Sedimentology and lithofacies of lacustrine shale: A case study from the Dongpu sag, Bohai Bay Basin, Eastern China

Journal of Natural Gas Science and Engineering 60 (2018) 174–189

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Sedimentology and lithofacies of lacustrine shale: A case study from the Dongpu sag, Bohai Bay Basin, Eastern China

T

Chuanyan Huanga,b,∗, Jinchuan Zhangb, Wang Huaa, Jiaheng Yuea, Yongchao Lua a b

Key Laboratory of Tectonics and Petroleum Resources of the Ministry of Education, China University of Geosciences, Wuhan, 430074, China China University of Geosciences, Beijing, 100083, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Lacustrine shale Mineral Lithofacies Depositional environment Dongpu sag Bohai bay basin

This paper analyzes the sedimentology and lithofacies of the lacustrine shale from the third member of the Eocene Shahejie Formation (Es3) in the Dongpu sag, Bohai bay basin, eastern China. The results show that lacustrine shale is heterogeneous in its sedimentary structure, lithology, mineralogy, lithofacies, and oil content. From the margin of the lake to its center, the depositional environment progresses from delta front to prodelta to deep water lake, and the primary sedimentary lithologies changes from interbedded mudstone and sandstone to mudstone with siltstone to mudstone with evaporite and carbonate rocks. The major deep water deposits are laminated shales. From the lake margin to the center, felsic mineral content decreases gradually, and clay mineral and pyrite content increases gradually. Felsic mineral content is the highest in the delta front shale, and clay mineral and pyrite content is highest in the deep water lake. Shale lithofacies also change with the depositional environment. The lithofacies of the delta front shale are primarily felsic-rich lithofacies: clay-rich carbonate-poor felsic shale (S-4) and a clay-rich, carbonate-poor felsic-rich mixed shale (MS-2). The lithofacies of the prodelta shale are primarily a carbonate-poor felsic-rich muddy shale (M-2), S-4, and MS-2. The lithofacies of deep water lake shales are primarily clay mineral-rich lithofacies: M-2, MS-2, and the clay-rich, carbonaterich, felsic-rich mixed shale (MS-3). The TOC and the types of organic matter also change with the depositional environment in the Dongpu sag. The results of this study show the sedimentary structures, lithology, mineral content, lithofacies, and spatial distribution of the lacustrine shale was not only controlled by the macro depositional environment and the local depositional environment, but also controlled by the source and transport (or the sediment transport path), water depth, and accommodation space.

1. Introduction In China, organic-rich black shale is widely distributed and is an important hydrocarbon source rock in lacustrine basins, such as the Songliao, the Bohai Bay, the Ordos, the Jungar, and other large basins (Zhang et al., 2012, 2014; Zou et al., 2013). Recent studies have determined that shale oil and gas potential in these lacustrine basins is very rich, especially for shale oil resources (Zhang et al., 2008, 2014; Zou et al., 2010; Zhang et al., 2015a). Shale is very complex (Manger and Curtis, 1991; Bowker et al., 2003; Loucks et al., 2007; Trabucho-Alexandre et al., 2011, 2012a; Abouelresh and Slatt, 2012). Even in the same depositional environment, shale has strong heterogeneity in terms of color, mineralogy, sedimentary structure, reservoir characteristics, and extent of oilhosting (Loucks et al., 2007; Zhang et al., 2009; Ross and Marc Bustin,

2009; Abouelresh and Slatt, 2012; Turner et al., 2016; Pawar et al., 2017; Atmani et al., 2017). Compared with marine basins, lacustrine basins are smaller, and the environment of depositions changes faster. The mineralogy and lithology of lacustrine shale are more complex (Zhang et al., 2009; Huang et al., 2015; Zhang et al., 2015a). Deng (1990) divided lacustrine shales from the third member of the Eocene Shahejie Formation (Es3) in Dongying sag, Bohai bay basin, eastern China into eight types based on lithofacies and mineralogy (Deng and Qian, 1990). This classification system for lacustrine shales severely restricted exploration for shale oil and gas. The purpose of this study is to analyze lacustrine shales from different environments of deposition within Es3, Dongpu sag, Bohai Bay basin in terms of their sedimentary structure, lithology, mineralogy, lithofacies, and oil-hosting. The study results will summarize the characteristics and factors controlling the lacustrine shale, which will

∗ Corresponding author. Key Laboratory of Tectonics and Petroleum Resources of the Ministry of Education, China University of Geosciences, Wuhan, 430074, China. E-mail address: [email protected] (C. Huang).

https://doi.org/10.1016/j.jngse.2018.10.014 Received 4 April 2018; Received in revised form 15 October 2018; Accepted 20 October 2018 Available online 25 October 2018 1875-5100/ © 2018 Published by Elsevier B.V.

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Fig. 1. (a) The study area of the Dongpu sag in the Bohai Bay Basin, Eastern China. Maps of the study area showing wells of cores and lines of section. (b) Cross section showing the structural framework of the Dongpu sag. Section location in Fig. 1a: A - B (modified from Oilfield, 1993; Liu and Jiang, 2013; Zhang et al., 2015). This investigation focuses upon the Es3 interval.

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Fig. 2. Generalized stratigraphy of the Dongpu sag (modified from Oilfield, 1993; Youliang 2005; Liu and Jiang, 2013; Zhang et al., 2015). This investigation focuses upon the Es3 (the srata in the red boxes).

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can be both massive and laminated, and are variously interbedded with siltstone, fine sandstone, calcite, and dolomite. In this paper, all black and fine sediment that particle size is less than 0.065 mm is defined as shale (Tourtelot, 1979; Trabucho-Alexandre et al., 2012b; TrabuchoAlexandre, 2015).

facilitate exploration for shale oil and gas in lacustrine settings. 2. Geological background 2.1. Basin development and paleoclimate The Bohai Bay Basin is a Cenozoic lacustrine rift basin located on the east coast of China with a total area of approximately 200,000 km2 (Fig. 1A) (Chang, 1991; Hao et al. 2009). The thickness of the Cenozoic strata is typically several hundred to several thousand meters, but locally, is more than ten thousand meters thick (in the Huanghua and Bozhong depressions). The Bohai Bay Basin is the most petroliferous basin in China and contains the greatest oil production (accounting for nearly one-third of the total oil production in China) (Chang, 1991; Hao et al. 2009; Huang et al., 2015). The basin experienced two major tectonic stages in its evolution: the Paleogene syn-rift stage and the Neogene post-rift stage (Chang, 1991; Hao et al. 2009; Huang et al., 2015). The Dongpu sag is located on the southwestern side of the Linqing depression (Fig. 1A). The sag experienced the same tectonic evolution as the basin overall: the Paleogene syn-rift stage and the Neogene postrift stage (Fig. 2), which produced thick Paleogene and Neogene sedimentary sections. The syn-rift Paleogene sediments are composed of the Eocene Shahejie Formation (Es) (further divided into four sections – the fourth, third, second, and first members of the Eocene Shahejie Formation, referred to as Es4, Es3, Es2, and Es1), and the Oligocene Dongying Formation (Ed). These formations were deposited in fluviallacustrine environments and are restricted to the grabens and halfgrabens (Fig. 1B). The post-rift sediments consist of the Miocene Guantao Formation (Ng), Miocene–Pliocene Minghuazhen Formation (Nm), and the Quaternary Pingyuan Formation (Qp) (Fig. 2). These formations are widespread and are dominated by fluvial deposits. Paleoclimate changed considerably during the syn-rift evolution of the Bohai Bay Basin. Arid conditions were dominant during the deposition of the Eocene Kongdian Formation (Ek in certain depressions) and the Es4, and a wet, northern subtropical climate was dominant during the deposition of Es3. Aridity increased from the late stage of Es3 deposition to Es2 and decreased thereafter, with a subtropical climate becoming dominant during the deposition of Es1 and Ed (Fig. 2) (Yang and Xu, 2004; Wang et al., 2015; Hao et al. 2011; Huang et al., 2015).

3.2. Sampling and test procedure For this paper, we collected cores from eight boreholes and approximately 128 shale samples from different interpreted environments of deposition from Es3 in the Dongpu sag. All of these shale cores were prepared and analyzed for sedimentary characteristics. These samples were analyzed for mineralogy by X-ray diffraction (XRD) analysis using a D8 DIS-COVER. Thin sections were prepared and Polarized light microscopy was used to observe micro-structure and shale oil-hosting. The temperature was held at 24 °C and the relative humidity was held at 35%. All of the data were generated by the PetroChina Research Institute of Petroleum Exploration and Development located at the Huabei Oilfield. 4. Sedimentology and description of lithofacies Shales in the lacustrine basin were deposited in deep water environments, prodeltas, and the subaqueous interdistributary area of the delta front. In this paper, shales are divided according to their depositional environment into deep water shales, prodelta shales, and delta front shales. 4.1. Sedimentary structure and lithology characteristics 4.1.1. Deep water lacustrine shale The deep water lacustrine shale is dark gray or black, and each individual layer is very thick. The shale lithologies are muddy shale, felsic shale, and calcareous shale (Fig. 3B), and these are mixed with thin siltstone (Fig. 3B) and salt rock layers (Fig. 3D). The main sedimentary structures are horizontal bedding and massive structure, but there are also minor small scale cross-stratification, slump deformation structures in the thin siltstone (Fig. 3C), and lamellation in the dark gray shale (Fig. 3A). Two groups developed approximately vertical fractures that are filled with calcite: for one group the fractures are along bedding, and for the other group, they are vertical to bedding.

2.2. Sedimentary environments 4.1.2. Prodelta shale The prodelta is another favorable environment for shale. The shale is predominantly black shale and thin siltstone interbedded with thin layers of fine sandstone. The siltstone layers are thicker than that of the deep water shale (Fig. 4). The major sedimentary structures are horizontal bedding (Fig. 4C and D), massive structure (Fig. 4B), and small cross-stratification (Fig. 4A). The shale with horizontal bedding is usually broken along bedding (Fig. 4C). The thin siltstone with small cross-stratification is the prodelta sheet sand, which incorporates some black lacerated mud-gravels (Fig. 4E), and displays slump deformation structures (Fig. 4F).

The depositional environment during the deposition of Es3 was dominated by deep lake conditions and braid river deltas; the water was fresh, and the Bohaidina-Parabohaidina formations, which are the main deep lake organic-rich black shales, were deposited in the Dongpu sag (Oilfield, 1993; Ji et al., 2005; Liu and Jiang, 2013; Zhang et al., 2015b). Three main sedimentary environments are recognized, namely: delta front, prodelta and deep water (lake). The delta front develops gray and brown mudstone interbedded with gray siltstone and fine sandstone, rich in bioturbation structures. The prodelta is characterized by black shale and thin siltstone with small cross stratification; occasionally slump deposits are observed in drill cores. Deep lake is rich in dark gray or black mudstone interbedded with lime shale, calcareous shale and thin siltstone, lacking in shelly fauna and bioturbation.

4.1.3. Delta front shale Delta front shale is mainly deposited in the subaqueous interdistributary area of the delta front. The most obvious feature of this shale is that versicolor mudstone, gray siltstone, and fine sandstone are interbedded. The thickness of each layer of shale and sand is approximately equal. The major sedimentary structures are horizontal bedding (Fig. 5B and F) and wavy cross-stratification (Fig. 5D). The shale with horizontal bedding is usually broken and incorporates many plant stem fossils (Fig. 5C). The gray siltstone displays numerous bioturbation structures, such as biological level caves (Fig. 5E), and abundant versicolor lacerated mud gravel (Fig. 5I).

3. Methodology 3.1. Nomenclature Shale does not have a single coherent definition, and such terms as mudstone, claystone, and shale are usually used interchangeably (Loucks et al., 2007; Boggs, 2009; Zou et al., 2010; Abouelresh and Slatt, 2012; Turner et al., 2016). In this study area, the black mudstones 177

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Fig. 3. Cores from W1 (borehole location in Fig. 1a), deep water lake. Lithology: deep gray mudstone and calcareous shale mixed with thin siltstone and salt rock. (a): black mudstone, horizontal bedding. (b): gray silty mudstone interbedded with thin black mudstone, horizontal bedding, cross stratification. Two groups of approximately vertical fractures that are filled with calcite: the fractures are along bedding, and the other group that are vertical to bedding. (c): gray siltstone, small scale cross-stratification, slump deformation structures. (d): thin salt rock layer.

low with an average content of only 9.7% and 8.3%, respectively (Fig. 6).

4.2. Mineralogy The shale is mainly composed of a variable mixture of clay minerals (e.g., illite, mixed layer illite-smectite, kaolinite, chlorite), quartz, feldspar, carbonates (e.g., calcite, dolomite, siderite), sulfides (mainly pyrite), amorphous material, and organic matter (Macquaker and Gawthorpe, 1993; Potter et al., 2005; Aplin and Macquaker, 2011; Könitzer et al., 2014).

4.2.2. Prodelta shale The dominant minerals in the prodelta shale are clay minerals with an average content of approximately 38.5% followed by quartz with a significantly a significantly increased content averaging approximately 31.3%. The average carbonate content is approximately 18.9%, and the average feldspar content is approximately 10.0%. The salt content is very low with an average content of approximately 0.9%. The pyrite content is even less with an average content of approximately 0.3%, and most of the samples contain no pyrite. The main clay minerals in the shale are composed of a mixed layer of illite-smectite (I-S) with an average content of 52.5% followed by illite with an average content of 32.1%. While the content of kaolinite is less than that of the deep water shale with an average content of 1.2%, with several samples containing no kaolinite; the chlorite content is higher than that of the deep water shale with an average content of approximately 14.2% (Fig. 7).

4.2.1. Deep water lacustrine shale XRD analysis and thin-section photography show that clay minerals are the dominant minerals in the deep water shale, and the average clay content is approximately 40.7%. That content is followed by carbonate, quartz, and feldspar: the average carbonate content is approximately 23.1%, the average quartz content is approximately 19.1%, and the average feldspar content is approximately 10.8%. There is also a small amount of evaporite, which is mainly gypsum and anhydrite with an average content of approximately 4.8%. The salt content varies greatly in different samples. Although pyrite content is very low with an average content of about only 1.5%, it is present in almost every sample. The clay minerals in the shale are mainly composed of mixed layers of illite-smectite (I-S), with an average content of 43.0% and illite with an average content of 39.0%. Kaolinite and chlorite content is very

4.2.3. Delta front shale The dominant minerals in the delta front shale are clay minerals and quartz. Clay mineral content is less than that of the deep water shale with an average content of only about 32.3%, while quartz content is 178

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Fig. 4. Cores from W2 (borehole location in Fig. 1a), prodelta. Lithology: gray mudstone mixed with thin gray siltstone and fine sandstone. (a): gray siltstone, crossstratification. (b): black mudstone, massive structure. (c): black shale, horizontal bedding, broken along bedding. (d): black shale, horizontal bedding. (e): gray fine sandstone, mixed with some black lacerated mud-gravels. (f): gray fine sandstone, deformable structure.

The pyrite content is low with an average content of approximately 0.5%, and most of the samples contain no pyrite. The main clay minerals in the shale are a mixed layer of illite-smectite (I-S) with an average content of 62.0% followed by illite, with an average content of

significantly increased compared to the deep water shale, with an average content of approximately 31.0%. The average carbonate content is approximately 19.2%, and the average feldspar content is approximately 11.7%. The average salt content is approximately 5.3%. 179

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Fig. 5. Cores from W3 (borehole location in Fig. 1a), delta front. Lithology: gray mudstone, interbedded with gray siltstone and fine sandstone. (a): black, gray, brown-red mudstone interbedded, horizontal bedding, usually broken. (b): black and gray mudstone interbedded, usually broken. (c): black shale, horizontal bedding, along bedding there deposited a lot of plant stem fossils. (d): gray siltstone, wavy cross stratification. (e): gray siltstone, developed a lot of bioturbation structure. (f): gray and black shale interbedded, horizontal bedded, usually broken along bedding. (g): gray siltstone, horizontal bedding. (h): gray fine sandstone, massive structure. (i): gray fine sandstone, mixed versicolor lacerated mud gravel.

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Fig. 6. Mineralogical constituent of shale samples from deep water lake in Es3. (a): dominant minerals constituent. CM: clay mineral, F: feldspar, CR: carbonate rock, SR: salt rock, P: pyrite, Q: quartz. (b): clay minerals constituent. K: kaolinite, C: chlorite, I: illite, I/S: mixed layer illite-smectite.

content > 50%), calcareous shale (C) (the carbonate content > 50%), muddy shale (M) (the clay minerals content > 50%), and mixed shale (MS) (each mineral content < 50%). Each lithofacies is further divided into 16 subcategories according to mineral content (Table 1, Fig. 9). Each subcategory is further divided according to sedimentary structures into two groups: laminated lithofacies and massive lithofacies. Ternary diagrams of the mineralogical constituents of shale show that most samples are either clay-rich, carbonate-poor, felsic-rich mixed shale (MS-2); clay-poor, carbonate-poor, felsic shale (S-3); clay-rich, carbonate-poor felsic shale (S-4); or carbonate-poor, felsic-rich muddy shale (M-2). Lithofacies of shales from different depositional environments, however, have their own different characteristics (Fig. 9). Most samples of the deep water shale are clay-rich, and the main lithofacies are carbonate-poor, felsic-rich muddy shale (M-2) and the clay-rich carbonate-poor felsic-rich mixed shale (MS-2) followed by the clay-rich carbonate-poor felsic shale (S-4); the clay-rich carbonate-rich felsic-rich mixed shale (MS-3); clay-rich, carbonate-rich felsic-poor mixed shale (MS-4); and clay-poor, felsic-rich calcareous shale (C-2) (Fig. 9A). Most samples of the delta front shale are felsic-rich, and the main

28.2%. Kaolinite and chlorite content are both low and average 4.7% and 5.1%, respectively (Fig. 8).

4.3. Lithofacies There is no uniform classification scheme on shale lithofacies. Classification schemes generally divide lithofacies based on sedimentary structures, lithology, or mineral compositions of the shale (Deng and Qian, 1990; Macquaker and Gawthorpe, 1993; Loucks et al., 2007; Abouelresh and Slatt, 2012; Trabucho-Alexandre et al., 2012; Turner et al., 2016). In this study shale lithofacies divisions are mainly based on the mineral content of the shale. The proportion of each lithofacies is also calculated to demonstrate the genetic links with the sedimentary environments (Table 1). Shale lithofacies divisions are derived from ternary diagrams of the mineralogical constituents and their content, which show relative proportions of clay minerals, carbonate (including calcite, dolomite, siderite and other carbonate minerals), and felsics (including quartz, feldspar, pyrite, phosphate and others) (Fig. 9). According to the clay-carbonate-felsic content with a cutoff of 50%, the shale can be divided into four lithofacies: felsic shale (S) (the felsic 181

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Fig. 7. Mineralogical constituent of shale samples from prodelta in Es3. (a): dominant minerals constituent. CM: clay mineral, F: feldspar, CR: carbonate rock, SR: salt rock, P: pyrite, Q: quartz. (b): clay minerals constituent. K: kaolinite, C: chlorite, I: illite, I/S: mixed layer illite-smectite.

Fig. 8. -Mineralogical constituent of shale samples from delta front in Es3. (a): dominant minerals constituent. CM: clay mineral, F: feldspar, CR: carbonate rock, SR: salt rock, P: pyrite, Q: quartz. (b): clay minerals constituent. K: kaolinite, C: chlorite, I: illite, I/S: mixed layer illite-smectite. 182

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Table 1 Types of shale lithofacies. category

Felsic shale (S)

total Calcareous shale (C)

total Muddy shale (M)

total Mixed shale (MS)

subcategory

Sedimentary environment(%)

clays

felsic

carbonate

name

delta front

predelta

deep lake

< 25% < 25% < 25% 25%∼50%

> 75% 50%∼75% 50%∼75% 50%∼75%

< 25% 25%∼50% < 25% < 25%

Silicastone (S-1) clay-poor carbonate-rich felsic shale (S-2) clay-poor carbonate-poor felsic shale (S-3) clay-rich carbonate-poor felsic shale (S-4)

< 25% < 25% < 25% 25%∼50%

< 25% 25%∼50% < 25% < 25%

> 75% 50%∼75% 50%∼75% 50%∼75%

Limestone (C-1) clay-poor felsic-rich calcareous shale (C-2) clay-poor felsic-poor calcareous shale (C-3) clay-rich felsic-poor calcareous shale (C-4)

> 75% 50%∼75% 50%∼75% 50%∼75%

< 25% 25%∼50% < 25% < 25%

< 25% < 25% < 25% 25%∼50%

Mudstone (M-1) carbonate-poor felsic-rich muddy shale (M-2) carbonate-poor felsic-poor muddy shale (M-3) carbonate-rich felsic-poor muddy shale (M-4)

< 25% 25%∼50% 25%∼50% 25%∼50%

25%∼50% 25%∼50% 25%∼50% < 25%

25%∼50% < 25% 25%∼50% 25%∼50%

clay-poor carbonate-rich felsic-rich mixed shale (MS-1) clay-rich carbonate-poor felsic-rich mixed shale (MS-2) clay-rich carbonate-rich felsic-rich mixed shale (MS-3) clay-rich carbonate-rich felsic-poor mixed shale (MS-4)

6% 3% 18% 21% 47% 0 6% 9% 0 15% 0 0 0 0 0 0 29% 9% 0 38%

4% 4% 11% 13% 33% 2% 0 2% 0 4% 0 15% 0 0 15% 2% 33% 9% 4% 48%

1% 2% 3% 9% 15% 2% 5% 1% 3% 1% 0 25% 1% 0 26% 4% 26% 11% 9% 50%

total

Fig. 9. Ternary diagrams of mineralogy by environment in Dongpu sag. (a) Lithofacies of deep water lake. (b) Lithofacies of prodelta. (c) Lithofacies of delta front. M: the clay minerals content. C: the carbonate content (including calcite, dolomite, ankerite, siderite). S: the felsic content (including quartz, feldspar, pyrite, phosphate, and anothers). According to the content of clays-carbonate-felsic ternary of 50%, shale can be divided into four lithofacies areas: the felsic shale (S) (the felsic content > 50%), the calcareous shale (C) (the carbonate content > 50%), the muddy shale (M) (the clay mineral content > 50%) and the mixed shale (MS) (each mineral content < 50%). On this basis, each lithofacies area can be further divided into subcategories according to minerals content and shale lithofacies are divided into 16 kinds of subcategories (Table 1).

for all samples decreases gradually with increasing clay content. In other words, shale oil-hosting decreases gradually with increasing clay content (Figs. 10 and 11). Even for the same lithofacies, intensity of fluorescence decreases with increasing clay content. For example, the intensity of fluorescence for Fig. 10B (MS-3, Clay mineral: 27.2%) and 11A (MS-3, Clay mineral: 20.8%) is higher than that of Fig. 10E (MS-2, 48.1%). The same characteristic is also observed for within the same subcategories of lithofacies. For example, the intensity of fluorescence in Fig. 10B (MS-3, Clay mineral: 27.2%), 10C (MS-3, Clay mineral: 33.1%), and 10D (MS-3, Clay mineral: 43.3%) is higher than others. If the sedimentary structure is similar, the intensity of fluorescence for muddy shale (M) is darker than that of other shale lithofacies, such as Fig. 10F (M-2, Clay mineral: 70.4%) and 11E (M-3, Clay mineral: 65%)these thin sections hardly fluoresce. If shale mineral content is similar, the intensity of fluorescence of laminated shale is higher than that of massive shale. For example, the fluorescence intensity of Fig. 10A and B (laminated shale) is higher than that of Fig. 11A and B (massive shale). The relationship between the intensity of fluorescence and felsic or carbonate content is not obvious. For example, for Fig. 11A, B, 11C, and 11D, the felsic content is similar, but the samples differ in their clay mineral content, so the oil-hosting in these samples is different. For Fig. 10B, C, and 10D, the felsic content of these samples is similar, but

lithofacies are clay-rich, carbonate-poor felsic-rich mixed shale (MS-2); clay-poor, carbonate-poor felsic shale (S-3); clay-rich, carbonate-poor felsic shale (S-4); and clay-rich carbonate-rich felsic-rich mixed shale (MS-3). Some samples, however, are clay-poor, felsic-rich calcareous shale (C-2); and clay-poor, felsic-poor calcareous shale (C-3) (Fig. 9C). Since the prodelta shale has characteristics of both the deep water shale and the delta front shale, the main lithofacies are clay-rich, carbonate-poor felsic-rich mixed shale (MS-2); carbonate-poor, felsic-rich muddy shale (M-2); and clay-rich, carbonate-poor felsic shale (S-4). These are followed by clay-rich, carbonate-rich felsic-rich mixed shale (MS-3); and clay-poor, carbonate-poor felsic shale (S-3) (Fig. 9B). 4.4. Fluorescence Oil will fluoresce after being irradiated with ultraviolet light. Different oil components and different oil content produce light with different brightness and color content (Riediger et al., 1990; Barwise et al., 1996; Yang et al., 1998). Generally, if the fluorescence intensity is higher, the sample is hosting more oil. Thin sections of samples from study area were observed using a fluorescence microscope (Figs. 10 and 11). This result shows that shale oil and gas were mainly hosted in the shale matrix and fractures. On the whole, the intensity of fluorescence 183

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Fig. 10. Thin sections and fluorescence of laminated shale. Boreholes location in Fig. 1a. A): sample from W3, the third member of the Eocene Shahejie Formation (Es3), delta front, 3495.24m. Dominant minerals: Quartz: 18.3%, Feldspar: 6.8%, Carbonate rock: 51.8%, Salt rock: 4.8%, Clay mineral: 17.1%, Pyrite: 1.2%. Lithofacies: C-3, microfractures along bedding filled with calcite. Fluorescence characteristics: yellowish green fluorescence of matrix and middle-dark, green fluorescence along microfractures and middle-dark. B): sample from W4, Es3, deep water lake, 3362m. Dominant minerals: Quartz: 10.8%, Feldspar: 9.4%, Carbonate rock: 42.4%, Salt rock: 3.6%, Clay mineral: 27.2%, Pyrite: 6.7%. Lithofacies: MS-3, microfractures along bedding filled with calcite. Fluorescence characteristics: yellowish green fluorescence of matrix and very dark, yellowish green fluorescence along microfractures in muddy shale and middle-bright, while no fluorescence along most of microfractures in calcareous shale, pale green fluorescence along part of microfractures and very dark. C): sample from W5, Es3, prodelta, 3561.01m. Dominant minerals: Quartz: 16.5%, Feldspar: 5.4%, Carbonate rock: 40.7%, Salt rock: 1.8%, Clay mineral: 33.1%, Pyrite: 2.5%. Lithofacies: MS-3. Fluorescence characteristics: yellowish green fluorescence of matrix and very dark, yellowish green fluorescence along part of microfractures and middle-dark. D): sample from W4, Es3, deep water lake, 3360.27m. Dominant minerals: Quartz: 13.8%, Feldspar: 9%, Carbonate rock: 30.4%, Salt rock: 1%, Clay mineral: 43.3%, Pyrite: 2.5%. Lithofacies: MS-3, microfractures along bedding filled with rich organic matter. Fluorescence characteristics: pale green fluorescence of matrix and dark, while no fluorescence along micro bedding with rich organic matter. E): sample from W6, Es3, deep water lake, 3563.65m. Dominant minerals: Quartz: 21.8%, Feldspar: 13.1%, Carbonate rock: 12.0%, Salt rock: 1.9%, Clay mineral: 48.1%, Pyrite: 3.1%.: Lithofacies: MS-2, a large number of microfractures filled with calcite. Fluorescence characteristics: no fluorescence of shale, green fluorescence along part of microfractures and very dark. F): sample from W7, Es3, prodelta, 3707.33m. Dominant minerals: Quartz: 17.7%, Feldspar: 5.7%, Carbonate rock: 1.6%, Salt rock: 2.3%, Clay mineral: 70.4%, Pyrite: 2.3%. Lithofacies: M-2, a large number of microfractures filled with calcite. Fluorescence characteristics: no fluorescence of whole shale, yellowish green fluorescence part of shale and very dark.

5. Discussion 5.1. Sedimentary characteristics of lacustrine shale The study results show that in the Dongpu sag, from the lake margin to the center of the lake, the depositional environment changed from delta front, via prodelta, to deep lake, corresponding to significant differences in lithology, mineralogy, and lithofacies. 5.1.1. Comparisons between sedimentary environments In the delta front, there was a shallow-water and relatively high energy environment, which constantly alternated between anoxic environment and oxic conditions. The depositional environment change quickly, so the shale from the delta front is gray and brown mudstone interbedded with gray siltstone and fine sandstone (Fig. 5A and B). The main sedimentary structures are wavy cross-stratification (Fig. 5d) and horizontal bedding (Fig. 5A, B and 5F). Since the delta front was close to basin margin, a large number of stems and plant debris was deposited in the shale (Fig. 5C). Shelly fauna and bioturbation were also rich, and a large number of bioturbation structures are usually found in delta front siltstone (Fig. 12C). The prodelta was in front of the delta and is connected to deep water. The main sedimentary structures here are horizontal bedding (Fig. 4C), and the main deposits are black shale and partly thin siltstone (Fig. 4), with small cross stratification (Fig. 4A). Because the prodelta was usually located below the slope break, delta front sand occasionally slumped and was redeposited by gravity at downslope locations, creating slump deformation structures (Fig. 4F). Early black shale deposits were eroded, and black lacerated mud gravels were redeposited in gravity flow sands (Fig. 12B). In deep water, the hydrodynamic environment was weak, and the bottom of the deep lake was anoxic, so shelly fauna and bioturbation were lacking. As a result, the deep water shale is dark gray or black

the samples differ in their clay mineral content and fractures. The fractures in Fig. 10D were filled by calcite and organic matter, and the fracture density and connectivity in Fig. 10C is very poor, whereas the fractures density and connectivity in Fig. 10B is good, so the intensity of fluorescence for Fig. 10B is the highest.

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mudstone interbedded with lime shale, calcareous shale, thin siltstone, and thin layers of salt (Fig. 3A and B). The main sedimentary structures are horizontal bedding and massive structure (Figs. 3 and 12A).

5.1.2. Comparisons of sedimentary lithofacies The dominant minerals in the delta front shale are quartz with an average content of approximately 31%, while the average clay content is only approximately 32.3% (Fig. 13A). The pyrite content is low and most samples contain no pyrite (Fig. 8A). Therefore, the delta front shale displays mainly felsic-rich lithofacies, which are the S-4 (21%) and the MS-2 (29%), followed by the S-3 (19%) (Fig. 9C and Table 1). The dominant minerals in the prodelta shale are clay minerals and quartz, with an average quartz content of approximately 31.3%, and an increased average clay mineral content of approximately 38.55% (Fig. 13A). The pyrite content is low, and almost no samples contain it (Fig. 7A). Therefore, the main lithofacies are the MS-2 (33%), the M-2 (15%) and the S-4 (13%) (Fig. 9B). The dominant minerals in the deep water lacustrine shale are clay minerals, with an average clay mineral content of more than 40%, but the average quartz content is smaller, at only 19.1%. Although pyrite content is low, averaging approximately 1.5% (Fig. 13A), almost every sample contains it (Fig. 6A). Therefore, the main lithofacies for the deep water lacustrine shale are the M-2 (25%) and the MS-2 (26%). The main clay minerals in the lacustrine shale are mixed layers of illite-smectite (I/S) and illite (I) (Fig. 13B). From the delta front to deep water, the I/S content gradually decreases, while the content of illite gradually increases. In addition to the depositional environment, clay mineral content and composition is important for diagenesis, which is mainly affected by clay mineral composition, temperature, pressure, pore water pH, and other factors (O'Neil and Kharaka, 1976; Dypvik, 1983; Moore and Reynolds, 1989). This study shows that from the sag margin to the lake center, the clay mineral content of shale gradually increased, and felsic mineral content gradually decreased (Fig. 13A). The main lithofacies of the delta front are felsic-rich (the S-4 and the MS-2); the main deep water lithofacies are clay-rich lithofacies (the M-2, and the MS-2).

5.1.3. Shale oil-hosting The thin sections of shale samples show that shale oil-hosting is related to minerals and fractures. If the clay mineral content is high, the shale oil-hosting is lower. Oil is mainly concentrated along fractures, especially bed-parallel fractures. If fractures are developed in shale, these fractures will improve shale oil-hosting. In other words, oilhosting is better in laminated shale than in massive shale, and fracture can further improve oil-hosting. If these fractures are filled by calcite or other minerals, the shale oil-hosting is reduced and there is almost no fluorescence along these fractures. The relationship between shale oilhosting and felsic or carbonate minerals is not obvious.

Fig. 11. Thin sections and fluorescence of massive shale. Boreholes location in Fig. 1a. A): sample from W3, Es3, delta front, 3535.55m. Dominant minerals: Quartz: 23.1%, Feldspar: 9.2%, Carbonate rock: 37.7%, Salt rock: 7.0%, Clay mineral: 20.8%, Pyrite: 2.2%. Lithofacies: MS-1. Fluorescence characteristics: green fluorescence of muddy shale and dark, yellowish green fluorescence of dolomitic crystals and middle-dark. B): sample from W4, Es3, deep water lake, 3367.41m. Dominant minerals: Quartz: 28.9%, Feldspar: 18.5%, Carbonate rock: 17.2%, Salt rock: 2.7%, Clay mineral: 30.1%, Pyrite: 2.6%.: Lithofacies: S4. Fluorescence characteristics: pale green fluorescence of argillaceous cement and very dark, Stars dotted yellowish green fluorescence of few clastic particles and middle-dark. C): sample from W3, Es3, delta front, 3468m. Dominant minerals: Quartz: 32.4%, Feldspar: 12.4%, Carbonate rock: 13.3%, Salt rock: 8.0%, Clay mineral: 33.9%, Pyrite: 0%.: Lithofacies: S-4, microfractures filled with calcite. Fluorescence characteristics: pale green fluorescence of calcite crystals and dark, no fluorescence of microfractures. D): sample from W4, Es3, deep water lake, 3353.8m. Dominant minerals: Quartz: 22.3%, Feldspar: 10.2%, Carbonate rock: 19.7%, Salt rock: 1.3%, Clay mineral: 45.2%, Pyrite: 1.3%. Lithofacies: MS-2. Fluorescence characteristics: pale green fluorescence of matrix and very dark. E): sample from W8, Es3, prodelta, 4020.14m. Dominant minerals: Quartz: 8%, Feldspar: 5%, Carbonate rock: 20%, Salt rock: 1.3%, Clay mineral: 65%, Pyrite: 0.7%. Lithofacies: M-3. Fluorescence characteristics: no fluorescence of matrix, only sporadic yellow fluorescence of cement and very dark.

5.1.4. Total organic carbon (TOC) and types of organic matter Research results showed that total organic carbon (TOC) content ranged from 0.6 to 1.4%, up to 2.0% (Zhang, 2013). From the margin to the lake center, TOC gradually increases that the TOC of the delta front shale is the lowest, and the TOC of the deep lake is the greater, especially in the area of salt deposition, where TOC is the highest. The spatial distribution of TOC is controlled by depositional environment. According to the prior research (Zhang, 2013), the main types of organic matter in Es3 shale from the Dongpu sag are Ⅰ1, Ⅱ1 and type Ⅱ2. The main type of organic matter in delta front shale is type Ⅱ2, while the main type of organic matter in deep water shale is type Ⅰ1. In other words, the types of organic matter in the shale are also controlled by the depositional environment. From the margin to lake center, the type of organic matter changes from Ⅱ2 to type Ⅰ1 (Tissot and welte, 1978). 185

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Fig. 12. Sedimentary structure and lithology of different environment shales. A) the deep water shale, B) the prodelta shale, C) the delta front shale.

oxic, shallow water environment. In highly productive lakes, neither stratification, nor permanent anoxic conditions are needed for the preservation of organic matter. Rapid burial removes the organic matter from the oxygen-rich sediment-water interface, thereby enhancing preservation (Stein, 1986, 1990; Trabucho-Alexandre, 2015). These results show that shales are controlled by these macro depositional environment factors, such as the paleoclimate, tectonics (accommodation space), and redox conditions. If the macro environment is the same during a depositional stage in one basin, what factors control the composition, structures, mineral, lithofacies and spatial distribution of shales? Macquaker and Gawthorpe (1993) considered that the depositional process controlled the petrologic characteristics and geographic distribution of black shales. They defined five mudstone lithofacies from

5.2. Controlling factors and distribution model of lacustrine shale Climate is an important factor controlling sedimentation in lacustrine basins. Climate determines the biota, temperature, and salinity of a lake. In humid and temperate climates, shale will be dark colored and rich in organic matter, including plant debris rafted in by rivers; in arid climates, shale will be lighter in color, lean on organic matter, and will be interbedded with evaporites (Trabucho-Alexandre, 2015). Accommodation space determines water depth, depositional environment, and the geographic distribution of shale (Carroll and Bohacs, 1999; Potter et al., 2005). Reducing environments are considered the key factor behind the deposition of black, organic-matter-rich mud (Jiang, 2003; Boggs, 2009). If organic matter is abundant and the sedimentation rate is rapid enough, black shale can be deposited, even in the case of an 186

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Fig. 13. Minerals average content of shale. A): dominant minerals average content, CM: clay mineral, F: feldspar, CR: carbonate rock, SR: salt rock, P: pyrite, Q: quartz. B): clay minerals average content. K: kaolinite, C: chlorite, I: illite, I/S: mixed layer illite-smectite.

(the M-2, and the MS-2). From the sag margin to the lake center, the types of organic matter changed from type Ⅱ2 to type Ⅰ1, and the total organic carbon content gradually increased. These characteristics of sedimentary structure, lithology, mineral content, lithofacies and the distribution of the lacustrine shales changed with the change in depositional environment (or along the sediment transport path). The observed spatial distribution and the sedimentary characteristics are similar to those of the Dongying sag (Deng and Qian, 1990; Bao-Li and Yao, 2007; Yang et al., 2015; Wang et al., 2016), and the case of southern England (Macquaker and Gawthorpe, 1994). The structure, lithology, mineral content, lithofacies, and the distribution of the lacustrine shales in the Dongpu sag depended on the local depositional environment, that is, on the source and transport (or the sediment transport path), water depth, accommodation space. According to the study results, a spatial distribution model of lacustrine shale is proposed (Fig. 14). At the same time, this paper preliminarily analyzed shale oilhosting. The study results show that the mineral content, sedimentary structure, fracture, and environment are a few important factors controlling shale oil-hosting. Shale oil-hosting is related to minerals and fractures. If the clay mineral content is high, shale oil-hosting is poor. Shale oil and gas were mainly concentrated along fractures, especially bed-parallel fractures. The study results are very simple, however, and the relationship among these factors is not resolved in this paper. Next, the authors will further analyze pore type and pore networks, which are major controlling factors for storage and permeability.

the Kimmeridge Clay Formation in southern England. Using these lithofacies data, and considering the relative length of the sediment transport path, water-column processes, absolute water depth, elastic dilution, and early-diagenetic processes, they derived a new depositional model and predicted the distribution of the mudstone lithofacies at a basin margin (Macquaker and Gawthorpe, 1994). Although other authors shared their own ideas about Macquakcr's research results (Wignall, 1994), the results about the spatial distribution of the mudstone lithofacies were found to be reasonable. From the basin margin to deep-water along the sediment transport path, different mudstone lithofacies were deposited in different settings and depositional environments. Since lacustrine basins are so much smaller than marine basins, lake sediments are extremely sensitive to changes in climate, accommodation space, and source (Bohacs et al., 2000; Carroll and Bohacs, 2001; Huang et al., 2015). In the case of the third and fourth members of the Eocene Shahejie Formation in the Dongying sag, Bohai Bay Basin (the paleoclimate was the same as the Dongpu sag), the research results show that organicmatter-rich lamellar shale lithofacies was primarily deposited in the deep or semi-deep, quiet, saline, and anoxic water (Yang et al., 2015). The organic-matter-rich stratiform shale lithofacies was mainly deposited in the semi-deep or shallow anoxic water by mechanical and chemical deposition (Bao-Li and Yao, 2007). The massive shale lithofacies was mainly deposited in the shallow water (Deng and Qian, 1990). On the whole, from the margin to lake center, in order, the following lithofacies were deposited: sandstone lithofacies, organic matter massive shale lithofacies, organic matter stratiform shale lithofacies, and organic matter-rich lamellar shale lithofacies. The lithofacies of the lacustrine shales was controlled by the depositional environment (Wang et al., 2016). Trabucho (2015) considered that shales are the product of both local and global conditions (Trabucho-Alexandre, 2015). In other words, the shales of one basin have the same characteristics as other shales controlled by global environment. They also have their own unique characteristics of local environment. Indeed, shales can be deposited by a variety of processes in almost any environment (Stow et al., 2001; Schieber, 2011; Trabucho-Alexandre et al., 2012b). This paper shows that from the sag margin to the lake center, along the sediment transport path, the depositional environment progressed in order from delta front, to prodelta, and deep water lake with partial salt deposition. In this progression, the percentage of sandstone gradually decreased, and sandstone layers gradually became thin; while the percentage of shale gradually increased and shale layers gradually became thick (Figs. 12 and 14). The clay mineral content of shale gradually increased, and felsic mineral content gradually decreased (Fig. 13A). The main lithofacies of the delta front are felsic-rich (the S-4 and the MS-2); the main deep water lithofacies are clay-rich lithofacies

6. Conclusions According to the shale samples from different depositional environments in the Es3 in the Dongpu sag, the research results shows the following: (1) In the Dongpu sag, from the margin to lake center (or deep water), the depositional environment changed from delta front to prodelta and deep water lake; main lithologies of lacustrine shale changed from mudstone interbedded with sandstone to mainly mudstone with siltstone, salt, and carbonate rock. Laminated shale was mainly deposited in the deep lake. Felsic mineral content decreased gradually, and clay mineral content increased gradually from the sag margin to the lake center. Felsic mineral content in the delta front shale is the highest, while clay mineral content and pyrite content are the highest in for deep water shale. Shale lithofacies also changed with the depositional environment. The lithofacies of the delta front are mainly felsic-rich lithofacies (S-4 and MS-2). The lithofacies of the prodelta are mainly M-2, S-4, and MS-2. The lithofacies of the deep water shale are mainly clay-rich lithofacies (the M-2, MS-2). TOC and organic matter type also changed with 187

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Fig. 14. Lacustrine shale spatial distribution model. From sag margin to lake center, environment changed from delta front to prodelta and deep water lake with part salt deposition. Sandstone percent in layer gradually decreased and sandstone layer gradually became thin, while shale percent gradually increased and shale layer gradually became thick. The clay minerals content of shale gradually became more and felsic minerals content gradually became less. The main lithofacies of delta front are felsic-rich lithofacies (the S-4 and the MS-2). The main lithofacies of deep lake are clay-rich minerals lithofacies (the M-2, and the MS-2). From sag margin to lake center, the types of organic matter changed from type Ⅱ2 to type Ⅰ1. The tatal organic carbon content gradually increased.

the depositional environment. (2) These results show that shale deposition is not only controlled by macro depositional environment, including paleoclimate, tectonics, and redox conditions, but also by the local depositional environment. The sedimentary structure, lithology, mineral content, lithofacies, and spatial distribution of the lacustrine shales were controlled by the local depositional environment, such as the source and transport (or the sediment transport path), water depth, and accommodation space.

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