al., 1984; Bohacs et al., 2000; Curtis, 2002; Schmoker, 2002).
75
The lacustrine Da’anzhai shale was formed under relatively dry climatic conditions that included both
76
arid and humid stages. In addition, the shale experienced a moderate degree of paleo-weathering with
77
stages of strong weathering, variable oxic to suboxic paleo-redox conditions that were weakly reducing,
78
and a predominantly freshwater setting interspersed with periods of saltwater.
M AN U
SC
73
4.2. Mineralogy properties and differences
80
As shown in Table 3, the Da’anzhai shale primarily contains clay (42.4%), quartz (27.7%) and
81
calcite (21.1%). The minerals in these lacustrine shales vary widely. Clay comprises the majority of the
82
shale and ranges from 16.3 to 68.4%, while the quartz content ranges from 5.6 to 53.4%. Carbonate
83
minerals, which mainly include calcite, dolomite and aragonite, range from 3.0 to 46.6% with an
84
average of 27.4%. The Da’anzhai shale contains more brittle minerals than the other lacustrine shales
85
and marine shales (Table 1 and Fig. 4). For example, the brittle mineral content of the Da’anzhai shale
86
ranges from 24.2 to 80.3%, with an average of 53.7%, indicating medium-high brittleness that is
87
favorable for natural fractures and hydraulic fracturing. Similar to other lacustrine shales, lacustrine
88
shales have more siliceous or calcareous interlayers than marine shales, which can cause the wider
AC C
EP
TE D
79
17
ACCEPTED MANUSCRIPT
ranges of carbonate and quartz minerals and improve the brittleness index and fracturing (Figs1 and
90
5I).Ternary diagrams were constructed to compare the mineral compositions of hot shales that have been
91
successfully exploited including lacustrine shales (Fig. 4a) and marine shales (Fig. 4b). Generally, the
92
comparison shows that the mineral composition of the lacustrine shales is more variable and complex
93
than in marine shales (Fig. 4). For example, the Da’anzhai shale has relatively wide ranges of carbonate
94
(1-70%), clay (15-70%) and quartz (5-50%) (Fig. 4a). In addition, the Da’anzhai shale has the wider
95
mineral ranges than other lacustrine shales, especially for the range of carbonate minerals.
M AN U
SC
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89
EP
97
TE D
96
Fig. 4. Ternary diagrams of mineral compositions. (a) Lacustrine shales: 1=Da’anzhai shale, 2=Jurassic shale in Sichuan
99
Basin (Jiang et al., 2017), 3=Triassic shale in Sichuan Basin(Jiang et al., 2017), 4=ES3 shale (Wang et al., 2015a). (b) Marine
AC C
98
100
shales: 5 Woodford shale (Jarvie et al. 2007 and Han et al. 2013), 6=Barnett shale (Jarvie et al. 2007 and Han et al. 2013),
101
7= Wufeng-Longmaxi shale (Wu et al., 2014), 8=Ohio shale (Jarvie et al. 2007 and Han et al. 2013).
102
Table 3 Minerals and TOC results for the Da’anzhai lacustrine shale.
103
18
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G10-3 2700.7 G10-4 2646.6 G4-1 2385.3 G4-2 2395.8
subquartz feldspar calcite dolomit aragonite siderite pyrite hematite total clay others carbonate brittleness member (%) (%) (%) e (%) (%) (%) (%) (%) (%) (%) (%) minerals (%) 44.2 3.0 52.8 3.0 47.2 Da1 22.1 1.1 0.9 1.2 6.3 68.4 2.1 24.2 Da13 37.5 2.0 2.9 2.6 55.0 2.9 40.4 Da13 1
4.2 2.1
G4-3 2405.7
Da Da1 Da1 Da13
28.1 28.2 35.2 25.8
2.5
G6-1 2537.1
Da13
21.4
2.6
G6-2 2580.1
8.7
1.2
47.6
19.5
1.1
0.8
J45-1 2632.4 J45-2 2656.0
Da3 Da1 Da13
12.5 23.4
0.6 6.1
27.9 3.9
36.3
0.4 0.4
0.7 1.1
J53-1 2827.3
Da13
31.9
4.1
1
14.4 55.3 14.3 17.5 69.4
6.4
17.0 13.4 12.5
4.5
4.6
0.7
1.6
1.2
Da13
8.5
J61-6 Li-1 Li-2 Li-3
2665.9 1713.8 1726.1 1734.3
Da1 Da1 Da1 Da13
25.1 30.4 24.2 40.7
1.8 2.6 1.0 2.3
15.6 9.8 28.9 3.1
1.0
P1-1 3215.9
Da13
23.3
0.9
0.6
1.0
P1-2 3221.2
Da13
34.8
4.1
P1-3 3239.2
Da13
30.3
14.9
P1-4 3258.6 S2-1 2836.1 S2-2 2849.9
3
Da Da1 Da1 Da13
23.2 19.8 40.3
51.8 48.2 10.7
Da3 Da3
18.9
S2-4
2889.0
S2-5 2891.6 X20-1 1844.4 X28-1 1952.2 X44-1 2089.4 X44-2 2093.0
Da1 Da13 Da13 Da13 Da13 Da13 1
53.4 49.4
3.6
LQ-1 3548.1 X8-1 1780.7
W8-1 2065.7
Da Da1
1.3
0.2 0.8 0.0 4.2
3.2
6.3 0.0
5.3 2.2
69.2 41.6 54.1
0.40 0.98
5.3
31.1
1.43
4.2
7.4
28.8
1.18
20.1
1.0
67.1
75.8
20.5 64.0
1.1 1.1
64.2 3.9
76.7 27.3
55.5
5.4
22.6 26.7 27.7 33.1
10.3
1.1
0.81
41.1 13.4 18.9
1.0 1.1 0.9 2.3
20.0 46.2 53.3 38.5 53.9
0.7
31.9
2.18
59.3 64.8 57.0 24.3
76.2 70.4 71.4 59.1
1.49
69.4
77.9
0.30
26.9 9.8 35.2 3.1
52.0 40.2 59.4 43.8
1.17 1.07 0.69
6.4
67.8
1.6
24.9
1.59
0.9
60.2
4.1
38.9
1.51
3.6
51.2
14.9
45.2
1.55
1.5 0.2 0.9
16.3 29.6 42.5
57.1 50.4 10.7
80.3 70.2 51.0
0.61 0.11 1.18
1.9 2.0
47.7
1.3
33.4
49.0
66.6
0.33
48.9
1.5
30.7
50.4
69.3
0.18 0.76
8.1
24.9 35.9
1.4
27.9
3.0
4.0
23.8
3.0
43.2
AC C
X44-3 2116.7
Da13
17.6
1.0
2.1
TE D
2867.1
44.9 9.5 42.7 6.8
0.9
TOC (%) 0.82 1.33
60.9
SC
J61-5 2706.8
S2-3
3.5
M AN U
Da1 Da13
16.9 5.6 14.4 34.8
26.6 54.9 38.4 64.8
3.1
2662.6 2664.4 2667.6 2698.1
Da Da1
3.5
7.4
25.6 8.5
EP
J61-1 J61-2 J61-3 J61-4
22.9
RI PT
depth (m) G10-1 2650.8 G10-2 2668.4 code
46.6 38.7
2.3
3.1 3.8 3.7 9.5
1.5
8.1
53.4 57.5
44.3 51.8
0.7
28.7 8.5
53.6 44.4
0.53
60.8
0.6
4.0
31.9
1.15
52.7
76.5
20.5
44.1
2.9
18.7
40.5
2.6
2.5
34.3
28.2
2.2
18.5
7.8
43.3
23.9
1.9
35.2
11.4
26.6
2.5
50.8
1.1
104
W8-2 2069.5
105
4.3. Characterization and controlling factors of lacustrine shale reservoirs
106
4.3.1. Microscopic characteristics
19
1.0
18.7
62.8
2.5
43.0
26.3
54.5
46.6
70.5
0.84
ACCEPTED MANUSCRIPT
108
AC C
EP
TE D
M AN U
SC
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107
109
Fig.5. Typical reservoirs images of lacustrine shale A:
Shells were stained using alizarin red to highlight the carbonate
110
minerals; the shells are well developed and preserved in terrestrial inputs and matrices. B: Greenish-yellow fluorescence.
111
Matrices with large amounts of terrestrial inputs between the shell grains have strong fluorescence intensity, indicating
112
effective reservoirs with good oiliness in the matrices. C: Lacustrine shale core sample with well-developed calcareous
113
interlayers, indicating a changeable sedimentary environment. D: Quartz particles with well-developed interparticle pores
20
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filled with clay minerals; quartz and clay minerals have well-developed intraparticle pores. E: Quartz particles have
115
well-developed intraparticle pores and interparticle pores; calcite minerals fill the storage spaces between quartz particles
116
(calcite cementation). F: Intraparticle pores located along the cleavage planes of clay particles; interparticle pores between
117
clay and carbonate minerals. G: OM with well-developed pores is accompanied by clay minerals. H: Calcite minerals fill the
118
spaces between particles (calcite cementation); quartz has well-developed intraparticle pores. I: Brittle minerals with
119
well-developed micro-cracks that are mainly filled by clay minerals. J: Model of different types of pores. K: Model of
120
carbonate mineral (e.g., calcite) diagenesis that results in reduced storage spaces.
SC
M AN U
121
RI PT
114
According to the International Union of Pure and Applied Chemistry (IUPAC) classification scheme, the pore types in unconventional reservoirs are divided by pore size into micropores, mesopores,
123
and macropores (<2 nm, 2-50 nm, and >50 nm, respectively) (IUPAC, 1994; Thommes et al., 2015).
124
Thin section observations of the shale indicate that the matrices fluoresce intensely (Fig. 5B). Nanoscale
125
reservoirs were examined using FE-SEM with argon-ion milling technology, and the minerals were
126
identified using EDS. Similar to other lacustrine shales, the pores and fractures in the Da’anzhai shale
127
are well developed and fall into four categories.
EP
(1) Interparticle pores: these pores represent the largest number of pores, and they have good
AC C
128
TE D
122
129
connectivity and form effective pore networks. The diameters generally range between 5 nm and 90 µm.
130
Many types of interparticle pores were observed, including pores between grains (Fig. 5D,E,H,J),
131
crystals and clay platelets (Fig. 5D,F,G,J), and pores at the edges of rigid grains (Fig. 5E,J).
132 133
(2) Intraparticle pores: these pores primarily formed through diagenesis, and some are primary. The diameters mainly range from 2 nm to 10 µm. The primary sub-types in the Da’anzhai shale are
21
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134
intraplatelet pores within clay aggregates (Fig. 5D,F,J), intercrystalline pores within pyrite framboids,
135
pores within fossil bodies and moldic pores after fossils (Fig. 5D,J). (3) Fracture pores: these pores are not controlled by individual particles and can be regarded as a
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136 137
type of pore that formed by rock deformation because of tectonic movements, sedimentation, diagenesis,
138
OM hydrocarbon generation and other geological effects (Fig. 5I,J).
(4) OM pores: these pores are found within OM (Fig. 5L) and are usually irregular, bubble-like, and
SC
139
oval. They generally range in diameter from 2 to 500 nm. Although most OM pores are incorrectly
141
regarded as isolated pores in two-dimensional planes, they are interconnected in three-dimensional space
142
and facilitate the development of reservoirs.
143
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Similar to other lacustrine and marine shale reservoirs, the Da’anzhai shale commonly contains well-developed pores and fractures, particularly intraparticle and interparticle pores (Curtis et al., 2002;
145
Chalmers et al., 2012; Milliken et al., 2013; Nie et al., 2015; Wang et al., 2015a; Zhou and Kang, 2016;
146
Jiang et al., 2017; Zhang et al., 2017). Especially, due to their rich terrestrial clastic mineral content, the
147
interparticle pores of lacustrine shales are better developed than those of marine shales. Most of the
148
intraparticle pores are nanoscale pores (<200 nm) (Fig. 5D,E,F) and the OM also contains nanopores
149
(Fig.5G). Microcracks can develop due to the high brittle mineral
150
However, these cracks are often filled by calcite and clay minerals due to the high clay content (42.4%)
151
(Fig.5I,J).
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TE D
144
22
content for the Da’anzhai shale.
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4.3.2. Nanoscale reservoirs characterized by LTNA
153
The LTNA test is the most widely used method of characterizing several parameters of nanoscale
154
pores in porous media (micropores and mesopores), includingVBJH, SBET, DA, ad/desorption isotherms
155
and PSD (Sing et al., 1985; IUPAC, 1994; Thommes et al., 2015; Wang et al., 2015c; Zhou et al., 2016;
156
Zhang et al., 2017).
157
Table 4 Comparisons of primary pore structure parameters measured by LTNA between the Da’anzhai shale and other
158
lacustrine shales
159
3
4
5
6
7
160
SBET (m2/g)
Sichuan
Jurassic Da'anzhai
1.75-10.69
Basin
Member
(6.92)
Upper Triassic
0.25-4.39
Chang 7 Member Xujiahe
Ordos Basin Sichuan Basin
3
(cm /100g)
D (nm)
1.19-4.10
7.21-24.71
(2.97)
(14.11)
0.02-0.90
6.67-18.40
(2.62)
(0.49)
(11.23)
0.62-14.70
0.22-2.97
4.91-13.93
(7.76)
(1.41)
(6.97)
Upper Triassic
1.46-5.24
0.72-1.40
9.40-25.00
Chang 7 Member
(2.94)
(1.01)
(16.40)
Bohai Bay
Shahejie
0.40-12.45
0.08-1.60
4.58-41.36
Basin
Formation
(3.62)
(0.62)
(16.84)
0.39-34.06
0.13-4.18
3.16-16.48
(10.92)
(1.59)
(9.56)
Upper Triassic
1.10-1.90
0.69-1.09
15.4-23.4
Chang 7 Member
(1.57)
(0.83)
(19.27)
Ordos Basin
Songliao
Formation
VBJH
TE D
2
strata
EP
1
location
Upper Cretaceous Qingshankou
AC C
NO.
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152
Basin
Ordos Basin
Formation
161
23
hysteresis
loops types
references
H3
H3 H3 H3 H3,H4
H2,H3
H3,H4
Jiang et al., 2016b Peng et al., 2016 Fu et al., 2015 Zhang et al., 2016 Wang et al., 2015b Yang et al., 2017
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162
164
Fig. 6. PSD curves obtained from the low temperature N2 adsorption isothermswith BJH pore sizes.
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163
The VBJH value of the Da’anzhai shale (2.97 cm3/100 g) is slightly higher than the values in other shales, and the SBET value (6.92 m2/g) is nearly equal to those of the other shales (Table 4). These values
166
indicate that the Da’anzhai shale can also have significant nanoscale storage space.
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To investigate the distribution and contributions of each pore type, the VBJH with respect to the PSD
168
was determined from the adsorption branches of the BJH model (Wang et al., 2015b; Jiang et al., 2016b;
169
Zhou et al., 2016; Zhang et al., 2017). The PSD plots show that pores with diameters between 3 and 5
170
nm make up the largest proportion of the total pore volume and that the proportions decrease with
171
increasing pore size, which is similar to
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some lacustrine shales (Fig. 6) (Jiang et al., 2016b).
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172
TE D
167
24
173 174
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Fig. 7. Comparisons between the nanopore structures of the Da’anzhai shale and other lacustrine shales. A, B, C: the Da’anzhai shale. D: Lacustrine shale of the Chang 7 Member in the Ordos Basin, China (Fu et al., 2015). E: Lacustrine shale
176
of the Songliao Basin, China (Wang et al., 2015b). F: Lacustrine shale of the Yanchang Formation in the Ordos Basin, China
177
(Jiang et al., 2016b; Wang et al., 2015c). G: Lacustrine shale of the Shahejie Formation in the Dongying Sag, China (Liu et
178
al., 2017). H: Lacustrine shale of the Shahejie Formation in the Zhanhua Sag, China. I: Lacustrine shale of the 5
179
Member of the Xujiahe Formation of Upper Triassic in Sichuan Basin, China (Peng, 2016).
180
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175
th
LTNA isotherms and hysteresis patterns can be used to investigate the physisorption mechanisms
181
and structural characteristics of shale pores (Sing et al., 1985; IUPAC, 1994; Thommes et al., 2015;
182
Zhou et al., 2016; Zhang et al., 2017). Adsorption isotherms are divided into five types (Types I-VI), and 25
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hysteresis loops are divided into four types (Types H1-H4) (Sing et al., 1985; IUPAC, 1994; Thommes et
184
al., 2015). Following this classification, the isotherms of the Da’anzhai shale samples are similar and
185
belong to Type П, and the hysteresis loops are characterized by Type H3 hysteresis loops that indicate
186
wedge-shaped pores (Sing et al., 1985; IUPAC, 1994; Thommes et al., 2015). However, compared to
187
other lacustrine shales, the Da’anzhai shale has relatively weak hysteresis loops and wide overlapping
188
ranges of adsorption and desorption isotherms (Fig. 7), which indicate many dead-end pores with highly
189
complex structures (De Boer, 1958; Nie et al., 2015).
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183
190
4.3.3. Factors that influence pore structure
191
SBET and VBJH are the two most basic LTNA parameters, and they can directly indicate the storage spaces of nanoscale reservoirs. Fig. 8a shows that the TOC values are positively correlated with SBET
193
and VBJH. However, the correlation is not strong because of the relatively low TOC values (average
194
0.97%). Fig. 8b shows that the lacustrine clay content is positively correlated with SBET and VBJH; this
195
correlation is mainly caused by the high clay content (42.4%) and well-developed intraparticle pores
196
(e.g., intraplatelet pores within clay aggregates) and interparticle pores (e.g., pores at the edges of
197
particles, crystals and clay platelets) (Figs. 5F, L). The quartz content is also positively correlated with
198
SBET and VBJH (Fig. 8c). Quartz particles in lacustrine shales usually occur within mineral matrices and
199
have well-developed interparticle pores at the edges of other minerals (e.g., clay and carbonate minerals).
200
The quartz particles also have well-developed nanoscale intraparticle pores. Additionally, the quartz
201
particles have no obvious cementation (Fig.5D). Conversely, the carbonate content is negatively
202
correlated with SBET and VBJH (Fig. 8d). Carbonate minerals, particularly calcite, can significantly affect
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26
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reservoir quality and generally has adverse effects (Wang et al., 2016).
Carbonate minerals (e.g.,
204
calcite) are easily dissolved and redistributed and can be formed in primary pores by various
205
mechanisms during the diagenesis processes (Figs. 5E, H, K) (Huang et al., 1984; Wang et al., 2016).
206
Based on qualitative observations using SEM, polarized light microscopy and fluorescence microscopy,
207
lacustrine carbonate minerals can effectively reduce the available storage space by diagenesis processes
208
including cementation, compaction, pressure-solution, recrystallization and replacement (Figs. 5E, H, K)
209
(Huang et al., 1984; Wang et al.,
210
2016).
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211
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Fig. 8. Correlation plots between nanopore storage space indicators (SBET and VBJH) and values of TOC and minerals.
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4.4. Characterization and controlling factors of OM in lacustrine shale
213
4.4.1. Characterization of the OM content
214
Water conditions play an important role in the formation of source rocks. Lacustrine shales
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primarily form in semi-deep to deep lakes with fresh to low salinity water, whereas marine shales
216
primarily form in reductive, salty, weakly alkaline and low energy water. These features give rise to
217
different source rock characteristics (Huang et al., 1984; Bohacs et al., 2000; Curtis, 2002; Schmoker,
218
2002; Chen et al., 2015; Yang et al., 2015; Jiang et al., 2016a). Due to these differences, we evaluate the
219
lacustrine OM using a terrestrial oil generation theory that was derived from studies of continental
220
petroliferous basins in China and plays a special role in global petroleum theories (Huang et al., 1984;
221
Lu and Zhang, 2008) (Table 5).
222
Table 5 Factors used to evaluate OM in lacustrine source rock (Huang et al., 1984; Lu and Zhang, 2008).
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TE D
lake types
TOC(%)
freash~brackish water salt water~high salt water
chloroform bitumen ‘A’ (%) S1 +S2 (mg/g)
non-source rock <0.4
EP
index
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223
SC
215
<0.2
<0.015
poor
source rock medium good
very good
0.4~0.6
>0.6~1.0
>1.0~2.0
>2.0
0.2~0.4
>0.4~0.6
>0.6~0.8
>0.8
0.015~0.050 <0.050~0.100 >0.100~0.200 <2
2~6
>6~20
>0.200 >20
The mean
224
values (and ranges) of the shale’s kerogen macerals consist of 32% exinites (29-35%), 27%
225
sapropelinites (23-32%) and 24% vitrinites (18-26%), which indicates that the OM is primarily derived
226
from plankton and bacteria. The δ13CPDB values (-29.1 to -29.5‰, average -29.3‰) support this finding
227
(Table 7) (Huang et al., 1984; Lu and Zhang, 2008). KI is an effective indictor of OM type. The OM
228
types (I, II1, II2, and III) can be identified using the KI ranges >>80-100, 80-40, 40-0 and <<0 to -100, 28
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respectively (Huang et al., 1984; Lu and Zhang, 2008). The KI values for the Da’anzhai shale vary from
230
2 to 15 with an average of 8.7, which indicates that the OM is chiefly sapropelic-humic type II2 and has
231
significant potential to generate hydrocarbons. The mean values (and ranges) of the organic elements C,
232
H, and O make up 46.59% (31.12-61.41%), 3.53% (2.09-4.66%) and 1.56% (0.35-1.88%) of the shale’s
233
OM, respectively. The high H/C ratios (0.79-1.05, average 0.90) and low O/C ratios (0.10-0.30, average
234
0.17) (Fig. 9a) indicate that the OM is type II and has relatively high hydrocarbon potential (Huang et al.,
235
1984; Lu and Zhang, 2008). In addition, the Tmax-HI index (Fig. 9b), which is an important evaluation
236
index for the OM type based on Rock-Eval pyrolysis, gives a similar result (Espitaliéet al., 1985; Lu and
237
Zhang, 2008; Misch et al., 2016). Generally, the Da’anzhai shale has similar OM types with other
238
lacustrine and marine shales and the dominant type is of type II (Table 1), which can support the
239
Da’anzhai shale hydrocarbon generation capacity.
240
The TOC values of the Da’anzhai shale vary from 0.11 to 2.18% (average 0.97%), these values are
241
generally lower than those measured in the world's major lacustrine and marine shales (Table 1 and Fig.
242
9). The Ro values of the Da’anzhai shale are similar or even higher than those of other lacustrine and
243
marine shales, which indicate that the OM is highly mature. This could also be deduced from the Tmax
244
values, which vary from 428 to 500 ℃ with an average of 449 ℃ (Table 6). Table 6 Analysis results for
245
Rock-Eval, Chloroform Bitumen ‘A’ and TOC.
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TE D
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229
29
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1.15
0.50 3.97 0.16 1.03 0.71 2.12 1.09 1.28 0.55 1.85 0.94 0.26 1.32 0.05 1.83
OPI 0.14 0.34 0.12 0.12 0.24 0.07 0.21 0.10 0.11 0.48 0.14 0.29 0.10 0.35 0.04
G10-2 G4-3 J53-1 Li-1 Li-2 P1-1
maceral (%) 13 exinite vitrinite inertinite δ CP DB Ro （%） KI （‰） sporo normal amorphinite resinite sporinite subtotal fusinite pollinite vitrinite 0.73-1.17 27 4 4 26 34 24 15 11 -29.2 (0.95) 1.13-1.65 27 3 2 30 35 18 20 11 -29.5 (1.43) 1.19-1.47 32 4 4 22 30 22 16 15 -29.2 (1.32) 0.78-1.08 28 6 6 19 31 26 15 9 -29.1 (0.95) 0.95-1.25 27 6 2 21 29 26 18 4 -29.5 (1.17) 0.88-1.28 23 5 6 22 33 26 18 2 -29.2 (1.02) sapropelinite
AC C 249 250
HI (mg/g) 60.60 298.77 40.65 117.53 49.69 142.48 93.29 119.80 79.33 116.50 61.96 43.18 112.14 97.82 159.28
PC (%) 0.05 0.50 0.02 0.10 0.08 0.19 0.12 0.12 0.05 0.30 0.09 0.03 0.12 0.01 0.16
D (%) 5.95 37.51 3.98 11.13 5.48 12.79 9.86 11.17 7.48 18.58 6.05 5.18 10.34 14.22 13.91
RI PT
0.08 2.03 0.02 0.14 0.22 0.17 0.29 0.15 0.07 1.70 0.16 0.11 0.14 0.03 0.09
S1 +S2 (mg/g) 0.58 6.00 0.18 1.18 0.94 2.29 1.38 1.43 0.62 3.55 1.09 0.38 1.46 0.09 1.92
SC
0.009 0.006 0.007 0.004 0.009 0.009 0.008 0.009 0.006 0.006 0.008 0.006 0.006 0.008 0.009
Tmax (℃) 456 444 461 439 428 443 441 446 445 452 453 448 444 500 439
HCI (mg/g) 11.04 153.11 7.30 16.61 16.33 11.67 25.49 14.82 10.77 107.39 10.88 19.28 12.41 73.45 8.27
Table 7 Results for Ro, macerals, carbon isotopes and organic elements.
code
248
0.24 0.11 0.27 0.01 0.40
S0 (mg/g) S1 (mg/g) S2 (mg/g)
TE D
247
0.02 0.25 0.42 0.40 0.35 0.31 0.16
TOC (%) 0.82 1.33 0.40 0.88 1.43 1.49 1.17 1.07 0.69 1.59 1.51 0.61 1.18
EP
246
G10-1 G10-2 G4-1 G4-2 G4-3 J61-4 Li-1 Li-2 Li-3 P1-1 P1-2 P1-4 S2-2 S2-5 XI44-2
chloroform bitumen A（%） 0.03
M AN U
code
Fig.9. Discrimination diagrams for the OM type.
30
organic elements C (%) H (%) O (%)
H/C
O/C
39.89
3.07
0.75
0.92
0.1
46.31
4.07
1.88
1.05
0.3
49.26
3.87
1.37
0.94
0.2
31.12
2.09
0.35
0.81
0.1
61.41
4.66
1.62
0.91
0.2
51.54
3.39
0.96
0.79
0.1
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251
Terrestrial oil-generation theory (Huang et al., 1984; Lu and Zhang, 2008) suggests the following conclusions regarding the lacustrine shale:
253
medium to very good, even though the values of S1+S2 (genetic potential) are poor to medium (Tables 5
254
and 6). (2) Because of the strong hydrocarbon expulsion of the high maturity OM (Ro>0.5-0.7%), the
255
evaluation criteria for high maturity OM should be reduced. The shale with high Ro values (0.95-1.43%)
256
is highly mature. (3) Lacustrine carbonate can also generate hydrocarbons, which is an important feature
257
but easy to overlook. The petroleum generation threshold of carbonate (TOC: 0.12-0.4%) is lower than
258
that of mudstone (TOC: 0.4-0.5%), which means that carbonates can produce hydrocarbons at lower
259
TOC values. Thus, the low TOC values of the high carbonate shale may not indicate a low hydrocarbon
260
generation capacity. Additionally, carbonate interlayers in shale can play a role similar to that of seal
261
rocks, which are conducive to the preservation of oil and gas. In summary, the results of this study
262
indicate that the Da’anzhai shale has some hydrocarbon generation capacity, which is also supported by
263
the clear fluorescence of the thin sections (Figs. 5B, C).
SC
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Based on microscopic observations of the Da’anzhai and lacustrine shales, the OM occurrence
EP
264
(1) The TOC and chloroform bitumen ‘A’ values are
RI PT
252
forms are generally similar to other lacustrine shales; they are mainly dispergated in shale matrices
266
(Fig.5) (Morad et al., 2010; Zou et al., 2011; He et al., 2012; Luo et al., 2013; Tian et al., 2014; Dang et
267
al., 2015; Xu et al., 2017). However, these lacustrine shales have more amorphous forms than marine
268
shales because of their different water conditions and biodegradation (Huang et al., 1984; Lu and Zhang,
269
2008; Tian et al., 2014; Xu et al., 2017). In addition, lacustrine OM is distributed in concentric belts
270
around the lake basin’s center, and OM abundance increases with lake depth. Thus, OM is generally
AC C
265
31
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more abundant in moderately deep to deep lake facies than in shallow lake facies, which can be
272
supported by the wide TOC range of lacustrine shales (Table 1 and Fig. 9) (Huang et al., 1984; Lu and
273
Zhang, 2008; Tian et al., 2014; Xu et al., 2017).
4.4.2. Factors that influence the OM content
Fig. 10. Correlation plots between TOC and V/Cr, V/Sc, Ni/Co and V/(V+Ni) values.
EP
276
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275
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274
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271
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277 278
Fig. 11. Correlation plots between TOC values and minerals.
279
TOC is commonly used to represent OM. As discussed in Section 4.1, the reducibility is positively correlated with the V/Cr, V/(V+Ni), and V/Sc ratios and negatively with the Ni/Co ratio. In addition,
281
pyrite is an important mineral in organic-rich sediments and is useful for reconstructing
282
paleoenvironmental conditions. A high pyrite content reflects a stable, high-salinity environment with
283
strong reducibility (Leventhal, 1983). The TOC values of the Da’anzhai shale are positively correlated
284
with the V/Cr, V/(V+Ni) and V/Sc ratios (Figs. 10a, b, c) and the pyrite content (Fig. 11a) and correlated
285
negatively with the Ni/Co ratio (Fig. 10d). These values indicate that paleo-redox conditions had a
286
significant effect on the formation and preservation of OM and that the strongly reducing conditions
287
were conducive to the preservation of OM (Zeng et al., 2015; Chen et al., 2016; Xu et al., 2017).
288 289
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280
Similar to other lacustrine shales, the TOC values of the Da’anzhai shale are positively correlated with the clay mineral content (Fig. 11b), which is due to the OM’s strong sorption ability of clay and its 33
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relatively high content for the lacustrine shales (Huang et al., 1984; Gingele et al., 2001; Dang et al.,
291
2015; Xu et al. 2017). The lacustrine clay minerals are small and rich in intraparticle and interparticle
292
pores, which creates storage space for OM (Figs.5D, F, G and J). The OM observed within the clay
293
minerals primarily has complex, dispergated and residual biological shapes, which indicate that the OM
294
and clay minerals are in close contact (Figs. 5G and J) (Huang et al., 1984; Lu and Zhang, 2008; Tian et
295
al., 2014; Xu et al., 2017). Conversely, the carbonate content is negatively associated with OM (Fig.
296
11c), which is similar to some lacustrine shales (Huang et al., 1984; Jarvis et al., 2001; Tian et al., 2014;
297
Wang et al., 2016). Carbonate minerals fill the pores via cementation, compaction, pressure-solution,
298
recrystallization and replacement , which effectively reduces the storage space for OM (Figs. 5E, H, K)
299
(Huang et al., 1984; Wang et al., 2016).. Furthermore, the high carbonate mineral content in the
300
lacustrine facies reflects relatively shallow water environments, such as the shore-shallow lake face and
301
shallow lacustrine facies, which are not favorable for the generation or storage of OM (Huang et al.,
302
1984; Wang et al., 2016; Xu et al., 2017). There is a weak correlation between the quartz content and
303
TOC values (Fig. 11d). The correlations between quartz and OM are not uniform; differences in the
304
correlation between quartz and TOC contents is related to the depositional environment of shale
305
reservoirs (Zeng et al., 2014; Liu et al., 2015; Ross and Bustin, 2007; Chalmers et al., 2012b; Tian et al.,
306
2013). Negative correlations are caused by the terrestrial quartz inputs (Zeng et al., 2014; Liu et al.,
307
2015), while the positive correlations are related to biogenic quartz (Ross and Bustin, 2007; Chalmers et
308
al., 2012b; Tian et al., 2013). Therefore, the quartz in the lacustrine shales may have both related to
309
biogenic and terrestrial origins. This could lead to the weak correlations between quartz and OM.
AC C
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290
34
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310 311
Based on the analyses in Figs. 10 and 11, the low TOC values of the Da’anzhai lacustrine shales were primarily caused by the oxygen paleoenvironment and carbonate minerals. The relatively shallow
313
and turbulent lake water formed a high oxygen paleoenvironment with greater inputs of terrestrial
314
nutrients, which was conducive to the breeding of shell organisms and the formation of carbonate
315
minerals but not to the preservation of OM. In summary, the oxygen paleoenvironment and high
316
carbonate mineral content are primarily related to the unique lacustrine facies, which was not conducive
317
to the formation of OM.
320
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4.5. Relationships between the mineralogy, reservoirs, OM and paleoenvironmental conditions of lacustrine shales
The mineral content of lacustrine shales is mainly related to the paleoenvironment. Strong
TE D
318
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312
paleo-weathering can lead to a greater number of weathering products, which is favorable to the
322
development of terrestrial minerals in lacustrine facies. Humid (rainy) paleoclimates with more water
323
can improve river transportation and reduce lake salinity, and low salinity paleoenvironments inhibit the
324
formation of authigenic carbonate. In addition, fine-grained terrestrial minerals, especially clay minerals,
325
inhibit the formation of authigenic carbonate (Leventhal, 1983; Huang et al., 1984; Bohacs et al., 2000;
326
Chi et al., 2003; Fu et al., 2015). High hydrodynamic conditions can effectively remove fine-grained
327
terrestrial minerals, which favors the formation of authigenic carbonate.
328 329
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321
The lacustrine sedimentary environment is complex and the lacustrine shale formation is easily influenced by the paleoenvironment. The two most important differences are as follows: first, the
35
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lacustrine paleoenvironments are complex and varied with the short detention time and strong variation
331
of water level (Huang et al., 1984; Chen et al., 2015). (Figs. 1 and 5C, Tables 2 and 6). Second, lakes are
332
close to the terrestrial sources with less water, which lead to rich terrestrial inputs with efficient impact
333
on shale (Figs. 1 and 5A,B, Tables 3 and 6) (Huang et al., 1984; Chen et al., 2015) (Huang et al., 1984;
334
Bohacs et al., 2000; Curtis, 2002; Schmoker, 2002). Lakes are generally located near paleo-uplift
335
environments with large topographical differences, and terrestrial debris can be transported directly into
336
a lake basin over short distances. These features result in rich, proximal and variable sources for
337
lacustrine facies. (Figs. 1, 5A,5B; Table 3). The above two characteristics can effectively lead to the
338
complex and changeable mineral contents of lacustrine shales (Table 3; Fig. 4). In addition, there are
339
some other common points of the lacustrine shales including relatively small thickness of monolayer,
340
more siliceous or calcareous intercalations, and poor comparability with strong heterogeneity (Table 1),
341
which are also the results of the two characteristics of lacustrine sedimentation environments (Tian et al.,
342
2014; Wang et al., 2015a; Zhu et al., 2016; Xu et al., 2017).
SC
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Similar to other lacustrine shales, the storage space of the Da’anzhai lacustrine shale is directly
EP
343
RI PT
330
controlled by minerals (Fig. 12). Because of the large amount of terrestrial clastic minerals, the
345
interparticle pores of lacustrine shales are well-developed. Developed interparticle pores and intraparicle
346
pores can guarantee a considerable amount of storage space. In addition, numerous siliceous or
347
calcareous interlayers in lacustrine shale formations can result from varying changeable
348
paleoenvironments with rich terrestrial inputs. These interlayers can increase the brittleness index and
349
facilitate the development of cracks.
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344
36
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Compared to other shales, the Da’anzhai lacustrine shale has a relatively low OM content (Table 6), which is mainly the result of more variable paleoenvironments with high oxygen content (Fig. 12)
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(Huang et al., 1984; Lu and Zhang, 2008; Tian et al., 2014; Xu et al., 2017). Different minerals have
353
different effects on storage spaces of OM, and minerals in lacustrine shales are mainly controlled by
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paleoenvironments (Fig. 12). In addition, the relatively low OM content of the Da’anzhai lacustrine
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shale is the reason for OM weak effects on storage spaces (Fig. 8a).
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In summary, minerals, reservoirs, and OM in lacustrine shales are closely associated with each
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other, and they were all controlled by the paleoenvironment which was the dominant factor and the link
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between these parameters (Fig.
12).
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Fig.12 Relationships between mineralogy, reservoirs, OM and paleoenvironment for lacustrine shale
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4. Conclusions
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1. There are some similarities between the Da’anzhai lacustrine shale and other lacustrine shales. (1)
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The two most important differences are that lacustrine shales formed in more complex and variable
366
paleoenvironments and have more abundant terrestrial inputs than marine shales. (2) The mineral
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compositions of lacustrine shales are variable and complex, and they have abundant terrestrial minerals. 37
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In addition, lacustrine shales have numerous siliceous or calcareous interlayers. (3) The interparticle
369
pores and interlayer pores of lacustrine shales are well-developed. Unlike other lacustrine shales, the
370
Da’anzhai shale has relatively weak hysteresis loops and wide overlapping ranges of adsorption and
371
desorption isotherms, indicating a greater number of dead-end pores with more complex structures. In
372
addition, the Da’anzhai shale has lower TOC values with more amorphous OM forms.
2. The minerals, reservoirs, and OM for lacustrine shales are closely associated with each other;
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these parameters are all controlled by the paleoenvironmental conditions in which the shales formed. (1)
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Highly variable lacustrine paleoenvironment lead to variable mineral contents and abundant terrestrial
376
inputs give rise to a high terrestrial mineral content in lacustrine shales, which can also create a higher
377
number of siliceous or calcareous intercalations relative to marine shales. (2) Terrestrial minerals (e.g.,
378
clay and quartz) can effectively improve the storage spaces, but the authigenic carbonate minerals (e.g.,
379
calcite) have the opposite effect. Well-developed interparticle pores in lacustrine shales are caused by
380
abundant particles. (3) Clay content has a positive effect on the formation and preservation of OM, while
381
higher oxygenation and carbonate contents have negative effects. Lakes generally have relatively
382
shallow water and a variable paleoenvironment, which leads to a higher oxygen content and reduces the
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OM abundance of lacustrine shales. The weak influence of OM on nanoscale storage spaces is due to the
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low TOC values of the lacustrine shales.
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3. Compared to other lacustrine shales, the Da’anzhai shale also demonstrates the potential for
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unconventional oil and gas exploration. (1) The lacustrine shale has well-developed storage spaces due
387
to the high number of interparticle pores, intraparticle pores, and fractures. Although the lacustrine shale
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has more dead-end pores and larger pores with more complex microstructures, the types of isotherms
389
(Type П) and hysteresis loops (Type H3) are generally similar to those of lacustrine shales. The values of
390
VBJH (1.19-4.10 m3/100 g, average 2.97 m3/100 g) and SBET (1.75-10.69 m2/g, average 6.92 m2/g) are
391
similar to other shales, indicating a high amount of storage space in nanoscale reservoirs. (2) Abundant
392
terrestrial minerals in lacustrine facies can support a greater number of interparticle pores and
393
intraparticle pores. Numerous siliceous or calcareous interlayers in lacustrine shales can also improve
394
the brittleness index, facilitating the development of fractures and creating a seal for oil and gas. (3)
395
Although the TOC values of the Da’anzhai shale are lower than other lacustrine shales, the OM still has
396
several advantages according to the evaluation criteria for continental source rocks, including moderate
397
values of chloroform bitumen ‘A’ and TOC, a high level of OM maturity and a favorable kerogen type
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(II).
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Acknowledgments
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Our study is supported by the Institute Program of PetroChina Research Institute of Petroleum
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Exploration and Development (Grant 2016yj01), and the National Natural Science Foundation of China
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(Grants 41272137 and 41572117). The authors also sincerely appreciate the support from the China
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Scholarship Council (CSC).
404
Nomenclature
405
TOC
406
Ro
Vitrinite reflectance
407
Tmax
Pyrolysis Temperature at Maximum Hydrocarbon Generation
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Total organic carbon, %
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SBET
Brunauer-Emmett-Teller specific surface area
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VBJH
Barrett-Joyner-Halenda total pore volume
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DA
Average pore diameter
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OM
Organic matter
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TE
Trace element
413
XRD
X-ray diffraction
414
FE-SEM
Field emission scanning electron microscopy
415
LTNA
Low temperature N2 adsorption
416
EDS
Energy dispersive spectrometry
417
S0
Gaseous hydrocarbons, mg/g
418
S1
Free hydrocarbons, mg/g
419
S2
Pyrolysis of hydrocarbons, mg/g
420
ICP-MS
Inductively coupled plasma-mass spectrometry
421
REE
Rare earth element, ppm
422
ΣHREE
Total content of heavy rare earth elements
423
ΣLREE
Total content of light rare earth elements
424
PSD
Pore size distribution
425
OPI
426
HI
Hydrogen index; HI=(S2/TOC)×100%, mg/g
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PC
Effective carbon content; PC=0.083×(S0+S1+S2), %
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Oil production index; OPI=S1/(S0+S1+S2)
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D
Degradation rate; D=(PC/TOC) ×100%, %
429
HCI
Hydrocarbon index; HCI=(S0+S1)/ TOC×100, mg/g
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KI
Kerogen type index; KI=Sapropelinite (%)×1+Exinite (%)×0.5+Vitrinite
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(%)×(-0.75)+Inertinite (%)×(-1) (Huang et al., 1984; Lu and Zhang, 2008)
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Geological and geochemical characterization of lacustrine shale: a case study of the Jurassic Da'anzhai Member shale in the central Sichuan Basin, Southwest China
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Qilu Xua,b,c, Bo Liuc,*, Yongsheng Maa, Xinmin Songd, Yongjun Wangd, Zhangxin Chenb a
School of Earth Sciences and Resources, China University of Geosciences(Beijing), Beijing 100083, China
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b
Chemical and Petroleum Engineering, University of Calgary, Calgary, Alberta T2N1N4, Canada
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c
Oil and Gas Research Center, Peking University, Beijing 100871, China
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d
PetroChina Research Institute of Petroleum Exploration and Development, Beijing 100083, China
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Abstract：
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Although numerous progresses have been achieved in characterizing marine shales, studies that are associated with lacustrine shales are limited.
To study lacustrine shales, samples were collected from
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the Jurassic Da'anzhai Member in the central Sichuan Basin of China, and their mineralogical, reservoir,
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OM, and paleoenvironmental characteristics were determined, as well as the relationships between them.
615
Analysis of trace elements reveals that the shales formed in paleoenvironments that were oxic to suboxic,
616
dry to humid, had moderate to strong weathering, and were characterized by fresh to salt water
617
conditions. These environments are more variable than those of marine shales. The paleoenvironmental
618
conditions and mineralogy of the shales, particularly the oxic to suboxic paleo-redox conditions, resulted
619
in the relatively low levels (0.11-2.18%, average 0.97%). However, based on evaluation criteria for
620
continental source rocks, the OM is of high quality because of its high level of maturity (Ro: 0.95-1.43;
621
Tmax: 428-500°C) and favorable kerogen type (II2). There are well-developed intraparticle pores,
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interparticle pores, and microcracks. The SBET (5.42-10.69 m2/g, average 6.92 m2/g) and VBJH (1.19-4.10
623
mL/100 g, average 2.97 mL/100 g) values also indicate good nanoscale storage space. Terrestrial
624
minerals (i.e., quartz and clay) and authigenic carbonate minerals (i.e., calcite) are, respectively,
625
positively and negatively correlated with the nanoscale storage space, .. Small pores (3-5 nm) dominate
626
the nanoscale storage space. The isotherms and hysteresis loops are of Type П and Type H3, respectively,
627
which indicates wedge-shaped pores. However, the hysteresis loops indicate that the lacustrine shale has
628
more dead-end pores and larger pores with more complex microstructures than other lacustrine
629
reservoirs. In general, the Da’anzhai lacustrine shale has the potential for unconventional oil and gas
630
exploration. The lacustrine shale’s mineralogy, reservoirs, and OM are closely related to each other, and
631
their differences are mainly caused by the paleoenvironmental conditions in which the shale formed.
633
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Keywords: Lacustrine shale; Paleoenvironment; Nanoscale reservoir; Organic matter; Minerals; Mechanisms.
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1. Introduction
635
The high commercial production of shale oil and gas in North America has made shale a focus of
EP
634
exploration in many countries and regions. However, the most widely developed shales were formed in
637
marine systems; few studies have focused on the characteristics of lacustrine shales. Additionally,
638
lacustrine shales are widely distributed in many areas, such as Africa, South America, Southeast Asia
639
and China (Huang et al., 1984; Bohacs et al., 2000; Curtis, 2002; Schmoker, 2002; Chen et al., 2015;
640
Jiang et al., 2016a; Yang et al., 2015).
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The most obvious characteristic of lacustrine shales is that they are subject to more variable paleoenvironments than marine shales. The formation environments of lacustrine and marine shales are
643
very different in terms of the water depth, energy, provenance, sedimentary type and biological effects
644
(Huang et al., 1984; Bohacs et al., 2000; Curtis, 2002; Schmoker, 2002). These features lead to
645
differences in their basic characteristics including mineralogy, organic matter (OM), and reservoir type
646
(Table 1). Minerals are the material basis of shales, and they affect not only OM generation but also
647
reservoir development (Huang et al., 1984; Bohacs et al., 2000; Curtis, 2002; Schmoker, 2002; Chen et
648
al., 2015; Yang et al., 2015; Jiang et al., 2016a). A major breakthrough in marine shale oil and gas
649
exploration was based on the recognition that shale acts not only as a source rock but also as a reservoir.
650
Unlike studies of conventional reservoirs, research on unconventional reservoirs has primarily focused
651
on nanoscale reservoirs. Previous studies of marine and lacustrine shales suggest that shale gas is mainly
652
controlled by a nanoscale pore system that is closely related to the mineral characteristics (Zou et al.,
653
2011; Morad et al., 2010; He et al., 2012; Luo et al., 2013; Dang et al., 2015; Chalmers et al., 2012;
654
Curtis et al., 2012; Jiang et al., 2015; Li et al., 2016; Jiang et al., 2017; Zhang et al., 2017). OM is the
655
basic material that generates hydrocarbons, and its type and maturity dominate the formation and
656
enrichment of shale oil and gas. The preservation and enrichment of OM in sediments is controlled by
657
many factors, including the primary productivity, paleo-redox conditions, nutrient availability, clastic
658
influx and minerals (Huang et al., 1984; Bohacs et al., 2000; Curtis, 2002; Schmoker, 2002; Chen et al.,
659
2015; Yang et al., 2015; Jiang et al., 2016a; Xu et al., 2017). In summary, the paleoenvironment,
660
minerals, reservoir, and OM, which are the basic characteristics of shales, have been the focuses of shale
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oil and gas studies, and these characteristics are often interrelated. Additionally, studies of the
662
characteristics and mechanisms have mainly focused on marine shales; fewer studies have examined
663
lacustrine shales.The lacustrine shale of the Jurassic Da’anzhai Member from the central Sichuan Basin
664
of China has been interpreted as fractured reservoirs, low permeability fractured reservoirs and tight
665
reservoirs. However, most studies have only focused on the conventional carbonate reservoirs, and the
666
shales have only been considered as source rocks for the carbonate reservoirs (Lu et al., 2014; Chen et
667
al., 2015). Encouragingly, the Da’anzhai shale of the Yuanba Block in the northwestern Sichuan Basin,
668
which has similar geological conditions to those of the central Sichuan Basin, hosts high-yield wells (He
669
et al. 2012; Zhu, et al. 2016). In addition, some lacustrine shales have yielded oil or gas, such as the
670
Triassic Chang 7 and Chang 9 Member in the Ordos Basin, the Paleogene strata (Es3 and Es4) in the
671
Bohai Bay Basin and the Cretaceous strata in the Songliao Basin (Qing-1) (Zou et al., 2011; Luo et al.,
672
2013; Dang et al., 2015; Li et al., 2015; Yang et al., 2015; Jiang et al., 2016b). These successful
673
developments strongly indicate that the Da’anzhai shale in the central Sichuan Basin is worth studying.
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In this study, we assessed the Da’anzhai lacustrine shale’s characteristics. We performed
675
paleoenvironmental evaluation using TE, mineralogical evaluation using XRD analysis, reservoir
676
evaluation using FE-SEMand LTNA analyses, and OM evaluation using analyses of the TOC,
677
chloroform bitumen ‘A’, Rock-Eval, carbon isotopes, kerogen macerals and organic elements. We also
678
compared the Da’anzhai shale to other lacustrine shales which have been successfully developed and
679
assessed the similarities and differences of lacustrine shales. In addition, we summarized the
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relationships between these characteristics and mechanisms affecting them. Finally, we judged the
681
development potential of the Da’anzhai shale.
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Table 1 Comparisons between basic parameters in the Da’anzhai shale and other successfully developed shales lacustrine shale
basin
Sichuan Basin, China
basin type
layer
foreland basin
Jurassic shale with
Chang 9
Es3
Es4
Qing-1
Barnett
Ordos Basin,
Ordos Basin,
Bohai Bay
Bohai Bay
China
China
Basin, China
Basin, China
polycyclic
polycyclic
superimposed
superimposed
downfaulted
downfaulted
basin
basin
basin
basin
basin
Triassic
Triassic
Paleogene
Paleogene
Cretaceous
Carboniferous
shale with
siliceous and
argillaceous
carbonaceous
limestone
shale, limestone,
interlayers
dolomite
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Central
Chang 7
Songliao
Fort Worth Basin,
Basin, China
Texas, US
downfaulted
SC
Da'anzhai
marine shale
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parameters
foreland basin
Ohio Appalachian Basin, Kentucky, US piedmont depression Devonian
shale with
shale with
shale with
shale with
siltstone
siltstone
siltstone
siltstone
interlayers
interlayers
interlayers
interlayers
1900~3200
1500~2500
1600~2600
1500~4200
1500~5200
1000~2500
1980~2590
510~1800
10~70
30~70
10~15
100~500
250~350
70~150
15~60
9~31
(20~80) /54
(37~53) /43
(29~56) /45
(25~80) /36
(30~80) /36
(30~80) /36
35~80
45~60
porosity (%)
0.20~5.92
1~12
permeability
0.002~0.9
TOC (%) Ro (%)
lithological
shells and
combination
carbonaceous interlayers
depth (m)
thickness (%)
minerals (%)
genetic type
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brittle
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effective
interbed of black shale and gray siliceous shale
10
2~8
1.3~9.3
6~12
4~5
4~7
0.01~0.3
(0.3~5.0) /2.97
<0.50
<0.50
<0.15
0.01
<0.1
(0.1~2.2) /1.0
(0.3~36) /8.3
(0.3~11.3) /3.1
(0.5~13.8) /3.5
(0.8~16.7) /3.2
(0.4~4.5) /2.2
2.0~7.0
0.5~4.7
0.9~1.5
0.8~1.2
0.9~1.3
0.4~1.2
0.4~2.0
0.5~1.5
1.1~2.2
0.3~1.3
pyrolysis gas
pyrolysis gas
pyrolysis gas
pyrolysis gas
Ⅰ-Ⅱ1
Ⅰ-Ⅱ
Ⅱ
Ⅱ
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pyrolysis gas
pyrolysis gas
pyrolysis gas
Ⅱ
Ⅰ-Ⅱ1
Ⅰ-Ⅱ1
biogenic and pyrolysis gas
kerogen type
Ⅰ-Ⅱ1
55
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al., 2013; Lu et al., 2014; Chen et al., 2015;
/2.00
0.31~1.24
8.50~9.91
Zou et al.,
Zou et al., 2011; Luo et al., 2013; Yang et al., 2015; Jiang et al., 2016a.
/2.92
2011; Luo et
Zou et al., 2011;
Zou et al.,
al., 2013; Shi et
Luo et al., 2013;
2011; Luo et
al., 2013; Yang
Dang et al.,
al., 2013; Li,
et al., 2015;
2015; Jiang et
2014; Jiang et
Jiang et al.,
al., 2016a
al., 2016a
2016a.
Zou et al.,
56
1.69~2.83
Schmoker, 2002;
Schmoker, 2002;
Bowker et al.,
Bowker et al.,
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references
(0.60~3.70)
2011; Luo et
2007; Jarvie et al.,
2007; Jarvie et al.,
al., 2013; Jiang
2007; Martini et
2007; Martini et
et al., 2016a
al., 2008; Lu et al.,
al., 2008; Lu et al.,
2014
2014.
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2011; Luo et
(1.68~4.25)
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Zou et al.,
0.92~2.91
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(m /t)
0.87~1.98
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3
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gas content
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2. Geological setting
686 687
EP
685
Fig. 1. Study location, geological map (Chen et al., 2015) and simplified stratigraphic column of the Jurassic Da’anzhai
Member in the central Sichuan Basin.
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The study area is located in the central Sichuan Basin, which includes the Nanchong and Suining
688
areas. It is bounded by the Huafu Mountains to the east and the Longquanshan basement faults to the
689
west, and it covers an area of 6×104 km2 (Fig. 1) (Lu et al., 2014; Chen et al., 2015). A lacustrine
690
environment dominated the Sichuan Basin during the Jurassic Period, and the Da’anzhai Member
691
represents the largest and deepest stage of this environment. The sedimentary facies of the Da’anzhai 57
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Member can be divided into shore-shallow lacustrine, shallow lacustrine and semi-deep lacustrine
693
subfacies, which include shell beach, calcareous flat, mud flat, and semi-deep lake microfacies. These
694
facies are distributed in concentric belts around the lake basin’s center in the Yilong-Pingchang areas
695
(Fig. 1) (Lu et al., 2014; Chen et al., 2015). From top to bottom, the Da’anzhai Member in the central
696
Sichuan Basin can be divided into main three sub-members: Da1, Da13 and Da3 (Fig. 1). The thickest
697
Da13 sub-member (30-50 m) is a set of widely distributed dark mudstones with lamellated fractures and
698
rich OM; this sub-member is considered the most important source rock for hydrocarbons. The Da3
699
sub-member deposits (5-14 m), which are initial deposition products, are primarily (mud) shell
700
limestones with dark gray mudstone interlayers. The Da1 sub-member (30-45 m thick) primarily
701
consists of shales and (mud) shell limestones; this sub-member contains the sedimentary products of the
702
period of basin uplift (Lu et al., 2014; Chen et al., 2015).
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3. Samples and experimental methods
704
3.1. Samples
705
Thirty-nine representative core samples were obtained from the Da’anzhai Member shale in the
EP
703
central Sichuan Basin based on drilling, core observations, stratigraphic data, and log responses (Fig. 1
707
and Table 3).
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3.2. Methods
709
All 39 shale samples were analyzed to determine their bulk mineralogy via XRD using a X'Pert Pro
710
MPD device with a working voltage of 40 kV and a current of 40 mA in the Key Laboratory of Orogenic
711
Belts and Crustal Evolution at Peking University. To investigate the clay mineral fraction in detail,
58
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712
textured mounts were measured four times at a goniometer rate of 0.5θ/minute with a registration range
713
from 2 to 42°2θ at the Beijing Research Institute of Uranium Geology. The micromorphology and oiliness of the samples were determined at the Key Laboratory of
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Marine Reservoir Evolution and Hydrocarbon Accumulation of the China University of Geosciences
716
(Beijing) using an Axio Scope A1 pol Fluorescence-Polarization Microscope. An INCA Synergy system
717
at the Key Laboratory of Orogenic Belts and Crustal Evolution at Peking University was used to analyze
718
the microstructure of each sample via FE-SEM and the mineral composition of each sample via EDS.
719
The samples were subjected to argon-ion polishing to obtain higher resolution images.
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LTNA analysis is capable of characterizing nanoscale pores in porous media, and adsorption tests were performed with dried samples (60-80 mesh) on an ASAP2020M device at the College of
722
Chemistry and Chemical Engineering of the China University of Petroleum (Beijing). N2 adsorption
723
isotherms were obtained at -196.15°C within a relative pressure range of 0.005 to 0.998. The BET
724
method for the equivalent specific surface areas and the BJH method for the total pore volumes were
725
applied in the calculations (Brunauer et al., 1938; Barrett et al., 1951).
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721
The OM richness was characterized by measuring the TOC values of 25 samples using a GHM-02
727
analyzer; 10% hydrochloric acid was used to remove carbonates, and the test sensitivity was 10-13 mg/g.
728
Chloroform bitumen ‘A’ was extracted from the crushed shale powders (<0.09 mm), and the weight was
729
calculated at the China University of Petroleum (Beijing). A Rock-Eval 6 instrument was used to
730
perform a pyrolysis analysis, which yielded the temperature of the Tmax, S0, S1, and S2. The OM maturity
731
was determined based on the Ro value, which was obtained using a reflected light microscope under oil
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immersion and calculated as the mean of 10-30 measurements taken for a single sample. Additionally,
733
measurements were taken to determine the isolated kerogen content and identify the organic kerogen
734
macerals, carbon isotope content and organic element content according to the (GB/T) 19144-2010,
735
(SY/T) 5125-2014, (GB/T) 18340.2-2010, (GB/T) 19143-2003 standards, respectively. The kerogen
736
measurements and the rock pyrolysis analysis were performed at the Keyuan Engineering Technology
737
Testing Center.
SC
The TE concentrations of 17 samples were determined using ICP-MS, which was performed at the
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739
Beijing Research Institute of Uranium Geology. The detailed sample processing procedure as well as the
740
analytical precision and accuracy followed the methods described for the (GB/T) 14506.28-2010
741
standard.
4. Results and discussions
743
4.1. Paleoenvironmental properties and differences
744
Lacustrine shales are different from marine shales mainly due to their formation paleoenvironments.
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TE D
742
Lakes are complex and dynamic systems, and the development of lacustrine shales is controlled more
746
significantly by the sedimentary environment (Huang et al., 1984; Bohacs et al., 2000; Curtis, 2002;
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Table 2 Trace elements of the Da’anzhai lacustrine shale. G10-2
G4-3
G6-2
J45-1
J45-2
J53-1
J61-1
J61-2
J61-3
J61-4
Li-1
Li-2
P1-1
S2-1
S2-3
S2-4
X44-1
Th(ppm)
11.70
10.40
5.43
4.32
12.00
10.60
1.72
4.13
7.74
10.20
11.30
9.50
11.60
2.31
10.50
6.21
13.70
Ni/Co
2.68
2.98
3.26
4.10
2.87
2.72
5.49
3.87
3.50
2.51
2.65
2.97
3.07
3.22
2.32
3.37
2.78
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code
1.76
1.58
1.44
1.50
1.64
1.64
2.47
1.56
1.33
1.69
1.60
1.50
1.61
1.77
1.30
1.41
1.45
0.78
0.80
0.63
0.63
0.78
0.78
0.67
0.68
0.70
0.77
0.78
0.74
0.78
0.61
0.68
0.68
0.77
V/Sc
9.78
8.70
6.88
7.73
9.36
8.91
13.81
8.41
7.94
9.03
7.93
7.21
9.22
9.80
8.54
7.14
7.81
Th/U
2.77
4.24
4.45
2.79
4.07
2.95
1.28
3.53
4.63
3.81
4.25
4.63
4.20
2.74
5.25
4.60
4.72
Sr/Cu
3.75
4.60
42.98
77.10
3.50
3.62
58.70
35.54
53.09
8.46
7.84
19.01
3.35
80.54
7.47
24.69
6.05
Sr/Ba
0.29
0.26
3.66
3.08
0.23
0.24
4.77
2.78
3.45
0.52
0.46
1.28
0.18
3.77
0.44
1.84
0.36
118.7
115.3
109.8
131.7
110.3
187.5
141.2
128.9
194.3
112.9
130.6
112.6
3
6
4
1
7
7
6
9
0
9
7
2
7.79
9.03
6.39
6.65
9.34
9.58
8.46
8.67
8.49
8.04
9.44
4.19
9.93
9.86
8.88
7.88
10.03
(La/Yb)N
6.65
7.75
6.57
8.37
6.82
8.04
9.85
9.82
11.14
8.00
7.50
4.35
7.49
10.88
8.62
7.67
9.05
(La/Sm)N
4.66
4.74
3.04
3.32
5.58
5.42
3.19
3.06
2.37
3.88
5.71
2.59
5.90
3.07
4.20
3.90
5.01
(Gd/Yb)N
0.99
1.04
1.52
1.97
0.77
0.97
2.04
2.01
2.92
1.45
0.84
1.41
0.79
2.10
1.35
1.38
1.17
(Ce/Yb)N
4.48
5.38
4.98
5.83
4.54
5.42
7.21
7.33
9.24
5.58
4.97
3.22
5.00
8.49
6.10
5.59
6.15
ΣLREE/ ΣHREE
88.73
M AN U
)
130.97
38.15
TE D
ΣREE(ppm
SC
V/Cr V/(V+Ni)
60.76
153.93
0.92
0.94
0.97
0.95
0.92
0.93
0.94
0.93
0.99
0.92
0.92
0.92
0.93
0.96
0.95
0.97
0.92
0.19
0.18
0.77
1.12
0.17
0.16
0.63
0.56
0.88
0.36
0.27
0.61
0.15
0.72
0.12
0.35
0.23
EP
δCe CSr(‰)
AC C
XN, where N refers to the chondrite-normalized value (Gromet et al., 1984); δCe=CeN/(LaN×PrN)1/2 (Wright et al.,1984; Murray et al.,1990).
61
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Schmoker, 2002). TEs can be used as effective quantitative indicators of the paleoenvironment by
2
comparing their type and abundance with standard values (Wright et al., 1984; Murray et al., 1990;
3
Manning, 1994; Ross et al., 1995; Kimura and Watanabe, 2001; Jarvis et al., 2001; Rimmer, 2004; Jones
4
and Roy et al., 2007).
5 6
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1
Fig. 2. Identification of paleoenvironments using TEs (a-c) and clay (d).
4.1.1. Paleo-redox conditions
8
Due to their responses to changing redox conditions, the ratios of redox-sensitive TEs, such as
AC C
9
EP
7
Ni/Co, V/Cr, V/Sc, Th/U, and δCe, are usually used to evaluate redox conditions. According to Jones
10
and Manning (1994), V/Cr ratios <2 indicate oxic conditions, V/Cr ratios from 2 to 4.25 indicate dysoxic
11
conditions, and V/Cr ratios >4.25 indicate suboxic to anoxic conditions. Kimura and Watanabe (2001)
12
found that the V/Sc ratio is positively related to oxic conditions. Jones and Manning (1994) showed that
13
Ni and Co are enriched in pyrite, and high Ni/Co ratios indicate anoxic conditions, Ni/Co ratios <5 62
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indicate oxic conditions, Ni/Co ratios between 5 and 7 indicate dysoxic conditions, and Ni/Co ratios >7
15
indicate suboxic to anoxic conditions. Similarly, Th/U ratios <1.5 represent anoxic conditions, Th/U
16
ratios between 1.5 and 3 represent dysoxic to suboxic conditions, and Th/U ratios >3 represent oxic
17
conditions (Jones and Manning, 1994). In addition, δCe values >1 (positive anomaly) and <0.95
18
(negative anomaly) suggest anoxic and oxic to suboxic conditions, respectively (Wright et al., 1984;
19
Murray et al., 1990).
SC
For the Da’anzhai shale in the central Sichuan Basin (Table 2), the ranges of the Ni/Co, V/Cr, V/Sc,
M AN U
20
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14
and Th/U ratios are relatively wide, from 2.32-5.49, 1.30-2.47, 6.88-13.81, and 1.28-5.25, respectively.
22
The values of δCe vary from 0.92 to 0.99 (average 0.94), which indicate a weakly negative anomaly
23
(Wright et al., 1984; Murray et al., 1990).Collectively, these proxies reveal that the shale was deposited
24
under variable paleo-redox conditions that were mainly oxic to suboxic (Fig. 2).
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21
4.1.2. Paleo-salinity
26
Sr/Ba values vary directly with the distance to a sea or lake coast; Sr/Ba ratios <1 suggest
EP
25
freshwater conditions, and Sr/Ba ratios >1 suggest salt water conditions (Jarvis et al., 2001; Kimura and
28
Watanabe, 2001). In addition, low Sr concentrations (0.1-0.3‰) indicate a freshwater setting, and high
29
values (0.8-1‰) represent a salt water setting (Jarvis et al., 2001; Kimura and Watanabe, 2001). Th
30
easily dissolves in acidic water and hydrolyzes to oxide or hydroxide sediments under alkaline
31
conditions; therefore, high Th values indicate a salt water environment (Jones and Manning, 1994).
32 33
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27
In the Da’anzhai shale samples (Table 2), the Sr/Ba ratios vary widely (0.18-4.77) with an average of 1.62. The Th content also varies widely (1.72-13.70 ppm) and averages 8.43 ppm, and the Sr 63
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34
concentrations vary from 0.12 to 1.12‰ (average 0.44‰). Generally, these proxies indicate variable
35
paleo-salinity conditions including both fresh and salt water settings (Fig. 2).
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4.1.3. Paleo-weathering
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36
37
39
Fig. 3. Chondrite normalized REE distribution curves for REE minerals. Chondrite values are from Gromet (1984).
When rocks are subjected to strong weathering, heavy rare earth elements (HREEs) dissolve and
TE D
38
migrate much more easily than light rare earth elements (LREEs). Therefore, high ΣLREE/ΣHREE
41
values indicate strong paleo-weathering conditions (Kimoto et al., 2006; Stevens and Quinton, 2008). In
42
addition, the (La/Yb)N, (La/Sm)N, (Gd/Yb)N and (Ce/Yb)N ratios, which are positively related to LREE
43
enrichment, are also positively correlated with paleo-weathering (Table 2) (Wright et al., 1984; Murray
44
et al., 1990; Ross et al., 1995; Roy et al., 2007).
AC C
45
EP
40
The ΣLREE/ΣHREE rates are between 4.19 and 10.03 (average 8.39), which indicate LREE
46
enrichment. The (La/Yb)N ratio is represented by the slope of the chondrite-normalized REE distribution
47
curve (Fig. 3), which indicates the degree of the graph’s inclination (Ross et al., 1995; Roy et al., 2007).
48
The (La/Yb)N values vary from 4.35 to 11.14 (average 8.15), which indicate that the curves are tilted to 64
ACCEPTED MANUSCRIPT
the right and that the samples are rich in LREE-acidic rocks (Fig. 3). The (La/Sm)N ratio, which
50
represents the degree of LREE fractionation, is positively related to the ΣLREE value (Ross et al., 1995;
51
Roy et al., 2007). The (La/Sm)N ratios range from 2.37 to 5.90 (average 4.10), which indicates a P(E)
52
model with abundant LREEs. The (Gd/Yb)N ratio, which indicates the degree of HREE fractionation, is
53
negatively correlated with the ΣHREE value (Ross et al., 1995; Roy et al., 2007). The (Gd/Yb)N values
54
vary from 0.77 to 2.92 (average 1.46), and the (Ce/Yb)N ratios vary from 3.22 to 9.24 (average of 5.85),
55
which indicate that the Da’anzhai shale is depleted in HREEs (Fig. 3). In summary, the REE results
56
(Table 2) indicate that the shale was deposited under moderate weathering conditions with periods of
57
strong weathering.
M AN U
SC
RI PT
49
4.1.4. Paleoclimate
59
∑REE values are closely related to paleoclimate conditions; that is, ∑REE values are higher in
TE D
58
warm and humid climates and lower in cold and dry climates (Wright et al., 1984; Murray et al., 1990;
61
Ross et al., 1995; Roy et al., 2007). The ΣREE values of the Da’anzhai shale vary widely from 38.15 to
62
194.3 µg/g (average 121.59 µg/g) (Table 2). The Sr/Cu ratio is another useful indicator of lacustrine
63
paleoclimate, with high values reflecting humid climates (Reheis, 1990; Chen et al., 2009).The Sr/Cu
64
ratios in the Da’anzhai shale also vary widely from 3.35 to 80.54%. In addition, illite usually forms in
65
dry conditions by the weathering depotassication of feldspar, mica and other aluminosilicate minerals.
66
Conversely, kaolinite forms in humid climates by the eluviation of feldspar, mica, and pyroxene.
67
Therefore, high illite/clay ratios indicate dry, saline waters with high K+ content, and high kaolinite/clay
68
ratios suggest humid climates (Vanderaveroet, 2000; Gingele et al., 2001). The illite/clay ratios of the
AC C
EP
60
65
ACCEPTED MANUSCRIPT
Da’anzhai shale are relatively high (between 26 and 86%, average 52%). However, the kaolinite/clay
70
ratios are relatively low (between 3 and 19%, average 11%) (Fig. 2d). These results indicate a variable
71
climate with both dry and humid periods as well as a drying trend from humid climates.
72
RI PT
69
In general, the TE contents and types t in lacustrine shales are more complex and varied than those in marine shales, which indicates that lacustrine shales tend to experience more variable
74
paleoenvironmental conditions (Huang et al., 1984; Bohacs et al., 2000; Curtis, 2002; Schmoker, 2002).
75
The lacustrine Da’anzhai shale was formed under relatively dry climatic conditions that included both
76
arid and humid stages. In addition, the shale experienced a moderate degree of paleo-weathering with
77
stages of strong weathering, variable oxic to suboxic paleo-redox conditions that were weakly reducing,
78
and a predominantly freshwater setting interspersed with periods of saltwater.
M AN U
4.2. Mineralogy properties and differences
TE D
79
SC
73
As shown in Table 3, the Da’anzhai shale primarily contains clay (42.4%), quartz (27.7%) and calcite
81
(21.1%). The minerals in these lacustrine shales vary widely. Clay comprises the majority of the shale
82
and ranges from 16.3 to 68.4%, while the quartz content ranges from 5.6 to 53.4%. Carbonate minerals,
83
which mainly include calcite, dolomite, and aragonite, range from 3.0 to 46.6% with an average of
84
27.4%. The Da’anzhai shale contains more brittle minerals than the other lacustrine shales and marine
85
shales (Table 1 and Fig. 4). For example, the brittle mineral content of the Da’anzhai shale ranges from
86
24.2 to 80.3%, with an average of 53.7%, indicating medium-high brittleness that is favorable for natural
87
fractures and hydraulic fracturing. Similar to other lacustrine shales, lacustrine shales have more
88
siliceous or calcareous interlayers than marine shales, which can cause the wider ranges of carbonate and
AC C
EP
80
66
ACCEPTED MANUSCRIPT
quartz minerals and improve the brittleness index and fracturing (Figs1 and 5I).Ternary diagrams were
90
constructed to compare the mineral compositions of hot shales that have been successfully exploited
91
including lacustrine shales (Fig. 4a) and marine shales (Fig. 4b). Generally, the comparison shows that
92
the mineral composition of the lacustrine shales is more variable and complex than in marine shales (Fig.
93
4). For example, the Da’anzhai shale has relatively wide ranges of carbonate (1-70%), clay (15-70%)
94
and quartz (5-50%) (Fig. 4a). In addition, the Da’anzhai shale has the wider mineral ranges than other
95
lacustrine shales, especially for the range of carbonate minerals.
M AN U
SC
RI PT
89
EP
97
TE D
96
Fig. 4. Ternary diagrams of mineral compositions. (a) Lacustrine shales: 1=Da’anzhai shale, 2=Jurassic shale in Sichuan
99
Basin (Jiang et al., 2017), 3=Triassic shale in Sichuan Basin(Jiang et al., 2017), 4=ES3 shale (Wang et al., 2015a). (b) Marine
AC C
98
100
shales: 5 Woodford shale (Jarvie et al. 2007 and Han et al. 2013), 6=Barnett shale (Jarvie et al. 2007 and Han et al. 2013),
101
7= Wufeng-Longmaxi shale (Wu et al., 2014), 8=Ohio shale (Jarvie et al. 2007 and Han et al. 2013).
67
ACCEPTED MANUSCRIPT
Table 3 Minerals and TOC results for the Da’anzhai lacustrine shale. feldspar
calcite
dolomite
aragonite
siderite
pyrite
hematite
(%)
(%)
(%)
(%)
(%)
(%)
(%)
(%)
G10-1
2650.8
Da1
44.2
G10-2
2668.4
Da13
22.1
1.1
0.9
2700.7
Da13
37.5
2.0
2.9
2646.6
1
28.1
4.2
22.9
1
G10-3 G10-4
Da
3.0 1.2
6.3 2.6
1.2
17.0
2385.3
Da
28.2
G4-2
2395.8
Da1
35.2
2.1
6.4
12.5
G4-3
2405.7
Da13
25.8
2.5
4.6
0.7
2537.1
Da13
21.4
2.6
2580.1
3
G6-2
Da
7.4
0.82
24.2
1.33
55.0
2.9
40.4
0.81
26.6
41.1
69.2
54.9
4.5
38.4
1.6
64.8
3.5
60.9
13.4
41.6
0.40
18.9
54.1
0.98
5.3
31.1
1.43
4.2
7.4
28.8
1.18
0.9
36.3
0.4
0.7
20.5
1.1
64.2
76.7
0.4
1.1
64.0
1.1
3.9
27.3
3.1
55.5
5.4
0.2
22.6
1.0
59.3
76.2
1.0
0.8
26.7
1.1
64.8
70.4
0.0
27.7
0.9
57.0
71.4
1.3
4.2
33.1
2.3
24.3
59.1
1.49
2.1
20.0
69.4
77.9
0.30
46.2
26.9
52.0
9.8
40.2
1.17
38.5
35.2
59.4
1.07
53.9
3.1
43.8
0.69
6.4
67.8
1.6
24.9
1.59
4.1
0.9
60.2
4.1
38.9
1.51
3.6
51.2
14.9
45.2
1.55
1.5
16.3
57.1
80.3
0.61
3.9
2827.3
Da13
31.9
4.1
2662.6
1
16.9
14.4
1
Da Da
5.6
55.3
1
44.9 9.5
J61-3
2667.6
Da
14.4
14.3
J61-4
2698.1
Da13
34.8
17.5
2706.8
Da13
8.5
69.4
2665.9
1
25.1
1.8
15.6
1
30.4
2.6
9.8
1.0
28.9
6.3
2.3
3.1
0.0
0.9
0.6
Da
1
1726.1
Da
24.2
Li-3
1734.3
Da13
40.7
3215.9
Da13
23.3
3221.2
Da13
34.8
3239.2
Da13
30.3
14.9
3258.6
3
23.2
51.8
Da
42.7 6.8
EP
Da
Li-2
P1-4
47.2
2.1
75.8
27.9
P1-3
3.0
68.4
67.1
6.1
P1-2
52.8
1.0
0.6
P1-1
(%)
20.1
12.5
1713.8
minerals
0.8
23.4
Li-1
(%)
1.1
Da
J61-6
(%)
(%)
19.5
Da13
J61-5
TOC
47.6
2656.0
2664.4
brittleness
1.2
2632.4
J61-2
carbonate
8.7
J45-2
J61-1
others
clay
1
J45-1
J53-1
3.5
TE D
G6-1
13.4
M AN U
G4-1
total
RI PT
(m)
sub-member
quartz
SC
code
depth
AC C
102
1.0
10.3 3.2
53.3 1.1
1.0
5.3
68
0.7
1.9
31.9
2.18
ACCEPTED MANUSCRIPT
2836.1
Da1
19.8
2849.9
1
Da
40.3
2867.1
Da13
17.6
47.7
1.3
S2-4
2889.0
3
Da
18.9
48.9
1.5
S2-5
2891.6
Da3
53.4
X20-1
1844.4
Da13
49.4
1952.2
1
Da
24.9
2089.4
Da13
35.9
X44-2
2093.0
Da13
27.9
3.0
4.0
X44-3
2116.7
Da13
23.8
3.0
43.2
LQ-1
3548.1
Da13
44.1
2.9
18.7
1780.7
Da13
40.5
2.6
2.5
2065.7
1
28.2
2.2
18.5
7.8
43.3
1
23.9
1.9
35.2
11.4
26.6
X8-1 W8-1 W8-2
2069.5
Da Da
29.6
0.9
42.5
50.4
70.2
0.11
10.7
51.0
1.18
33.4
49.0
66.6
0.33
30.7
50.4
69.3
0.18
2.0
8.1 1.4
2.3
25.6
3.1
8.5
RI PT
46.6
3.8
3.7
SC
X44-1
0.2
9.5
2.5
M AN U
X28-1
10.7
TE D
S2-3
3.6
2.2
EP
S2-2
48.2
AC C
S2-1
69
53.4
38.7
1.5
8.1
57.5
44.3
0.7
28.7
53.6
8.5
44.4
0.53 1.15
51.8
60.8
4.0
31.9
20.5
52.7
76.5
34.3
18.7
62.8
2.5
43.0
26.3
54.5
46.6
70.5
50.8
0.6
1.1
1.0
0.76
0.84
ACCEPTED MANUSCRIPT
4.3. Characterization and controlling factors of lacustrine shale reservoirs
104
4.3.1. Microscopic characteristics
105
AC C
EP
TE D
M AN U
SC
RI PT
103
106
Fig.5. Typical reservoirs images of lacustrine shale. A: Shells were stained using alizarin red to highlight the carbonate
107
minerals; the shells are well developed and preserved in terrestrial inputs and matrices. B: Greenish-yellow fluorescence.
108
Matrices with large amounts of terrestrial inputs between the shell grains have strong fluorescence intensity, indicating
70
ACCEPTED MANUSCRIPT
effective reservoirs with good oiliness in the matrices. C: Lacustrine shale core sample with well-developed calcareous
110
interlayers, indicating a changeable sedimentary environment. D: Quartz particles with well-developed interparticle pores
111
filled with clay minerals; quartz and clay minerals have well-developed intraparticle pores. E: Quartz particles have
112
well-developed intraparticle pores and interparticle pores; calcite minerals fill the storage spaces between quartz particles
113
(calcite cementation). F: Intraparticle pores located along the cleavage planes of clay particles; interparticle pores between
114
clay and carbonate minerals. G: OM with well-developed pores is accompanied by clay minerals. H: Calcite minerals fill the
115
spaces between particles (calcite cementation); quartz has well-developed intraparticle pores. I: Brittle minerals with
116
well-developed micro-cracks that are mainly filled by clay minerals. J: Model of different types of pores. K: Model of
117
carbonate mineral (e.g., calcite) diagenesis that results in reduced storage spaces.
SC
M AN U
118
RI PT
109
According to the International Union of Pure and Applied Chemistry (IUPAC) classification scheme, the pore types in unconventional reservoirs are divided by pore size into micropores, mesopores,
120
and macropores (<2 nm, 2-50 nm, and >50 nm, respectively) (IUPAC, 1994; Thommes et al., 2015).
121
Thin section observations of the shale indicate that the matrices fluoresce intensely (Fig. 5B). Nanoscale
122
reservoirs were examined using FE-SEM with argon-ion milling technology, and the minerals were
123
identified using EDS. Similar to other lacustrine shales, the pores and fractures in the Da’anzhai shale
124
are well developed and fall into four categories.
EP
AC C
125
TE D
119
(1) Interparticle pores: these pores represent the largest number of pores, and they have good
126
connectivity and form effective pore networks. The diameters generally range between 5 nm and 90 µm.
127
Many types of interparticle pores were observed, including pores between grains (Figs. 5D,E,H,J),
128
crystals and clay platelets (Figs. 5D,F,G,J), and pores at the edges of rigid grains (Figs. 5E,J).
71
ACCEPTED MANUSCRIPT
129
(2) Intraparticle pores: these pores primarily formed through diagenesis, and some are primary. The diameters mainly range from 2 nm to 10 µm. The primary sub-types in the Da’anzhai shale are
131
intraplatelet pores within clay aggregates (Figs. 5D,F,J), intercrystalline pores within pyrite framboids,
132
pores within fossil bodies and moldic pores after fossils (Figs. 5D,J).
133
RI PT
130
(3) Fracture pores: these pores are not controlled by individual particles and can be regarded as a type of pore that formed by rock deformation because of tectonic movements, sedimentation, diagenesis,
135
OM hydrocarbon generation and other geological effects (Figs. 5I,J).
M AN U
136
SC
134
(4) OM pores: these pores are found within OM (Fig. 5L) and are usually irregular, bubble-like, and oval. They generally range in diameter from 2 to 500 nm. Although most OM pores are incorrectly
138
regarded as isolated pores in two-dimensional planes, they are interconnected in three-dimensional space
139
and facilitate the development of reservoirs.
140
TE D
137
Similar to other lacustrine and marine shale reservoirs, the Da’anzhai shale commonly contains well-developed pores and fractures, particularly intraparticle and interparticle pores (Curtis et al., 2002;
142
Chalmers et al., 2012; Milliken et al., 2013; Nie et al., 2015; Wang et al., 2015a; Zhou and Kang, 2016;
143
Jiang et al., 2017; Zhang et al., 2017). Especially, due to their rich terrestrial clastic mineral content, the
144
interparticle pores of lacustrine shales are better developed than those of marine shales. Most of the
145
intraparticle pores are nanoscale pores (<200 nm) (Figs. 5D,E,F) and the OM also contains nanopores
146
(Fig.5G). Microcracks can develop due to the high brittle mineral content for the Da’anzhai shale.
147
However, these cracks are often filled by calcite and clay minerals due to the high clay content (42.4%)
148
(Figs. 5I,J).
AC C
EP
141
72
ACCEPTED MANUSCRIPT
4.3.2. Nanoscale reservoirs characterized by LTNA
150
The LTNA test is the most widely used method of characterizing several parameters of nanoscale
151
pores in porous media (micropores and mesopores), including VBJH, SBET, DA, ad/desorption isotherms
152
and PSD (Sing et al., 1985; IUPAC, 1994; Thommes et al., 2015; Wang et al., 2015c; Zhou et al., 2016;
153
Zhang et al., 2017).
154
Table 4 Comparisons of primary pore structure parameters measured by LTNA between the Da’anzhai shale and other
155
lacustrine shales
4
5
6
7
156 157
SC
Sichuan
Jurassic Da'anzhai
1.75-10.69
Basin
Member
(6.92)
M AN U
3
SBET (m2/g)
VBJH 3
(cm /100g)
D (nm)
1.19-4.10
7.21-24.71
(2.97)
(14.11)
0.02-0.90
6.67-18.40
(0.49)
(11.23)
0.22-2.97
4.91-13.93
Upper Triassic
0.25-4.39
Chang 7 Member
(2.62)
Sichuan
Xujiahe
0.62-14.70
Basin
Formation
(7.76)
(1.41)
(6.97)
1.46-5.24
0.72-1.40
9.40-25.00
Ordos Basin
Ordos Basin
TE D
2
strata
Upper Triassic
Chang 7 Member
(2.94)
(1.01)
(16.40)
Bohai Bay
Shahejie
0.40-12.45
0.08-1.60
4.58-41.36
Basin
Formation
(3.62)
(0.62)
(16.84)
0.39-34.06
0.13-4.18
3.16-16.48
(10.92)
(1.59)
(9.56)
Upper Triassic
1.10-1.90
0.69-1.09
15.4-23.4
Chang 7 Member
(1.57)
(0.83)
(19.27)
Songliao Basin
Upper Cretaceous
EP
1
location
Qingshankou Formation
AC C
NO.
RI PT
149
Ordos Basin
73
hysteresis
loops types
references
H3
H3 H3 H3 H3,H4
H2,H3
H3,H4
Jiang et al., 2016b Peng et al., 2016 Fu et al., 2015 Zhang et al., 2016 Wang et al., 2015b Yang et al., 2017
RI PT
ACCEPTED MANUSCRIPT
158
160
Fig. 6. PSD curves obtained from the low temperature N2 adsorption isotherms with BJH pore sizes.
SC
159
The VBJH value of the Da’anzhai shale (2.97 cm3/100 g) is slightly higher than the values in other shales, and the SBET value (6.92 m2/g) is nearly equal to those of the other shales (Table 4). These values
162
indicate that the Da’anzhai shale can also have significant nanoscale storage space.
M AN U
161
To investigate the distribution and contributions of each pore type, the VBJH with respect to the PSD
164
was determined from the adsorption branches of the BJH model (Wang et al., 2015b; Jiang et al., 2016b;
165
Zhou et al., 2016; Zhang et al., 2017). The PSD plots show that pores with diameters between 3 and 5
166
nm make up the largest proportion of the total pore volume and that the proportions decrease with
167
increasing pore size, which is similar to some lacustrine shales (Fig. 6) (Jiang et al., 2016b).
EP AC C
168
TE D
163
74
169 170
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 7. Comparisons between the nanopore structures of the Da’anzhai shale and other lacustrine shales. A, B, C: the
Da’anzhai shale. D: Lacustrine shale of the Chang 7 Member in the Ordos Basin, China (Fu et al., 2015). E: Lacustrine shale
172
of the Songliao Basin, China (Wang et al., 2015b). F: Lacustrine shale of the Yanchang Formation in the Ordos Basin, China
173
(Jiang et al., 2016b; Wang et al., 2015c). G: Lacustrine shale of the Shahejie Formation in the Dongying Sag, China (Liu et
174
al., 2017). H: Lacustrine shale of the Shahejie Formation in the Zhanhua Sag, China. I: Lacustrine shale of the 5
175
Member of the Xujiahe Formation of Upper Triassic in Sichuan Basin, China (Peng, 2016).
176
AC C
EP
171
th
LTNA isotherms and hysteresis patterns can be used to investigate the physisorption mechanisms
177
and structural characteristics of shale pores (Sing et al., 1985; IUPAC, 1994; Thommes et al., 2015;
178
Zhou et al., 2016; Zhang et al., 2017). Adsorption isotherms are divided into five types (Types I-VI), and 75
ACCEPTED MANUSCRIPT
hysteresis loops are divided into four types (Types H1-H4) (Sing et al., 1985; IUPAC, 1994; Thommes et
180
al., 2015). Following this classification, the isotherms of the Da’anzhai shale samples are similar and
181
belong to Type П, and the hysteresis loops are characterized by Type H3 hysteresis loops that indicate
182
wedge-shaped pores (Sing et al., 1985; IUPAC, 1994; Thommes et al., 2015). However, compared to
183
other lacustrine shales, the Da’anzhai shale has relatively weak hysteresis loops and wide overlapping
184
ranges of adsorption and desorption isotherms (Fig. 7), which indicate many dead-end pores with highly
185
complex structures (De Boer, 1958; Nie et al., 2015).
M AN U
SC
RI PT
179
186
4.3.3. Factors that influence pore structure
187
SBET and VBJH are the two most basic LTNA parameters, and they can directly indicate the storage spaces of nanoscale reservoirs. Fig. 8a shows that the TOC values are positively correlated with SBET
189
and VBJH. However, the correlation is not strong because of the relatively low TOC values (average
190
0.97%). Fig. 8b shows that the lacustrine clay content is positively correlated with SBET and VBJH; this
191
correlation is mainly caused by the high clay content (42.4%) and well-developed intraparticle pores
192
(e.g., intraplatelet pores within clay aggregates) and interparticle pores (e.g., pores at the edges of
193
particles, crystals and clay platelets) (Figs. 5F, L). The quartz content is also positively correlated with
194
SBET and VBJH (Fig. 8c). Quartz particles in lacustrine shales usually occur within mineral matrices and
195
have well-developed interparticle pores at the edges of other minerals (e.g., clay and carbonate minerals).
196
The quartz particles also have well-developed nanoscale intraparticle pores. Additionally, the quartz
197
particles have no obvious cementation (Fig.5D). Conversely, the carbonate content is negatively
198
correlated with SBET and VBJH (Fig. 8d). Carbonate minerals, particularly calcite, can significantly affect
AC C
EP
TE D
188
76
ACCEPTED MANUSCRIPT
reservoir quality and generally has adverse effects (Wang et al., 2016). Carbonate minerals (e.g., calcite)
200
are easily dissolved and redistributed and can be formed in primary pores by various mechanisms during
201
the diagenesis processes (Figs. 5E, H, K) (Huang et al., 1984; Wang et al., 2016). Based on qualitative
202
observations using SEM, polarized light microscopy and fluorescence microscopy, lacustrine carbonate
203
minerals can effectively reduce the available storage space by diagenesis processes including
204
cementation, compaction, pressure-solution, recrystallization and replacement (Figs. 5E, H, K) (Huang
205
et al., 1984; Wang et al., 2016).
207
AC C
206
EP
TE D
M AN U
SC
RI PT
199
Fig. 8. Correlation plots between nanopore storage space indicators (SBET and VBJH) and values of TOC and minerals.
77
ACCEPTED MANUSCRIPT
4.4. Characterization and controlling factors of OM in lacustrine shale
209
4.4.1. Characterization of the OM content
210
Water conditions play an important role in the formation of source rocks. Lacustrine shales
RI PT
208
primarily form in semi-deep to deep lakes with fresh to low salinity water, whereas marine shales
212
primarily form in reductive, salty, weakly alkaline and low energy water. These features give rise to
213
different source rock characteristics (Huang et al., 1984; Bohacs et al., 2000; Curtis, 2002; Schmoker,
214
2002; Chen et al., 2015; Yang et al., 2015; Jiang et al., 2016a). Due to these differences, we evaluate the
215
lacustrine OM using a terrestrial oil generation theory that was derived from studies of continental
216
petroliferous basins in China and plays a special role in global petroleum theories (Huang et al., 1984;
217
Lu and Zhang, 2008) (Table 5).
218
Table 5 Factors used to evaluate OM in lacustrine source rock (Huang et al., 1984; Lu and Zhang, 2008). lake types freash~brackish
salt water~high salt water
AC C
chloroform bitumen ‘A’ (%)
non-source rock
source rock poor
medium
good
very good
0.4~0.6
>0.6~1.0
>1.0~2.0
>2.0
<0.2
0.2~0.4
>0.4~0.6
>0.6~0.8
>0.8
<0.015
0.015~0.050
<0.050~0.100
>0.100~0.200
>0.200
<2
2~6
>6~20
>20
<0.4
EP
TOC(%)
water
TE D
index
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S1+S2(mg/g)
219
The mean values (and ranges) of the shale’s kerogen macerals consist of 32% exinites (29-35%),
220
27% sapropelinites (23-32%) and 24% vitrinites (18-26%), which indicates that the OM is primarily
221
derived from plankton and bacteria. The δ13CPDB values (-29.1 to -29.5‰, average -29.3‰) support this
222
finding (Table 7) (Huang et al., 1984; Lu and Zhang, 2008). KI is an effective indictor of OM type. The 78
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OM types (I, II1, II2, and III) can be identified using the KI ranges >>80-100, 80-40, 40-0 and <<0 to
224
-100, respectively (Huang et al., 1984; Lu and Zhang, 2008). The KI values for the Da’anzhai shale vary
225
from 2 to 15 with an average of 8.7, which indicates that the OM is chiefly sapropelic-humic type II2
226
and has significant potential to generate hydrocarbons. The mean values (and ranges) of the organic
227
elements C, H, and O make up 46.59% (31.12-61.41%), 3.53% (2.09-4.66%) and 1.56% (0.35-1.88%)
228
of the shale’s OM, respectively. The high H/C ratios (0.79-1.05, average 0.90) and low O/C ratios
229
(0.10-0.30, average 0.17) (Fig. 9a) indicate that the OM is type II and has relatively high hydrocarbon
230
potential (Huang et al., 1984; Lu and Zhang, 2008). In addition, the Tmax-HI index (Fig. 9b), which is an
231
important evaluation index for the OM type based on Rock-Eval pyrolysis, gives a similar result
232
(Espitaliéet al., 1985; Lu and Zhang, 2008; Misch et al., 2016). Generally, the Da’anzhai shale has
233
similar OM types with other lacustrine and marine shales and the dominant type is of type II (Table 1),
234
which can support the Da’anzhai shale hydrocarbon generation capacity.
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The TOC values of the Da’anzhai shale vary from 0.11 to 2.18% (average 0.97%), these values are
236
generally lower than those measured in the world's major lacustrine and marine shales (Table 1 and Fig.
237
9). The Ro values of the Da’anzhai shale are similar or even higher than those of other lacustrine and
238
marine shales, which indicate that the OM is highly mature. This could also be deduced from the Tmax
239
values, which vary from 428 to 500 ℃ with an average of 449 ℃ (Table 6).
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Table 6 Analysis results for Rock-Eval, Chloroform Bitumen ‘A’ and TOC.
code
bitumen A
TOC
Tmax
S1+S2
(℃)
(mg/g)
S0 (mg/g)
S1 (mg/g)
S2 (mg/g)
0.82
0.009
0.08
0.50
456
1.33
0.006
2.03
3.97
444
(%)
（%） G10-1
0.03
G10-2 G4-1
0.02
0.40
0.007
0.02
0.16
461
G4-2
0.25
0.88
0.004
0.14
1.03
439
0.42
1.43
0.009
0.22
0.71
428
0.40
1.49
0.009
0.17
2.12
443
Li-1
0.35
1.17
0.008
0.29
1.09
441
Li-2
0.31
1.07
0.009
0.15
1.28
Li-3
0.16
0.69
0.006
0.07
D
HCI
(mg/g)
(%)
(%)
(mg/g)
0.58
0.14
60.60
0.05
5.95
11.04
6.00
0.34
298.77
0.50
37.51
153.11
0.18
0.12
40.65
0.02
3.98
7.30
1.18
0.12
117.53
0.10
11.13
16.61
0.24
49.69
0.08
5.48
16.33
0.07
142.48
0.19
12.79
11.67
1.38
0.21
93.29
0.12
9.86
25.49
446
1.43
0.10
119.80
0.12
11.17
14.82
0.55
445
0.62
0.11
79.33
0.05
7.48
10.77
0.006
1.70
1.85
452
3.55
0.48
116.50
0.30
18.58
107.39
0.24
1.51
0.008
0.16
0.94
453
1.09
0.14
61.96
0.09
6.05
10.88
P1-4
0.11
0.61
0.006
0.11
0.26
448
0.38
0.29
43.18
0.03
5.18
19.28
S2-2
0.27
1.18
0.006
0.14
1.32
444
1.46
0.10
112.14
0.12
10.34
12.41
S2-5
0.01
0.008
0.03
0.05
500
0.09
0.35
97.82
0.01
14.22
73.45
XI44-2
0.40
0.009
0.09
1.83
439
1.92
0.04
159.28
0.16
13.91
8.27
1.15
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1.59
P1-2
EP
241 242
PC
2.29
AC C
P1-1
HI
0.94
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chloroform
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Table 7 Results for Ro, macerals, carbon isotopes and organic elements. maceral (%)
Li-1
Li-2
P1-1
(1.43) 1.19-1.47 (1.32) 0.78-1.08 (0.95) 0.95-1.25 (1.17) 0.88-1.28 (1.02)
27
4
4
26
34
24
27
3
2
30
35
18
32
4
4
22
30
22
28
6
6
19
31
27
6
2
21
29
23
5
6
22
33
vitrinite
KI
fusinite
15
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1.13-1.65
sporinite
pollinite
subtotal
normal
resinite
（‰）
11
SC
J53-1
(0.95)
sporo
amorphinite
organic elements
δ13CPDB
inertinite
C (%)
H (%)
O (%)
H/C
O/C
-29.2
39.89
3.07
0.75
0.92
0.1
20
11
-29.5
46.31
4.07
1.88
1.05
0.3
16
15
-29.2
49.26
3.87
1.37
0.94
0.2
26
15
9
-29.1
31.12
2.09
0.35
0.81
0.1
26
18
4
-29.5
61.41
4.66
1.62
0.91
0.2
26
18
2
-29.2
51.54
3.39
0.96
0.79
0.1
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0.73-1.17
vitrinite
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G10-2
Ro（%）
exinite
EP
code
sapropelinite
AC C
243 244
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Fig.9. Discrimination diagrams for the OM type.
247
Terrestrial oil-generation theory (Huang et al., 1984; Lu and Zhang, 2008) suggests the following
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conclusions regarding the lacustrine shale: (1) The TOC and chloroform bitumen ‘A’ values are medium
250
to very good, even though the values of S1+S2 (genetic potential) are poor to medium (Tables 5 and 6).
251
(2) Because of the strong hydrocarbon expulsion of the high maturity OM (Ro>0.5-0.7%), the
252
evaluation criteria for high maturity OM should be reduced. The shale with high Ro values (0.95-1.43%)
253
is highly mature. (3) Lacustrine carbonate can also generate hydrocarbons, which is an important feature
254
but easy to overlook. The petroleum generation threshold of carbonate (TOC: 0.12-0.4%) is lower than
255
that of mudstone (TOC: 0.4-0.5%), which means that carbonates can produce hydrocarbons at lower
256
TOC values. Thus, the low TOC values of the high carbonate shale may not indicate a low hydrocarbon
257
generation capacity. Additionally, carbonate interlayers in shale can play a role similar to that of seal
258
rocks, which are conducive to the preservation of oil and gas. In summary, the results of this study
259
indicate that the Da’anzhai shale has some hydrocarbon generation capacity, which is also supported by
260
the clear fluorescence of the thin sections (Figs. 5B, C).
AC C
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Based on microscopic observations of the Da’anzhai and lacustrine shales, the OM occurrence forms are generally similar to other lacustrine shales; they are mainly dispergated in shale matrices
263
(Fig.5) (Morad et al., 2010; Zou et al., 2011; He et al., 2012; Luo et al., 2013; Tian et al., 2014; Dang et
264
al., 2015; Xu et al., 2017). However, these lacustrine shales have more amorphous forms than marine
265
shales because of their different water conditions and biodegradation (Huang et al., 1984; Lu and Zhang,
266
2008; Tian et al., 2014; Xu et al., 2017). In addition, lacustrine OM is distributed in concentric belts
267
around the lake basin’s center, and OM abundance increases with lake depth. Thus, OM is generally
268
more abundant in moderately deep to deep lake facies than in shallow lake facies, which can be
269
supported by the wide TOC range of lacustrine shales (Table 1 and Fig. 9) (Huang et al., 1984; Lu and
270
Zhang, 2008; Tian et al., 2014; Xu et al., 2017).
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Fig. 10. Correlation plots between TOC and V/Cr, V/Sc, Ni/Co and V/(V+Ni) values.
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Fig. 11. Correlation plots between TOC values and minerals.
276
TOC is commonly used to represent OM. As discussed in Section 4.1, the reducibility is positively
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correlated with the V/Cr, V/(V+Ni), and V/Sc ratios and negatively with the Ni/Co ratio. In addition,
278
pyrite is an important mineral in organic-rich sediments and is useful for reconstructing
279
paleoenvironmental conditions. A high pyrite content reflects a stable, high-salinity environment with
280
strong reducibility (Leventhal, 1983). The TOC values of the Da’anzhai shale are positively correlated
281
with the V/Cr, V/(V+Ni) and V/Sc ratios (Figs. 10a, b, c) and the pyrite content (Fig. 11a) and correlated
282
negatively with the Ni/Co ratio (Fig. 10d). These values indicate that paleo-redox conditions had a
283
significant effect on the formation and preservation of OM and that the strongly reducing conditions
284
were conducive to the preservation of OM (Zeng et al., 2015; Chen et al., 2016; Xu et al., 2017).
AC C
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Similar to other lacustrine shales, the TOC values of the Da’anzhai shale are positively correlated with the clay mineral content (Fig. 11b), which is due to the OM’s strong sorption ability of clay and its
287
relatively high content for the lacustrine shales (Huang et al., 1984; Gingele et al., 2001; Dang et al.,
288
2015; Xu et al. 2017). The lacustrine clay minerals are small and rich in intraparticle and interparticle
289
pores, which creates storage space for OM (Figs. 5D,F,G,J). The OM observed within the clay minerals
290
primarily has complex, dispergated and residual biological shapes, which indicate that the OM and clay
291
minerals are in close contact (Figs. 5G and J) (Huang et al., 1984; Lu and Zhang, 2008; Tian et al., 2014;
292
Xu et al., 2017). Conversely, the carbonate content is negatively associated with OM (Fig. 11c), which is
293
similar to some lacustrine shales (Huang et al., 1984; Jarvis et al., 2001; Tian et al., 2014; Wang et al.,
294
2016). Carbonate minerals fill the pores via cementation, compaction, pressure-solution,
295
recrystallization and replacement, which effectively reduces the storage space for OM (Figs. 5E,H,K)
296
(Huang et al., 1984; Wang et al., 2016). Furthermore, the high carbonate mineral content in the
297
lacustrine facies reflects relatively shallow water environments, such as the shore-shallow lake face and
298
shallow lacustrine facies, which are not favorable for the generation or storage of OM (Huang et al.,
299
1984; Wang et al., 2016; Xu et al., 2017). There is a weak correlation between the quartz content and
300
TOC values (Fig. 11d). The correlations between quartz and OM are not uniform; differences in the
301
correlation between quartz and TOC contents are related to the depositional environment of shale
302
reservoirs (Zeng et al., 2014; Liu et al., 2015; Ross and Bustin, 2007; Chalmers et al., 2012b; Tian et al.,
303
2013). Negative correlations are caused by the terrestrial quartz inputs (Zeng et al., 2014; Liu et al.,
304
2015), while the positive correlations are related to biogenic quartz (Ross and Bustin, 2007; Chalmers et
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305
al., 2012b; Tian et al., 2013). Therefore, the quartz in the lacustrine shales may have both related to
306
biogenic and terrestrial origins. This could lead to the weak correlations between quartz and OM. Based on the analyses in Figs. 10 and 11, the low TOC values of the Da’anzhai lacustrine shales
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were primarily caused by the oxygen paleoenvironment and carbonate minerals. The relatively shallow
309
and turbulent lake water formed a high oxygen paleoenvironment with greater inputs of terrestrial
310
nutrients, which was conducive to the breeding of shell organisms and the formation of carbonate
311
minerals but not to the preservation of OM. In summary, the oxygen paleoenvironment and high
312
carbonate mineral content are primarily related to the unique lacustrine facies, which was not conducive
313
to the formation of OM.
316
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4.5. Relationships between the mineralogy, reservoirs, OM and paleoenvironmental conditions of lacustrine shales
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The mineral content of lacustrine shales is mainly related to the paleoenvironment. Strong paleo-weathering can lead to a greater number of weathering products, which is favorable to the
318
development of terrestrial minerals in lacustrine facies. Humid (rainy) paleoclimates with more water
319
can improve river transportation and reduce lake salinity, and low salinity paleoenvironments inhibit the
320
formation of authigenic carbonate. In addition, fine-grained terrestrial minerals, especially clay minerals,
321
inhibit the formation of authigenic carbonate (Leventhal, 1983; Huang et al., 1984; Bohacs et al., 2000;
322
Chi et al., 2003; Fu et al., 2015). High hydrodynamic conditions can effectively remove fine-grained
323
terrestrial minerals, which favors the formation of authigenic carbonate.
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324
The lacustrine sedimentary environment is complex and the lacustrine shale formation is easily influenced by the paleoenvironment. The two most important differences are as follows: first, the
326
lacustrine paleoenvironments are complex and varied with the short detention time and strong variation
327
of water level (Huang et al., 1984; Chen et al., 2015) (Figs. 1 and 5C, Tables 2 and 6). Second, lakes are
328
close to the terrestrial sources with less water, which lead to rich terrestrial inputs with efficient impact
329
on shale (Figs. 1 and 5A,B, Tables 3 and 6) (Huang et al., 1984; Chen et al., 2015) (Huang et al., 1984;
330
Bohacs et al., 2000; Curtis, 2002; Schmoker, 2002). Lakes are generally located near paleo-uplift
331
environments with large topographical differences, and terrestrial debris can be transported directly into
332
a lake basin over short distances. These features result in rich, proximal and variable sources for
333
lacustrine facies. (Figs. 1, 5A, 5B; Table 3). The above two characteristics can effectively lead to the
334
complex and changeable mineral contents of lacustrine shales (Table 3; Fig. 4). In addition, there are
335
some other common points of the lacustrine shales including relatively small thickness of monolayer,
336
more siliceous or calcareous intercalations, and poor comparability with strong heterogeneity (Table 1),
337
which are also the results of the two characteristics of lacustrine sedimentation environments (Tian et al.,
338
2014; Wang et al., 2015a; Zhu et al., 2016; Xu et al., 2017).
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Similar to other lacustrine shales, the storage space of the Da’anzhai lacustrine shale is directly
340
controlled by minerals (Fig. 12). Because of the large amount of terrestrial clastic minerals, the
341
interparticle pores of lacustrine shales are well-developed. Developed interparticle pores and intraparicle
342
pores can guarantee a considerable amount of storage space. In addition, numerous siliceous or
343
calcareous interlayers in lacustrine shale formations can result from varying changeable
87
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344
paleoenvironments with rich terrestrial inputs. These interlayers can increase the brittleness index and
345
facilitate the development of cracks. Compared to other shales, the Da’anzhai lacustrine shale has a relatively low OM content (Table 6),
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which is mainly the result of more variable paleoenvironments with high oxygen content (Fig. 12)
348
(Huang et al., 1984; Lu and Zhang, 2008; Tian et al., 2014; Xu et al., 2017). Different minerals have
349
different effects on storage spaces of OM, and minerals in lacustrine shales are mainly controlled by
350
paleoenvironments (Fig. 12). In addition, the relatively low OM content of the Da’anzhai lacustrine
351
shale is the reason for OM weak effects on storage spaces (Fig. 8a).
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In summary, minerals, reservoirs, and OM in lacustrine shales are closely associated with each
353
other, and they were all controlled by the paleoenvironment which was the dominant factor and the link
354
between these parameters (Fig. 12).
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355
357 358
Fig.12 Relationships between mineralogy, reservoirs, OM and paleoenvironment for lacustrine shale
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4. Conclusions
1. There are some similarities between the Da’anzhai lacustrine shale and other lacustrine shales. (1)
359
The two most important differences are that lacustrine shales formed in more complex and variable
360
paleoenvironments and have more abundant terrestrial inputs than marine shales. (2) The mineral
361
compositions of lacustrine shales are variable and complex, and they have abundant terrestrial minerals. 88
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In addition, lacustrine shales have numerous siliceous or calcareous interlayers. (3) The interparticle
363
pores and interlayer pores of lacustrine shales are well-developed. Unlike other lacustrine shales, the
364
Da’anzhai shale has relatively weak hysteresis loops and wide overlapping ranges of adsorption and
365
desorption isotherms, indicating a greater number of dead-end pores with more complex structures. In
366
addition, the Da’anzhai shale has lower TOC values with more amorphous OM forms.
2. The minerals, reservoirs, and OM for lacustrine shales are closely associated with each other;
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367
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362
these parameters are all controlled by the paleoenvironmental conditions in which the shales formed. (1)
369
Highly variable lacustrine paleoenvironment lead to variable mineral contents and abundant terrestrial
370
inputs give rise to a high terrestrial mineral content in lacustrine shales, which can also create a higher
371
number of siliceous or calcareous intercalations relative to marine shales. (2) Terrestrial minerals (e.g.,
372
clay and quartz) can effectively improve the storage spaces, but the authigenic carbonate minerals (e.g.,
373
calcite) have the opposite effect. Well-developed interparticle pores in lacustrine shales are caused by
374
abundant particles. (3) Clay content has a positive effect on the formation and preservation of OM, while
375
higher oxygenation and carbonate contents have negative effects. Lakes generally have relatively
376
shallow water and a variable paleoenvironment, which leads to a higher oxygen content and reduces the
377
OM abundance of lacustrine shales. The weak influence of OM on nanoscale storage spaces is due to the
378
low TOC values of the lacustrine shales.
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3. Compared to other lacustrine shales, the Da’anzhai shale also demonstrates the potential for
380
unconventional oil and gas exploration. (1) The lacustrine shale has well-developed storage spaces due
381
to the high number of interparticle pores, intraparticle pores, and fractures. Although the lacustrine shale
89
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has more dead-end pores and larger pores with more complex microstructures, the types of isotherms
383
(Type П) and hysteresis loops (Type H3) are generally similar to those of lacustrine shales. The values of
384
VBJH (1.19-4.10 m3/100 g, average 2.97 m3/100 g) and SBET (1.75-10.69 m2/g, average 6.92 m2/g) are
385
similar to other shales, indicating a high amount of storage space in nanoscale reservoirs. (2) Abundant
386
terrestrial minerals in lacustrine facies can support a greater number of interparticle pores and
387
intraparticle pores. Numerous siliceous or calcareous interlayers in lacustrine shales can also improve
388
the brittleness index, facilitating the development of fractures and creating a seal for oil and gas. (3)
389
Although the TOC values of the Da’anzhai shale are lower than other lacustrine shales, the OM still has
390
several advantages according to the evaluation criteria for continental source rocks, including moderate
391
values of chloroform bitumen ‘A’ and TOC, a high level of OM maturity and a favorable kerogen type
392
(II).
393
Acknowledgments
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Our study is supported by the Institute Program of PetroChina Research Institute of Petroleum
395
Exploration and Development (Grant 2016yj01), and the National Natural Science Foundation of China
396
(Grants 41272137 and 41572117). The authors also sincerely appreciate the support from the China
397
Scholarship Council (CSC).
398
Nomenclature
399
TOC
400
Ro
Vitrinite reflectance
401
Tmax
Pyrolysis Temperature at Maximum Hydrocarbon Generation
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Total organic carbon, %
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SBET
Brunauer-Emmett-Teller specific surface area
403
VBJH
Barrett-Joyner-Halenda total pore volume
404
DA
Average pore diameter
405
OM
Organic matter
406
TE
Trace element
407
XRD
X-ray diffraction
408
FE-SEM
Field emission scanning electron microscopy
409
LTNA
Low temperature N2 adsorption
410
EDS
Energy dispersive spectrometry
411
S0
Gaseous hydrocarbons, mg/g
412
S1
Free hydrocarbons, mg/g
413
S2
Pyrolysis of hydrocarbons, mg/g
414
ICP-MS
Inductively coupled plasma-mass spectrometry
415
REE
Rare earth element, ppm
416
ΣHREE
Total content of heavy rare earth elements
417
ΣLREE
Total content of light rare earth elements
418
PSD
Pore size distribution
419
OPI
420
HI
Hydrogen index; HI=(S2/TOC)×100%, mg/g
421
PC
Effective carbon content; PC=0.083×(S0+S1+S2), %
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Oil production index; OPI=S1/(S0+S1+S2)
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D
Degradation rate; D=(PC/TOC) ×100%, %
423
HCI
Hydrocarbon index; HCI=(S0+S1)/ TOC×100, mg/g
424
KI
Kerogen type index; KI=Sapropelinite (%)×1+Exinite (%)×0.5+Vitrinite
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(%)×(-0.75)+Inertinite (%)×(-1) (Huang et al., 1984; Lu and Zhang, 2008)
426
References
427
Barrett, E.P., Joyner, L.G., Halenda, P. P., 1951. The determination of pore volume and area distributions in porous
429
substances. I. Computations from nitrogen isotherms. J. Am. Chem. Soc. 73(1), 373-380.
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Bohacs, K.M., Carroll, A.R., Neal, J.E., Mankiewicz, P.J., 2000.Lake-basin type, source potential, and
430
hydrocarbon character: an integrated sequence-stratigraphic-geochemical framework. Lake basins through
431
space and time: AAPG Studies in Geology 6, 3-34.
435 436
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309-319.
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Brunauer, S., Emmett, P.H., Teller, E.J., 1938. Adsorption of gases in multimolecular layers. Am. Chem. Soc. 60,
Chalmers, G.R., Bustin, R.M., Power, I.M., 2012. Characterization of gas shale pore systems by porosimetry,
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ACCEPTED MANUSCRIPT Highlights
1. The Da'anzhai Member is systematically studied as a shale system for the first time.
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2. The mechanisms of lacustrine shale paleoenvironment and mineral are discussed.
3. The mechanisms of lacustrine shale reservoirs and OM are discussed.
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4. The relationships between paleoenvironment, mineral, reservoir and OM are
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discussed.