Depositional and compositional controls on diagenesis of the mixed siliciclastic-volcaniclastic sandstones: A case study of the Lower Cretaceous in Erennaoer Sag, Erlian Basin, NE China

Journal Pre-proof Depositional and compositional controls on diagenesis of the mixed siliciclasticvolcaniclastic sandstones: A case study of the Lower Cretaceous in Erennaoer Sag, Erlian Basin, NE China Wei Wei, Xiaomin Zhu, Karem Azmy, Shifa Zhu, Mingwei He, Shuyang Sun PII:

S0920-4105(19)31271-9

DOI:

https://doi.org/10.1016/j.petrol.2019.106855

Reference:

PETROL 106855

To appear in:

Journal of Petroleum Science and Engineering

Received Date: 26 July 2019 Revised Date:

3 December 2019

Accepted Date: 21 December 2019

Please cite this article as: Wei, W., Zhu, X., Azmy, K., Zhu, S., He, M., Sun, S., Depositional and compositional controls on diagenesis of the mixed siliciclastic-volcaniclastic sandstones: A case study of the Lower Cretaceous in Erennaoer Sag, Erlian Basin, NE China, Journal of Petroleum Science and Engineering (2020), doi: https://doi.org/10.1016/j.petrol.2019.106855. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier B.V.

CRediT author statement Wei Wei: Conceptualization, Writing- Original draft preparation, Writing- Reviewing and Editing, Funding acquisition. Xiaomin Zhu: Supervision, Conceptualization, Karem Azmy: WritingReviewing and Editing, Shifa Zhu: Conceptualization, Mingwei He: Software, Validation. Shuyang Sun: Software, Validation

Wei et al 1

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Depositional and compositional controls on diagenesis of the

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mixed siliciclastic-volcaniclastic sandstones: a case study of the

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Lower Cretaceous in Erennaoer sag, Erlian basin, NE China

4

Wei Weia,b,c, Xiaomin Zhub,c, Karem Azmyd, Shifa Zhub,c, Mingwei Hee, Shuyang Sunf

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a

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Chinese Academy of Sciences, Beijing 100029, PR China

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b

College of Geosciences, China University of Petroleum, Beijing 102249, PR China

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c

State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum,

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Beijing 102249, PR China

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d

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Canada

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e

CNOOC Research Institute Co. Ltd, Beijing, 100027, PR China

15

f

Virginia Polytechnic Institute and State University, Blacksburg, 24060, USA

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Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics,

Department of Earth Sciences, Memorial University of Newfoundland, St. John’s, NL, A1B 3X5,

Wei et al 2

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Abstract

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A succession of tight-oil mixed siliciclastic-volcaniclastic sandstones (MSVS) deposited in the

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Lower Cretaceous in the Erlian Basin has increasingly attracted attention for its significant

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discovery of oil and gas. The mixed compositions have a significant influence on diagenesis and

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reservoir properties. Understanding the relationship between lithofacies and diagenesis is of great

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value to providing an understanding of hydrocarbon enrichment potential in MSVS. The current

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investigation utilizes a multi-method approach including core analysis, petrography, SEM,

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microthermometry, and stable carbon- and oxygen-isotope geochemistry to better understand the

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mixed sandstones’ lithofacies and controls on diagenesis. Three main sandstone lithofacies are

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composed of non-tuffaceous sandstone deposited in fan delta front (FFD-NTss), non-tuffaceous

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sandstone deposited in pro-fan delta (PFD-NTss), and tuffaceous sandstone deposited in pro-fan

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delta and lake (PFD-Tss). The results suggest that the early diagenetic stage of the MSVS

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includes the formation of chlorite and subhedral micritic-crystalline dolomite-1 cement (DI). The

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late stage includes the precipitation of quartz and calcite and subhedral to euhedral

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microcrystalline dolomite-2 cements (DII). FFD-NTss suffered from strong compaction due to

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abundant silt-sized and clay matrix whereas PFD-NTss was influenced by abundant calcite

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cementation due to weak compaction and more rigid debris. PFD-Tss contains abundant volcanic

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materials, alteration of which not only provided abundant ions but enhanced methanogensis

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during the shallow burial, therefore, leading to formation of early diagenetic DI, analcime, and

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chlorite. Clay minerals are predominant in FFD-NTss whereas the late diagenetic calcite and

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dolomite cements are common in PFD-NTss that suffered from weak compaction. With the

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alteration of volcanic material and clay mineral transformations, the late diagenetic carbonate

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and clay minerals are widely distributed in the PFD-Tss.

Wei et al 3

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Key words: diagenesis; authigenic carbonate; volcaniclastic; Erennaoer sag; Erlian Basin

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1. Introduction

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Recent success in exploration and development of unconventional reservoirs (e.g., tight

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source rock/fine-grained rock) has raised a dramatic increase in research on lithostratigraphic

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features. The mixed siliciclastic-volcaniclastic sediments (MSVS) formed by volcanic debris and

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ash mixing with marine/lacustrine deposits (Kolata et al., 1987; Huff et al., 1992; Königer and

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Lorenz, 2002; Huff 2008; Fisher and Schmincke, 2012), have been found to be unconventional

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reservoirs containing high hydrocarbon potentials (Myrow and Landing, 1992; Manville et al.,

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2009; Riccomini et al., 2016), such as the middle Permian Lucaogou Formation of the Junggar

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Basin in China (Wu et al., 2016) and the Pilmatué Member of the Agrio Formation at Loma La

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Torre area, northern Neuquén Basin, Argentina (Diego and Sebastián., 2019). However, these

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formations were not of interest for petroleum geologists so far and considered only in igneous

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petrographic studies (Friedman and Sanders, 1978; Schmincke, 1982; Allen, 2001). A few

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decades ago, these sediments came in focus of sedimentological studies (Friedman and Sanders,

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1978; Schmincke, 1982; Allen, 2001), but yet little attention was given to the evaluation of

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reservoir quality and diagenesis (Surdam and James, 1979; Hay et al. 1987; De Ros et al., 1997;

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Marfil et al., 1998; Zhu et al., 2016).

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Unlike conventional sandstones, the component of MSVS is more complicated, because of

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their various compositions resulting from the alteration of volcanic materials and the

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precipitation of diagenetic minerals (Wei et al., 2016a,b, 2018, 2019). In the diagenetic minerals,

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carbonate plays an important role in the MSVS properties. The earlier-formed, pore-filling

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carbonate cements could increase the pressure resistance of reservoirs and then prevent

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compaction and benefit later mineral dissolution (Wei et al., 2018, 2019). On the other hand,

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compared to non-tuffaceous mudstones (av. 1.14% in Arshan Formation in Errennaoer sag),

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TOC contents of tuffaceous mudstones close to MSVS are usually higher (av. 2.82% in Arshan

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Formation in Errennaoer sag), which are related to volcanic ash input (Wei et al., 2019).

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Therefore, MSVS has higher accumulation and hydrocarbon potentials, and tuffaceous materials

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and diagenesis have a significant influence on reservoir properties. However, the relationship

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between tuffaceous materials and diagenesis has not studied.

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Recently, a set of MSVS in the Lower Cretaceous has increasingly attracted attention of

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scientists because of a significant oil and gas discovery in the Erennaoer Sag, the Erlian Basin,

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China. Previous studies focused on sedimentology and stratigraphy of the Erennaoer Sag rather

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than the reservoirs of those rocks. In this study, we take MSVS in the Erennaoer Sag as an

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example to study influence of tuffaceous materials on diagenesis. It aims to: (1) identify the

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lithology, detrital, and diagenetic compositions of MSVS in the Arshan Formation; and (2)

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analyze the influence of volcanic material on early diagenetic alteration.

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2. Geological setting

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The Erlian Basin is one of the largest intracontinental Meso-Cenozoic basins in Northeast

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China (Fig. 1a; Dou et al., 1998; Lin et al., 2001; Dou and Chang, 2003; Bonnetti et al., 2014;

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Ding et al., 2015; Guo et al., 2018, 2019; Wei et al., 2018, 2019). The Erennaoer Sag is

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lacustrine sedimentary basin, located in the western Erlian Basin, of medium economic potential

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(the proven reserves are about 152.88 million barrels of oil; Fig. 1a). It is an asymmetric graben

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sag with an area of 1800 km2. The boundary faults strike NE, and the faults within the sag that

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affect the basement and sedimentary strata also strike NE. The Naoxi sub-sag (Fig. 1b; study

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area) has the leading oil and gas plays.

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The depositional-structural history of Erennaoer Sag includes pre-rift (Jurassic), syn-/post-rift

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(Early Cretaceous), and uplift (Late Cretaceous to present), associated with Yanshanian and

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Himalayan Movements (Lin et al., 2001). The main stage of rifting (syn-rift) occurred during the

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early Cretaceous (135–112 Ma), when a series of lacustrine sediments were deposited as a

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transgressive-regressive cycle (Ding et al., 2015). It consists of the Arshan (K1a) and Tengger

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(K1t) formations (Fig. 1c), which are the main oil/gas-bearing strata with abundant volcanic

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intercalations forming MSVS. Unconformably overlying Paleozoic metamorphic rocks and

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granites, K1a mainly consists of coarse-grained alluvial fan and fan-delta deposits and fine-

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grained offshore-lacustrine deposits with a thickness of 100–900 m. K1t is composed of

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lacustrine grey-black shales interbedded with fan-delta sandstones and siltstones with a thickness

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of 400-2100 m. During the K1a and K1t periods, the sag was deposited mainly in anoxic

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hypersaline lacustrine environments and experienced rapid subsidence. With frequent volcanic

98

eruptions in the periphery of the sag, some volcanic ash fell into the sag and mixed with

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terrigenous tuffaceous particles (older reworked volcaniclastic deposits) and clay minerals, and

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then MSVS was deposited.

101

After the deposition of Tengge'er Formation, the sag entered into post-rift stage (112 Ma to

102

present). At the end of the deposition of Saihantala Formation, the sag achieved the maximum

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burial depth and then suffered tectonic uplift. The Lower Cretaceous sedimentary rocks are

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unconformably overlain by thin Tertiary to Quaternary sandy conglomerates.

105

3. Methods

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A total of 110 sandstone core samples, collected from 15 wells and covering the depth

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between 600 m and 2200 m in the Arshan Formation, were analyzed. Samples were thin-

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sectioned and examined petrographically using a FEICA LEICA DFC550 polarizing microscope.

Wei et al 6

109

Thin sections were saturated with blue epoxy for visual estimation of porosity. Also, the thin

110

sections were stained with a mix of Alizarin red-S and potassium ferricyanide solutions for

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identification of calcites and dolomites (Dickson, 1965). 300 points on slice were counted to

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analyze the composition and textures of diagenetic minerals. Examination of diagenetic minerals

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and pore space characteristics was analyzed using a scanning electron microscopy (SEM).

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Porosity and permeability of MSVS are

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X-ray diffraction (XRD) patterns were collected for mineral identification and quantification

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on a Rigaku Ultima IV diffractometer with Co–Kα radiation and a step size of 0.02° 2θ. Bulk

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rock and clay preparation followed the analytical methods described in Duane and Robert, 1997.

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Powder samples were prepared to analyse the bulk mineralogy, whereas oriented samples (<2

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µm size-fraction) were used to identify the detailed clay mineralogy. Measurements were

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performed under air-dried and ethylene-glycolated conditions. The latter treatment causes

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interlayer expansion of swelling clays, allowing the recognition of discrete smectite and mixed-

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layer phases.

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Furthermore, ten representative MSVS samples comprised of one single type of carbonate

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cements were microsampled using a microdrill under a binocular microscope for stable carbon-

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and oxygen isotope analyses. The powdered sandstone samples (15 mg each) were run using a

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Thermo-Fisher mass spectrometer at China University of Petroleum (Beijing). Analytical errors

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of better than 0.05‰ (2σ) for the analyses were determined by repeated measurements of IAEA-

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C O-1 (δ13C VPDB = 2.492‰ and δ18O VPDB = -2.4‰), IAEA-CO-8 (δ13C VPDB = 5.764‰ and δ18O

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VPDB

= -22.7‰), and GBW4405 (δ13C VPDB = 0.57‰ and δ18O VPDB = -8.49‰) during each run.

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Permeability to air measurements were performed in a Hassler type core holder with 400 psi

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confining stress. Pore volumes was determined by helium injection using a porosimeter. Porosity

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and grain densities were also calculated by helium injection.

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4. Results

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4.1. PETROLOGY

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4.1.1. Lithofacies and petrography

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The Arshan Formation sandstones in the Lower Cretaceous were fan delta-lacustrine

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sediments (Lin et al., 2001; e.g., Wei et al., 2019). Due to the limitation of drilled-well locations,

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the studied Arshan sandstones are mainly distributed in the fan delta front, pro-fan delta, and

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lacustrine facies (Fig. 2, Wei et al., 2019). Petrographic examinations revealed three types of

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lithofacies: non-tuffaceous sandstone deposited in fan delta front (FFD-NTss), non-tuffaceous

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sandstone deposited in pro-fan delta (PFD-NTss), and tuffaceous sandstone deposited in pro-fan

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delta and lake (PFD-Tss; Table 1).

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FFD-NTss is dominantly composed of arkose and lithic arkose (Folk, 1974, 1980; Garzanti,

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2018; Fig. 3). It consists mainly of fine- to coarse-grained (100 µm to 1 mm) sandstones with

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low angle cross-stratification (~ 10 cm to 50 cm thick). The sorting is medium to poor, and the

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color is grey. The grains consist predominantly of plagioclase (44.2±7.45 vol.%, n=8), quartz

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(36.5±7.04 vol.%, n=8) and lithic fragment (19.2±9.54 vol.%, n=8). The matrix is dominantly

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composed of clay minerals (up to 35%), silt-sized quartz, plagioclase, and lithic fragments.

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Carbonate cements are locally observed (up to 20 vol.%), including calcite (av. 9.1 vol.%) and

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dolomite (av. 2.5 vol.%).

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PFD-NTss is dominantly composed of arkose and lithic arkose (Folk, 1974, 1980; Garzanti,

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2018; Fig. 3). It mainly consists of medium- to fine-grained (40 to 500 µm) sandstone and

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siltstone interbedded with mudstone. The sorting is medium, and the color is grey. It is a

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typically fining- and thinning-upward channelized unit (thicker than 5 cm) with small-scale cross

Wei et al 8

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bedding. It predominantly contains quartz (45±8.93 vol.%, n=29), plagioclase (42±8.48 vol.%,

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n=29) and lithic fragment (12.5±10.89 vol.%, n=29). Carbonate cements are locally observed (up

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to 38 vol.%) and comprise mainly of calcite cement.

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PFD-Tss is dominantly composed of lithic arkose (Fig. 3; e.g., Wei et al., 2019). It mainly

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consists of grey to grey-green, fine- to very fine-grained tuffaceous silt- to sandstone (50 to 200

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µm) with massive, wavy laminated or deformed stratification. It consists of plagioclase

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(43.3±5.94 vol.%, n=7), quartz (33.6±8.01 vol.%, n=7) and lithic fragment (3±8.98 vol.%, n=7).

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Carbonate cement is conventional (17.82±11.79%, n=28) in PFD-Tss and is composed of

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dolomite (av. 16 vol.%) and calcite (av. 6.4 vol.%). The matrix consists of clay minerals (up to

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43 vol.%), volcanic crystal, and glassy fragments (Table 1).

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4.1.2. Porosity and Permeability

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The measured core porosity of MSVS ranges from 1% to 36% (14.0±8.0 %, n=245), and the

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measured core permeability ranges from 0.002 mD to 878 mD (av. 253.7 mD, n=215). Vertically,

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the porosity and permeability show a rapidly decrease with the burial depth (Fig. 4). However,

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the reservoir quality varies among different lithofacies sandstones. The average porosity and

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permeability in the FFD-NTss, PFD-NTss, and PFD-Tss are 14.58±5.67% and ~158.01mD,

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18.11±8.12% and ~300.34 mD, and 7.04±5.87% and ~0.61 mD, respectively (Table 1).

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4.2. DIAGENETIC MINERALS

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Diagenetic minerals in the Arshan sandstones mainly consist of carbonate cement, authigenic

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quartz and clay minerals. However, the composition and content of diagenetic minerals are

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various among different lithofacies.

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4.2.1. Carbonate cement

Wei et al 9

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Carbonate cements, including calcite and dolomite, are the most dominant and ubiquitous

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diagenetic mineral in the MSVS. Calcite cement is the dominant pore-occluding cement in PFD-

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NTss, but also occurs in some parts of FFD-NTss and PFD-Tss. It occurs as micro- to coarse

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crystals (up to 200 µm) filling inter- and intragranular dissolution pores, replacing detrital grains

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(Fig. 5) and engulfing quartz overgrowth or deformed mica (Fig. 5a,b). Partial replacement of

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detrital grains by calcite cement can further enlarge the area of the cement and force the detrital

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grains apart into a loosely packed fabric (Fig. 5c).

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Dolomite cement is common in PFD-Tss and occasionally occurs in the non-tuffaceous

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sandstones (FFD-NTss and PFD-NTss). Two types of dolomite cement occur in the MSVS.

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Dolomite-1 cement (DI) mainly occurs as subhedral, micritic crystals (around 10 µm; Figs. 5d,e

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and 6a), locally aggregating patchily within the tuffaceous matrix or replacing detrital grains (Fig.

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5d,e; e.g., Wei et al., 2019). Dolomite-2 cement (DII) mainly occurs as subhedral to euhedral,

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microcrystalline crystals (10 to 100 µm; Fig. 5e), replacing detrital grains or pore-filling among

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detrital grains. Petrographic examinations indicate that the DI occurs mainly in the Tss but DII in

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all types of the Tengge’er sandstones.

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4.2.2. Quartz cement

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Quartz cement mainly occurs as overgrowths (up to 5 vol.%) on detrital quartz grains with a

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thickness of 5 to 30 um (Fig. 5b) or engulfed by calcite cements in the PFD-NTss. It rarely

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occurs in FFD-NTss and PFD-Tss where abundant chlorite and illite occur. Additionally, quartz

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cement occurs as automorphic quartz (< 2 vol.%), intergrowth-filling in small masses of

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interstitial or matrix clay in the PFD-Tss (Fig. 6b).

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4.2.3. Clay minerals

Wei et al 10

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Four types of clay minerals were identified based on the XRD study of the <2 µm size

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fraction and bulk fraction, namely: kaolinite, chlorite, Mixed-layered illite-smectite (MLIS) and

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illite. Minor amounts of kaolinite (13±17.41% in total clay mineral contents) occur mainly as

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booklets of vermicularly stacked pseudohexagonal crystals. Kaolinite always fills intergranular

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pores in the MSVS. It has a higher content in the non-tuffaceous sandstones and shows a

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decreasing trend with progressive burial (Fig. 7).

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Booklet and foliaceous shaped chlorite are mainly occured in primary pores (Fig. 6c) and on

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grain surfaces (14±14.08% in total clay mineral contents). Chlorite mainly occurs as continuous

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clay-coating surrounding quartz grains in the PFD-Tss and FFD-NTss in the absence of quartz

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overgrowth (Fig. 8a). On the contrary, it dominantly occurs as discontinuous or thin clay coating

209

attaching the quartz grains or quartz overgrowth in the FFD-NTss (Fig. 8b). XRD data shows

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that the chlorite is abundant in the shallow depth and decreases with burial depth increasing in

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the PFD/FFD-Tss. Conversely, it shows a trend with progressive burial in the PFD-Tss (Fig. 7).

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Needle shaped illite is mainly presents in primary pores and sometimes on grain surfaces

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and in feldspar dissolution pores, locally bridging pore-throats (Fig. 6a,d). Illite is an important

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clay mineral in the MSVS (52.4±28.61 wt.% in total clay mineral contents). The XRD analysis

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shows that the illite has an increasing trend with burial (Fig. 7). MLIS occurs as honeycomb-

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textured and lath-like crystalline (Fig. 6d), approximately 30%-60% illite. It is common in the

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tightly packed tuffaceous and argillaceous matrix. MLIS is present in lesser abundance

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(20.6±18.41 wt.% in total clay mineral contents).

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4.2.4. Minor cements

Wei et al 11

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Analcime cement occurs as subhedral, fine-grained crystals occluding intergranular pores in

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the PFD-Tss. Also, it is characterized by dissolution and replaced by carbonate cements (Fig.

222

8c,d).

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4.3. COMPACTION AND DISSOLUTION

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MSVS has suffered various degrees of compaction, resulting in concavo-convex grain

225

contacts (Figs. 5 and 8) and bending of flexible grains such as micas (Fig. 5a). Chemical

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compaction leads to the sutured contacts between detrital grains (Fig. 5a,b). The FFD-NTss and

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PFD-Tss have suffered heavier compaction than PFD-NTss with abundant carbonate cements.

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The petrographic examination of thin sections and SEM studies indicate that the pores in the

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MSVS are dominantly secondary (Fig. 8a,e,f) but rarely primary (5%). The primary pores are

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mainly filled with authigenic minerals such as calcite cement (Fig. 8f). The intragranular

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dissolution pores are formed by the dissolution of detrital feldspars and lithic fragments (Fig.

232

8a,e,f). The intergranular porosities are mainly developed by dissolution of calcite cements (Fig.

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8f; e.g., Xiong et al., 2015) and dissolution of the tuffaceous matrix (Fig. 8a) such as PFD-Tss.

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4.4. ISOTOPIC COMPOSITION OF CARBONATE CEMENTS

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The δ13C and δ18O values of the calcite cement vary from +1‰ to +3.1‰ PDB and from -17.6‰

236

to -15.3‰ PDB, respectively. However, dolomite cement has heavier and wider ranges from +2

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to +6.1‰ PDB and from -20.5‰ to -5.7‰ PDB, respectively. Dolomite-1 (DI) has relatively

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heavier δ13C (~ 6‰ PDB) and δ18O (~ -8‰ PDB) values relative to those of DII, ~ 2.5‰ PDB

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and -17.7‰ PDB, respectively (Fig. 9, Table 2).

240

5. Discussion

241

5.1. DIAGENETIC SEQUENCES

Wei et al 12

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The diagenetic history of MSVS is mainly constructed based on petrography. Early

243

diagenesis includes compaction, precipitation of DI, and chlorite, whereas late diagenesis

244

includes precipitation of quartz cements, late calcite and DII cement, illitization of kaolinite and

245

chlorite, and dissolution (Fig. 10).

246

5.2. ORIGIN OF CARBONATE CEMENTS

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The δ18O values of carbonate could reflect its temperature of crystallization (Longstaffe,

248

1987). Based on the formula calculating the depositional temperature of calcite and dolomite

249

(Craig, 1965):

250

T(℃) = 16.9-4.2 (δ18OPDB-δ18OSMOW)+0.13(δ18OPDB-δ18OSMOW )2

251

The δ18OPDB is the oxygen isotope values of carbonate cement, ‰ PDB, δ18OSMOW is the

252

oxygen isotope values of pore water in equilibrium with calcite and dolomite, ‰ SMOW. The

253

estimation approach of δ18OSMOW value is discussed in another paper (Wei et al., 2019).

254

Applying the calculation equation to calculate the precipitation temperatures of carbonate

255

cements, the result shows that DI mainly precipitated from 30°C to 50 °C, DII mainly

256

precipitated from 60 to 100°C and the precipitation temperatures of calcite mainly range from 50

257

to 80 °C (Table 2).

258

Based on previous studies, the heavy δ13C of DI cement (up to 6.1% PDB; Table 2) is

259

probably caused by bacterial mediation during methanogenesis (Wei et al., 2018, 2019). The

260

tuffaceous material might enhance biogenic blooms and methanogenesis under near-surface

261

conditions (Magara 2003; Delmelle et al., 2007; Sam Boggs, 2009; Langmann et al., 2010; Lin et

262

al., 2011; Baldermann et al., 2015, 2019). During microbial-mediated methanogenesis,

263

preferentially released via methane outgassing, which produces solutions that are relatively

12

C is

Wei et al 13

264

enriched in the heavy (13C) isotopomer from which the isotopically heavy carbonate cement (DI)

265

could precipitate at the shallow burial (30 - 50°C).

266

The δ13C values of calcite and DII cements are mainly around 2.5‰, which are largely higher

267

than DI, indicating the involvement of some organic carbon (Irwin et al., 1977; Curtis, 1977;

268

Macaulay et al., 2000; Surdam et al., 1989; Xi et al., 2019a; Wei et al., 2019). Ro of Arshan

269

mudstone is between 0.4% and 1.1% (Wei et al., 2019), representing that the Arshan mudstones

270

are in the stage of abundant organic acid and CO2 generation. Therefore, the organic CO2 would

271

be the major carbon source for calcite and DII precipitation. Additionally, the δ18O values of

272

calcite and DII tend to be more depleted (from -15‰ to -20‰, Table 2), thus reflecting the

273

influence of higher temperature related to increasing burial diagenesis. Therefore, calcite (50 -

274

80°C) and DII (60 - 100°C) are mainly late diagenetic carbonate cements formed during the time

275

of hydrocarbon generation and postdating quartz overgrowth. The dissolution of the early

276

diagenetic cements or fragments could also enhance precipitation of later carbonate cements

277

(Kim et al. 2007).

278

5.3. ORIGIN OF OTHER AUTHIGENIC MINERALS

279

Petrographic examinations indicate that quartz overgrowth is associated with quartz

280

dissolution at grain contacts or feldspar dissolution, suggesting internal Si4+ sources. On the

281

other hand, the alteration of volcanic ash released silica (Bien et al., 1958), contributing to the

282

formation of quartz.

283

Analcime can directly occur in alkaline lake deposits at shallow depths without volcaniclastic

284

material (Hay, 1966; Surdam, 1977; Gall and Hyde, 1989; Remy and Ferrell, 1989; Renaut,

285

1993). Under such circumstance, the analcime is characterized by subhedral, micro crystal. In the

286

Arshan MSVS, the grain of analcime is coarser in the PFD-Tss, suggesting that its origin is

Wei et al 14

287

unlikely direct authigenic precipitation. However, with the attendance of volcaniclastic minerals,

288

the coarse-crystalline analcime could also occur at shallow depths at a relatively low temperature

289

(Boggs and Seyedolali, 1992; Wei et al., 2018). The hydration of volcanic glass may release Ca2+,

290

which can bind with HCO3− to form early carbonate cements provided that a source of

291

bicarbonate is available (Boles and Surdam, 1979). This is consistent with the common

292

occurrence of analcimes occurring as coarse crystals and coexisting with calcite cement and

293

feldspar in the study area. Therefore, the alteration of volcanic materials could lead to the

294

deposition of analcime in the PFD-Tss, although large amounts have been replaced or dissolved

295

with burial (Fig. 8c).

296

XRD analysis and petrographic examinations indicate that the chlorite is abundant in two

297

intervals of depth, in the PFD-Tss in the shallow settings and in the non-tuffaceous sandstones in

298

the deep settings (Fig. 7). As for PFD-Tss, the chlorite content is abnormally higher at shallow

299

depth and decreases with increasing burial depth (Fig. 7). It is assumed that abundant chlorite at

300

shallow depth is probably related to exotic sources such as smectite or plagioclase (Sam Boggs,

301

2006). We assume that the source of authigenic chlorite in the PFD-Tss was mainly related with

302

alternation of volcanic materials and clay mineral transformation during the shallow depth

303

(Honess and Jeffries, 1940; Giles and De Boer, 1990; Higgs et al., 2007).

304

5.4. COMPOSITION CONTROLS ON THE DIAGENETIC ALTERATIONS

305

As the latest studies showed, the Arshan Formation was deposited in anoxic hypersaline

306

lacustrine environments (Luo et al. 2018; Wei et al. 2018, 2019). MSVS was mainly distributed

307

in the fan-delta and lacustrine sedimentary facies (Wei et al., 2019). Based on above studies,

308

products of early diagenesis depend on depositional environments and sandstone compositions,

Wei et al 15

309

as seen in the different compositions of DI, analcime and clay minerals in different lithofacies

310

(Fig. 11).

311

FFD-NTss comprise medium-scale cross-bedding conglomeratic to fine-grained sandstone

312

front and were mainly deposited in the fan delta. It is characterized by medium-poor sorting and

313

massive argillaceous matrix, leading to less primary porosity and permeability. Due to

314

occurrence of abundant plastic minerals such as silt-sized and clay matrix, FFD-NTss suffered

315

from strong compaction, leading to massive reduction of reservoir porosity (e.g., Xi et al., 2016).

316

The relationship of compaction and cementation to destruction of reservoir porosity shows that

317

FFD-NTss are apparently plotted in the compaction field (Fig. 12), indicating significant loss of

318

primary porosity by compaction.

319

PFD-NTss contains more quartz and feldspar debris and less tuffaceous and argillaceous

320

matrix, which have strong compressive strength leading to less compaction. It was mainly

321

deposited in the pro-fan delta, It is apparently plotted in the both compaction and cementation

322

fields (Fig. 12, Houseknecht, 1987), indicating that its initial porosity was destroyed by

323

compaction and cementation (Figs. 5a-c).

324

PFD-Tss were dominated by pro-fan delta and shallow lacustrine deposits and mainly

325

deposited in low-energy environment. It is characterized by massive tuffaceous matrix.

326

Generally, volcanic materials contain a variety of unstable constituents (e.g. fine-grained lithic

327

fragments, glassy fragments, mafic minerals, plagioclase) that are readily devitrified, altered and

328

replaced during weathering and diagenesis (Fisher and Schmincke, 1984). For example, the

329

alteration of volcanic minerals could release big amounts of ions such as Fe2+ and Mg2+ for the

330

occurrence of analcime in PFD-Tss in shallow burial settings (Fig. 8c). It is the earliest

331

diagenetic mineral formed by the alteration of volcanic material. Alteration of volcanic materials

Wei et al 16

332

also favor organic matter and methanogenesis (Zhu et al., 2016, Wei et al., 2018, 2019),

333

especially during the anoxic hypersaline lacustrine environment in Arshan period. With the ions

334

from volcanic material alteration and methanogenesis, DI as a major early diagenetic mineral

335

occurred in PFD-Tss. In addition, chlorite coats attaching quartz and feldspar grains were a

336

product of alternation of volcanic materials, occurring in the early diagenetic stage. The

337

formation of chlorite consumed Fe2+ and Mg2+, and released Ca2+ and SiO2 (Boles and Franks,

338

1979; Xi et al., 2019b), leading to quartz and calcite cementation. PFD-Tss plots in a wide field

339

along the diagonal line (Fig. 12) suggests that the low porosity was mainly cause by both

340

compaction and cementation.

341

In the late diagenetic stage, quartz overgrowth and calcite cement occurred preferentially in

342

PFD-NTss with little matrix and clay coating, but partially occurred in FFD-NTss and PFD-Tss

343

due to abundant clay matrix and authigenic chlorite. When organic matter matured (hydrocarbon

344

generating temperature was around 90℃; Liu 2014; Zuo et al. 2016; Wei et al., 2019), organic

345

acids were released from mudstones through the MSVS. The generated CO2 and hydrocarbons

346

increased H+ activity (acidity) in the pore fluids, dissolving early carbonate cement and detrital

347

debris. The dissolution of early carbonate cements is common in PFD-NTss, and the dissolution

348

of volcanic matrix and lithic fragments preferentially occurs in adjacent PFD-Tss. On the other

349

hand, matrix-rich, lower permeability FFD-NTss is less impacted by dissolution since it is far

350

from lacustrine environments. With progressive burial, late replacive carbonates (e.g. DII)

351

occurred in all lithofacies, indicating that deep-burial diagenetic fluid composition is similar

352

throughout Arshan Formation.

Wei et al 17

353

6. Conclusions

354

(1) Three lithofacies are recognized in the Arshan Formation of the Erennaoer Sag: non-

355

tuffaceous sandstone deposited in fan delta front (FFD-NTss), non-tuffaceous sandstone

356

deposited in pro-fan delta (PFD-NTss), and tuffaceous sandstone deposited in pro-fan delta and

357

lake (PFD-Tss).

358

(2) The dominant early diagenesis in the Arshan sandstones includes compaction, chlorite

359

and DI cementation. Subsequent late diagenetic processes include precipitation of quartz cements,

360

the formation of calcite and DII, and illitization of kaolinite and chlorite.

361

(3) Depositional environments and detrital compositions have a significant effect on the early

362

diagenesis of the Arshan sediments, controlling the degrees of compaction, varieties and amounts

363

of early diagenetic minerals. FFD-NTss suffers from strong compaction due to abundant silt-

364

sized and clay matrix. Conversely, PFD-NTss is subjected to weak compaction due to more rigid

365

debris. DI, analcime and chlorite cements are notable in PFD-Tss associated with the alteration

366

of volcanic materials.

367

(4) Late diagenesis is influenced by multiple factors including the detrital composition of

368

sediments, early diagenetic minerals, and burial temperatures. Late carbonate and clay minerals

369

were caused by dissolution of early calcite cement and clay mineral transformations. However,

370

the formation of late diagenetic minerals in PFD-Tss is associated with the dissolution of

371

volcanic materials and detrital grains.

372

Acknowledgments

373

We thank Huabei Oilfield Company of CNPC for their indispensable help providing well

374

data, rock core samples and permission to publish these data. This study was supported by the

375

Natural Science Foundation of China (grant number 41802176).

Wei et al 18

376

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546

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548

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Figure Captions

554

Fig. 1 (a) Generalized geological map of Erennaoer Sag showing the tectonic units and location

555

of study wells. (b) Structural profile. This section represents true thickness; the Upper

556

Cretaceous was not precipitated due to great tectonic uplift and thus is absent. (c) Stratigraphic

557

column. (modified from Lin et al. 2001, Zuo et al. 2016, and Luo et al. 2018).

558

Fig. 2 Section of sedimentary facies of the mixed lithological sandstones

559

Fig. 3 Quartz feldsparelithic grain (QFL) diagram showing the composition of the detrital grains

560

of the mixed lithological sandstone ((Q = quartz grain, F = feldspar grain, L = lithic grain;

561

modified after Folk, 1974, Garzanti, 2018).

562

Fig. 4 The distribution of reservoir quality with the burial depth in the different lithofacies.

563

Carbonate cement includes calcite and dolomite cements.

564

Fig. 5 Photomicrographs showing carbonate minerals of the Arshan sandstones. (a) Blended

565

mica (Mica) engulfed by calcite cement (Cal), Well N35, 1605.7 m, cross nicols (XN). (b)

566

Quartz overgrowth (Qc) engulfed by calcite cement, Well N35, 1605.7 m, plane light (PP). (c)

567

Calcite cement (Cal) occluding intergranular pores or replacing detrital grains, Well N35,

568

1595.72 m, PP. (d) DI occurring with tuffaceous matrix and replaced by calcite cement (Cal),

569

Well N97, 1039.2 m, XN. (e) DI engulfing debris grains and replaced by DII, and followed by

570

calcite cement, Well N36, 1649.7 m, XN. (f) Calcite (Cal) occluding dissolution pores and

571

replacing feldspar grains, Well N30, 1338.26 m, PP.

572

Fig. 6 SEM images of the Arshan sandstones. (a) Subhedral crystalline DI and illite (I) in

573

tuffaceous sandstones, Well N36, 1649.7 m; (b) Amorphous silica (Si) in tuffaceous sandstones,

574

Well N53, 1540.1 m; (c) Booklet and foliaceous crystalline chlorite (Ch), Well N30, 1739.9 m;

Wei et al 27

575

(d) Lath-like MLIS (mixed-layer illite/smectite) and needle-like illite (I), Well N36, 1649.7 m. P-

576

pore, F-feldspar.

577

Fig. 7 The relative amount of clay cement with respect to burial depth in the different lithofacies

578

Fig. 8 Photomicrographs showing diagenetic minerals features of the Arshan sandstones. (a)

579

Matrix dissolution micropores (P) and chlorite (Ch) occluding intergranular pores, Well N126,

580

1591.06 m, PP. (b) Chlorite (Ch) coating engulfing quartz (Q) and feldspar (F) grains Well N19,

581

1483.36 m, PP. (c) Analcime (An) occluding intergranular pores, and partially replaced by

582

calcite cement and followed dissolved, Well N22, 1545.09 m, PP; (d) is the cross nicols of (c); I

583

Feldspar (F) dissolution pores and intergranular dissolution pores (P), Well N97, 1039.2 m, PP.

584

(f) Calcite dissolution pores (P) and lithic fragment (L) dissolution pores, Well N23, 2202.92 m,

585

PP.

586

Fig. 9 (a) Distribution of δ13C and δ18O of carbonate cements in the mixed lithological sandstone

587

with respect to burial depth. (b) δ18O-δ13C diagram of various carbonate cements in the mixed

588

lithological sandstone

589

Fig. 10 Diagenetic alterations of MSVS in Arshan Formation

590

Fig. 11 Diagenetic model displaying the evolution pathways and spatial and temporal

591

distribution of diagenetic minerals in the Arshan Formation, the diagenesis marked red means the

592

major diagenetic process. (the cross-section is modified from Lin et al., 2001)

593

Fig. 12 Diagram showing the intergranular volume vs. calcite cement volume in the Arshan

594

sandstones (Houseknecht, 1987)

595

Wei et al 28

596

Tables

597

Table 1

598

Reservoir characteristics in each sedimentary facies and statistics of the reservoir porosity.

599

Table 2

600

Mineralogical and isotopic composition of carbonate cements in the Arshan sandstones. Cal-

601

calcite, DI-dolomite-1, DII-dolomite-2

602

Table 1 Reservoir characteristics in each sedimentary facies and statistics of the reservoir porosity. Lithofacies

PFD-NTss

PFD-Tss

Fine-grained sandstone and siltstone

Tuffaceous sandstone, tuffaceous siltstone

Graded, small-scale cross-bedding, wavy laminated

Wavy laminated, massive, deformed, structureless,

10-50

5-20

5-10

11.5±10.29 (8)/1-35a

3.22±2.28

(20)/1-8

14.45±10.41 (16)/3-43.1

Carbonate cement (%)

5.78±5.48 (29)/0.1-18.9

9.68±5.34 (146)/0-65.7

17.82±11.79 (28)/2.4-51.1

Porosity (%)

14.58±5.67 (62)/4.8-33.5

18.11±8.12 (163)/1.7-36.5

7.04±5.87 (20)/1-20

Permeability (mD)

158.01 (41)/0.064-939

300.34 (160)/0.002-5704

0.61 (14)/0.01-5.42

Location

Proximal channel

Distal channel and pro-fan delta

Pro-fan delta and shallow lacustrine

Lithology

Structure

FFD-NTss Conglomeratic sandstone to fine-grained sandstone Graded, medium-scale cross-bedding, massive

Thickness of layer (cm) Clay matrix (%)

a

11.5±10.29 (19)/1-35

a

mean±stdev (count number)/max content- min content

Table 2 Mineralogical and isotopic composition of carbonate cements in the Arshan sandstones. Cal-calcite, DI-dolomite-1, DII-dolomite-2 Carbonate

δ13C ‰

δ18O ‰

Temperature

content (%)

(PDB)

(PDB)

(℃)

Well

Minerals

N36

80%Cal+20%Do

27

1.00

-17.60

104.38

N47-1

83%Cal+17%Do

35.6

3.10

-15.30

86.75

N11

80%Cal+20%Do

18

1.20

-16.30

94.25

N47-2

20%Cal+80%DI

15.6

6.00

-5.70

28.12

N47-3

20%Cal+80%DI

3.7

6.10

-9.30

47.28

N71

10%Cal+90%DI

17.5

2.60

-9.00

45.55

N84X

30%Cal+70%DI

30.2

5.20

-7.90

39.43

N126-2

30%Cal+70%DII

58.1

2.80

-15.10

85.28

N120-1

20%Cal+80%DII

45.5

2.60

-20.50

128.59

N120-2

100%DII

34.3

2.00

-17.50

104.38

Temp means the paleotemperature of carbonate cement (Craig, 1965).

Diagenesis of the siliciclastic-volcaniclastic sandstones Influences of depositional environments and detrital compositions on diagenesis Influences of volcanic materials on volcaniclastic sandstones diagenesis

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Wei Wei, Xiaomin Zhu, Karem Azmy, Shifa Zhu, Mingwei He, Shuyang Sun