Effects of paleo sedimentary environment in saline lacustrine … · 2020-03-30 · quality source rocks in the Dongpu Depression. 1. Introduction Hydrocarbon accumulation in the

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Effects of paleo sedimentary environment in saline lacustrine basin on organic matter accumulation and preservation: A case study from the Dongpu Depression, Bohai Bay Basin, China

Ling Tang a,b, Yan Song a,b,*, Xiongqi Pang a,**, Zhenxue Jiang a,b, Yingchun Guo c,d,***, Hongan Zhang e, Zhihong Pan f, Hang Jiang g

a State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing, 102249, China b Unconventional National Gas Institute, China University of Petroleum, Beijing, 102249, China c Key Laboratory of Paleomagnetism and Tectonic Reconstruction, Ministry of Natural Resources, Beijing 100081, China d Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China e Zhongyuan Oilfield Branch, SINOPEC, Puyang City, Henan 457001, China f Department of Physics, University of Alberta, Canada g Oil and Gas Resources Strategy Research Center, Beijing, 100034, China

A R T I C L E I N F O

Keywords: Saline lacustrine basin Organic matter Enrichment and preservation Sedimentary environment Dongpu depression Shahejie formation

A B S T R A C T

The Dongpu Depression has abundant oil and gas resources, but the distribution of hydrocarbon resources and enrichment degrees of organic matter (OM) in the northern and southern Dongpu Depression have significant differences due to the diverse sedimentary environment. To gain deep insight into the effects of sedimentary environment on OM accumulation and preservation, a series of experiments including element geochemistry, organic geochemistry and isotope geochemistry were performed on core samples collected from the fresh, brackish and saline regions of the third member of the Eocene Shahejie Formation (Es3). The lacustrine closure, paleo-salinity, redox conditions, hydrodynamic conditions, paleo-climate and paleo-productivity were quanti-tatively evaluated. Controlling factors and OM enrichment models were evaluated and established. The results show that the Es3 Formation was deposited as the lake experienced closed saline to transitional and then fresh lake stages. The lacustrine deposits were accompanied by climate changes from dry to humid and strongly rifting during Es3 deposition. The weak hydrodynamic conditions (Zr/Rb) and paleo climate indices (δ18O, climate index, Fe/Mn, Sr/Cu and Al/Mg) imply that moderate humid and dry climate can encourage OM development and preservation. The paleo productivity indices (δ13C, P/Al, P/Ti and P content) are positively related to TOC indicating that high productivity promotes OM accumulation. All the paleo salinity indices (Z value, Sr/Ba and B/Ga) have positive correlations with TOC content, suggesting that OM enrichment increases with salinity to some extent. The positive relationships between the redox indices (V/Cr, V/Sc, U/Th, and δU) and TOC indicate that redox conditions are the most important for OM preservation. This study not only advances the theory of OM enrichment mechanism in saline basins, but also provides guidance for predicting the distributions of high- quality source rocks in the Dongpu Depression.

1. Introduction

Hydrocarbon accumulation in the saline lacustrine sedimentary environment is particularly common in the Cenozoic lacustrine basins in

China (Xu et al., 2019), such as those in the Shahejie Formation (Es) of the Dongying and Dongpu Depressions in the Bohai Bay Basin (Zhang et al., 2011, 2012), Qianjiang Formation in the Jianghan Basin (Zhang et al., 2003), Gancaigou Formation in the Qaidam Basin (Feng et al.,

* Corresponding author. State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing, 102249, China. ** Corresponding author. *** Corresponding author. Key Laboratory of Paleomagnetism and Tectonic Reconstruction, Ministry of Natural Resources, Beijing 100081, China.

E-mail addresses: [email protected] (L. Tang), [email protected] (Y. Song), [email protected] (X. Pang), [email protected] (Z. Jiang), [email protected] (Y. Guo), [email protected] (H. Zhang), [email protected] (Z. Pan), [email protected] (H. Jiang).

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https://doi.org/10.1016/j.petrol.2019.106669 Received 28 December 2018; Received in revised form 1 September 2019; Accepted 7 November 2019

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2013; Wang et al., 2015) and Fengcheng Formation in the Mahu Depression of Junggar Basin (Cao et al., 2015). Among the 200 petro-liferous basins in the world, 58% of oil fields are related to salt-bearing strata (Ma et al., 2000). Recently, hydrocarbon geochemistry in saline lacustrine basins has become a popular and difficult topic of study. Significant achievements have been made in the relationship between gypsum-salt rocks and hydrocarbon formation-migration-accumulation (Kirkland and Evans, 1981; Volozh et al., 2003; Manzi et al., 2005; Feng et al., 2013; Gao et al., 2015). However, there are few studies on the organic matter (OM) enrichment and preservation mechanism of source rocks in a saline lacustrine rift basin (SLRB) (Hu et al., 2018).

The OM enrichment mechanism in a saline lacustrine environment is the result of several factors, including the OM source, settling and preservation, which are commonly related to high OM input (Zhu et al., 2005b; Gallego et al., 2007; Xu et al., 2015), oxygen-deficiency in bot-tom water with a stratified water column (Ingall et al., 1993; Bentum et al., 2009) or their combined effects (Tyson, 2005). While there are several factors influencing OM enrichment, the productivity model and preservation model are widely used but not limited to the lacustrine environment (Arthur and Sageman, 1994; Carroll and Bohacs, 1999; Bohacs et al., 2000; Hofmann et al., 2000; Burdige, 2007; Passey et al., 2010). The former model emphasizes productivity (Pedersen and Cal-vert, 1990; Zhang et al., 2005), whereas the latter model stresses pres-ervation under stagnant saline and brackish water as the key to forming abundant OM (Demaison and Moore, 1980; Arthur and Sageman, 1994; Makeen et al., 2015). Smith and Bustin (1998) demonstrated that both increased productivity and improved preservation can be essential and complementary factors affecting organic-rich mud deposition in the Williston Basin. Mort et al. (2007) investigated that the preservation

model may explain the recorded TOC mass accumulation rate values. Meyers and Ishiwatari (1993) proposed that OM abundance in a lake was the result of the interaction of the lake basin structure, lake basin morphology and OM depositional process. Liu et al. (2016) considered that the degree of OM enrichment first increased and then decreased with increasing water salinity in the lacustrine shales of the Qaidam Basin. Lu et al. (2013) found that the OM enrichment increased with increasing water salinity and reducing conditions in shales of the Dongpu Depression. Zhang et al. (2017a,b) considered the high bio-logical productivity and the strong anoxic environment resulting from salt-water layering as the first-order constraints controlling OM enrichment in the Dongpu Depression. However, previous studies on the lacustrine OM enrichment mechanism mostly followed the explanations for marine OM enrichment, and focused less on the depositional envi-ronment differences between marine and lacustrine settings (Zhang et al., 2016). In addition, for the SLRB in the Dongpu Depression, the paleo-salinity conditions may have been particularly important for OM enrichment, the lacustrine closure, redox conditions, paleo productivity, hydrodynamic conditions, and paleo climate condition cannot be ignored. Comprehensive analyses of these factors influencing OM enrichment have rarely been performed, and an OM enrichment model in the Es3formation of the depression has not been established.

The Dongpu Depression is a typical continental saline lacustrine rift basin in China, with three dominant depositional environments (saline, brackish, and fresh) (Hu et al., 2018). Generally, the OM enrichment degree is the highest in the saline region, lower in the brackish region, and the lowest in the fresh region. Previously, many studies have been carried out on sedimentary environment and source rock evaluations (Chen et al., 2003; Ji et al., 2005; Liu et al., 2014). These previous

Fig. 1. Geological setting of the study area. (a) The location of the Bohai Bay Basin in China. (b) The location of the Dongpu Depression, which is marked with red lines. (c) Structural features of the Dongpu Depression and locations of sampling wells in the study area (after Chen et al., 2003; Tang et al., 2019). (d) NW-SE trending, cross section, whose location is shown in Fig. 1c (after Su et al., 2006). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

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studies showed that evaporites were the most important factors con-trolling the deposition of high-quality source rocks and that paleo-salinity controlled the OM enrichment in the Dongpu Depression (Lu et al., 2013; Zhang et al., 2017a,b). However, the mechanisms of OM accumulation and preservation in the Dongpu Depression have seldom been discussed (Lu et al., 2013); how the saline sedimentary environ-ment influences OM enrichment in the depression has also not been addressed. Therefore, it is imperative to study OM accumulation and preservation in the Es3 Formation.

In this study, core samples of Es3 Formation were selected from the saline water region (SWR) in the northern area, the brackish water re-gion (BWR) in the central area, and the fresh water region (FWR) in the southern area (Chen et al., 2003, 2012). Experiments including total organic carbon (TOC) analysis, major and trace element measurement, carbon and oxygen (C–O) isotopes of carbonates and X-ray diffraction (XRD) analysis were conducted on these core samples. The lacustrine closure was analyzed and the paleosalinity, redox conditions, hydro-dynamic conditions, climate and paleo productivity were quantitatively evaluated in the SWR, BWR and FWR during Es3 deposition. Then the main factors controlling OM enrichment and an OM enrichment model were evaluated and established. This study reinforces the theory of OM enrichment mechanism and demonstrates OM enrichment model in the

SLRBs, and improves understanding of the hydrocarbon generation mechanism, resource evaluation and distribution of high-quality source rocks prediction in the Dongpu Depression and other SLRBs in the world.

2. Geological setting

The Dongpu Depression is located in the southern Bohai Bay Basin, and is a typical continental SLRB abundant in oil and gas with this Bohai Bay Basin (Fig. 1a and b) (Zhang et al., 2017a,b; Hu et al., 2018). At present, the exploration area covers 5.3 � 103 km2, and more than 12.37 � 108 tons (8.66 � 109 bbl) of oil reserves and 3.68 � 1011 m3 of gas reserves have been found in this area (Hu et al., 2018; Xu et al., 2019). The Es3 layer (the third member of Eocene Shahejie Formation) is the main hydrocarbon source rock, and holds the main oil and gas res-ervoirs (Su et al., 2006; Zhang et al., 2017a,b; Tang et al., 2019). However, nearly 93.7% oil and 80% gas are enriched in the northern saline region, suggesting an extremely uneven distribution between the north and south (Chen et al., 2003; Gao et al., 2011, 2015; Liu et al., 2014). The Dongpu Depression is surrounded by the Luxi uplift to the east, Neihuang uplift to the west, Lankao uplift to the south and eastern depression of the Linqing subbasin to the north (Fig. 1c) (Chen et al., 2003; Du et al., 2008; Liu et al., 2014).

Fig. 2. Lithostratigraphy and depositional characteristics of the Dongpu Depression and their correspondence with tectonic movements and relative lake levels (after Ji et al., 2005).

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Table 1 TOC content, Carbon and Oxygen isotopes and saline minerals content of Es3 Formation in the Dongpu Depression.

Sample NO. Depth (m) Formation Lithofacies Region TOC (%) δ13CV-PDB (‰) δ18OV-PDB (‰) Z Value Gypsum (%) Anhydrite (%) Halite (%) Total saline minerals content (%)

1-PS8 4269.49 Es3U Mudstone FWR 0.25 � 4.5 � 7.8 114.20 0.1 / / 0.1 2-PS8 4271.39 Es3U Mudstone FWR 0.44 � 1.7 � 7.5 120.08 / 1.1 / 1.1 3-PS8 4557.22 Es3M Mudstone FWR 0.61 � 2.0 � 7.9 119.27 / / / 0.0 4-PS8 4561.68 Es3M Mudshale FWR 0.83 � 1.6 � 5.6 121.23 1.0 / / 1.0 5-PS8 4585.34 Es3M Mudshale FWR 1.15 0.6 � 9.2 123.95 0.9 1.0 / 1.9 6-PS8 4588.12 Es3M Shale FWR 1.22 � 0.7 � 3.7 124.02 0.9 / / 0.9 7-PS18-8 3165.55 Es3U Shale SWR 0.78 0.1 � 6.6 124.22 2.1 0.2 / 2.3 8-PS18-8 3170.20 Es3U Shale SWR 0.88 � 6.5 � 6.0 111.00 0.2 10.7 2.9 13.8 9-PS18-8 3174.65 Es3U Shale SWR 0.46 � 6.3 � 6.8 111.01 / 16.0 2.2 18.2 10-PS18-8 3179.80 Es3U Shale SWR 0.54 � 5.8 � 6.1 112.38 / 1.7 3.9 5.6 11-PS18-8 3182.54 Es3U Oil shale SWR 1.2 � 1.1 � 8.1 121.01 2.2 / 3.2 5.4 12-PS18-8 3187.10 Es3U Oil shale SWR 5.64 � 1.2 � 6.3 121.71 3.0 / 1.9 4.9 13-PS18-8 3193.00 Es3U Oil shale SWR 1.64 � 3.4 � 6.7 117.00 1.5 0.5 3.8 5.8 14-Hu96 3883.56 Es3M Oil shale SWR 0.74 � 2.6 � 6.6 118.69 1.7 / 0.3 2.0 15-Hu96 3885.80 Es3M Oil shale SWR 2.09 � 0.5 � 7.4 122.59 1.7 / / 1.7 16-Hu96 3886.69 Es3M Oil shale SWR 2.16 0.3 � 9.1 123.38 4.7 / / 4.7 17-Hu96 3888.01 Es3M Oil shale SWR 2.4 � 0.1 0.0 127.10 0.8 / 1.0 1.8 18-Hu96 3885.55 Es3M Oil shale SWR 2.6 � 0.1 � 8.2 123.01 3.1 / 0.3 3.4 19-Hu96 3888.39 Es3M Oil shale SWR 1.15 � 0.4 � 2.5 125.24 2.8 0.8 / 3.6 20-Hu96 4040.30 Es3M Oil shale SWR 0.39 � 7.5 1.2 112.54 2.8 0.8 / 3.6 21-Wen250 3604.80 Es3M Mudstone SWR 0.12 � 3.9 � 6.4 116.13 1.3 / / 1.3 22-Wen250 3671.30 Es3M Mudstone SWR 0.22 � 5.1 � 6.8 113.47 / 1.5 / 1.5 23-Wen250 3675.50 Es3M Mudstone SWR 1.32 � 0.4 � 9.0 122.00 6.3 0.3 / 6.6 24-Wen250 3678.50 Es3M Mudstone SWR 0.83 � 5.5 � 2.9 114.59 11.9 / 0.6 12.5 25-Wen250 3680.60 Es3M Mudstone SWR 1.26 � 0.7 0.2 125.97 6.1 0.7 / 6.8 26-Wen250 3682.00 Es3M Mudstone SWR 1.54 � 2.4 � 8.9 117.95 7.0 0.6 / 7.6 27-Wen201 3672.75 Es3L Mudstone SWR 2.04 1.8 � 8.1 126.95 3.6 0.8 / 4.4 28-Wen201 3675.05 Es3L Mudstone SWR 1.24 0.7 � 6.5 125.50 3.0 / / 3.0 29-Wen201 3676.95 Es3L Mudstone SWR 2.06 0.2 � 7.2 124.12 4.7 / / 4.7 30-Wen201 3678.25 Es3L Oil shale SWR 1.39 � 2.0 � 6.6 119.92 6.9 / / 6.9 31-Wen201 3680.00 Es3L Mudshale SWR 2.08 0.4 � 8.3 123.99 2.8 / / 2.8 32-PS4 3894.80 Es3M Mudshale BWR 0.47 � 3.2 � 6.8 117.36 1.5 0.4 / 1.9 33-PS4 4188.05 Es3M Mudstone BWR 0.27 � 4.2 1.4 119.40 0.8 1.0 / 1.8 34-PS4 4188.70 Es3M Mudstone BWR 1.85 � 4.7 2.8 119.07 / / / 0.0 35-PS4 4999.70 Es3L Shale BWR 0.8 1.9 � 10.8 125.81 2.4 0.4 / 2.8 36-PS4 5002.77 Es3L Shale BWR 0.7 1.8 � 9.3 126.36 1.6 / / 1.6 37*-Mao2 / Es3U Limestone BWR / 0.4 � 1.0 127.54 / / / / 38*-Mao2 / Es3U Limestone BWR / � 3.9 � 6.5 116.18 / / / / 39*-Mao2 / Es3U Limestone BWR / � 5.0 � 8.2 112.94 / / / / 40*-Ming1 / Es3U Dolomite BWR / � 2.1 � 9.1 118.48 / / / / 41*-Wei20 / Es3L Dolomite BWR / � 1.1 � 8.9 120.68 / / / / 42*-Wei185 / Es3L Dolomite BWR / 1.3 � 0.7 129.58 / / / / 43*-Wei185 / Es3L Dolomite BWR / 2.0 � 7.5 127.60 / / / /

Note: *provided by Ren et al. (2000); FWR: Fresh Water Region; SWR: Saline Water Region; BWR: Brackish Water Region; Z¼ 2:048ðδ13Cþ50Þþ0:498ðδ18Oþ50Þ (Keith and Weber, 1964), Total saline minerals content ¼ Gypsum þ Anhydrite þ Halite.

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The Paleogene strata in the Dongpu Depression consist of a lacustrine sediment suite with salt-bearing clastic rocks, which is dominated by fluvial and lacustrine facies with sand and mudstone deposits (Chen et al., 2003; Hu et al., 2018) (Fig. 2). The thickness of the Paleogene strata is nearly 6000 m, including the Kongdian Formation (Ek), Sha-hejie Formation (Es) and Dongying Formation (Ed). The Es3 member with a thickness of 3000 m can be divided into upper (Es3U), middle (Es3M) and lower (Es3L) submembers (Du et al., 2008; Hu et al., 2018; Tang et al., 2019). The Es3 member formed during the main rifting stage, at which time the water was semideep to deep lacustrine and deltaic facies were dominant (Gao et al., 2012). The lithology in Es3 mainly comprises gray mudstone, light gray siltstone, gypsum-salt rocks, and oil shale, which are interbedded with argillaceous dolomite and gypsum mudstone (Ji et al., 2005). In the northern Dongpu Depression, four sets of gypsum-salt strata (Es1 salt, Es3U salt, Es3M salt and Es3L salt) were depos-ited (Chen et al., 2003). The Es3L salt is the thickest and most widely distributed (Qu et al., 2003; Ji et al., 2005). The sedimentary centers of gypsum-salt rocks are located in Wenliu and Weicheng, with the thick-ness reaching 950 m in the north, but no salt rocks were deposited in the south (Su et al., 2006; Liu et al., 2014; Gao et al., 2015). The northern area had a typical saline lacustrine environment, the central area had a briskish water environment and the southern area had a fresh water environment (Chen et al., 2012; Tang et al., 2019) (Fig. 1c). The north saline lacustrine environment developed good-fair source rocks, which are mainly composed of sapropelic and humic kerogen, with an average TOC content of 1.54% and chloroform bitumen “A” of 0.182% (Chen et al., 2012; Tang et al., 2017). Conversely, the southern fresh lacustrine environment developed poor-moderate source rocks, with an average TOC content of 0.34% and chloroform bitumen “A” (0.0244%), which is mainly composed of partial humic mixed and mixed type kerogens (Chen et al., 2012; Tang et al., 2017).

3. Samples and experimental methods

3.1. Samples selection

Thirty-six core plug samples of the Es3 Formation were obtained from six wells (Hu96, PS18-8, Wen201, Wen250, PS4 and PS8) drilled in

the Dongpu Depression, which can represent the three different sedi-mentary environments; the SWR (Hu96, PS18-8, Wen250 and Wen201), BWR (PS4) and FWR (PS8). A series of experiments including TOC analysis, major and trace element measurement, inorganic C–O isotopes of carbonates and XRD analysis were performed. In addition, for further study, more than 618 groups of element analytical data were collected from Zhongyuan Oil Field Company, and analytical 7 groups of C–O isotopes were cited from previous research to make a better explanation for this study (Ren et al., 2002) (Table 1).

3.2. Experimental methods

The TOC content was determined with a LECO CS-400 analyzer in the State Key Laboratory of China University of Petroleum (Beijing). First, these core plug samples were crushed into powers (80 mesh), and 10% hydrochloric acid was then applied to remove carbonates. Next, the hydrochloric acid was washed away with deionized water until neutral, the samples were dried in an oven for 2 h (80 �C), and finally the samples were tested on the machine with a test sensitivity of 10� 13 mg/g. The major and trace elements were measured in the Analytical Laboratory of the Beijing Research Institute of Uranium Geology (ALBRIUG) using a PW2404 Wavelength dispersive X-ray fluorescence spectrometer. The powder samples (nearly 1 g) were preheated in a muffle oven at 1000 �C for 90 min to remove OM and carbonates, and the weight loss was recorded. Later, approximately one-half of the ashed sample was mixed with 8 times as much lithium tetraborate (Li2B4O7) and fused into glass- beads, which were analyzed by X-ray fluorescence (XRF, Rigaku 100e). The XRD analysis was performed with a Rigaku D/max-2500PC analyzer under conditions of 26 �C and 31% humidity. The test conditions were as follows: Cu target, K radiation; 40 kV tube voltage, and 30 mA tube current.

For the inorganic C–O isotopes of carbonate, the phosphoric acid (H3PO4) method was used with an MF-ISOPRIME mass spectrometer. First, OM was removed from the powder samples (200 mesh) by hydrogen peroxide, and the samples were baked at 105 �C for 12 h. Then, 20 mg of sample was sent to the reactor with 4 mL H3PO4. Next, the CO2 was collected and analyzed using an isotope mass spectrometer. The GBW0445 standard was used to ensure the accuracy of the tests, and

Fig. 3. (a) Frequency histogram of the Mn/Sr ratios of the Es3 Formation in the Dongpu Depression. (b) Frequency histogram of δ13C. (c) Frequency histogram of δ18O. (d) Cross plot of δ13C and δ18O values for Es3 Formation, which has a weak correlation coefficient (R2 ¼ 0.1778).

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the unit standard was V-PDB with an error range below 0.1‰ for the C–O isotopes obtained in the experiments.

3.3. Sample preservation assessment

Previous studies showed that isotope exchange reactions occur in the late diagenesis process for lacustrine carbonate rocks, especially for the oxygen isotopes, which are was more susceptible to epigenetic alteration processes, thus causing significant decreases in δ13C and δ18O and losing part or all of the original information (Derry et al., 1994; Kaufman and Knoll, 1995). Therefore, assessing the original conservation of samples is particularly essential before analyzing the depositional environment with C–O isotopes. Derry et al. (1992) reported that, when the Mn/Sr ratio is less than 10 (a more stringent standard is less than 2–3), car-bonate can usually retain its original isotope composition (Derry et al., 1994; Kaufman and Knoll, 1995). In Fig. 3a, the Mn/Sr ratios of all samples are less than 10. The Mn/Sr ratios of approximately 93.7% of the samples are less than 3, indicating that these samples basically maintain their original C–O isotope compositions. Previous research showed that when δ18O ranges from � 10‰ to 5‰, δ13C may change slightly. When δ18O is lower than � 10‰, δ13C may change considerably (Derry et al., 1994; Kaufman and Knoll, 1995). From Fig. 3b and c, δ18O varies between � 10.8‰ and 2.8‰ with an average of � 5.93‰ (only one sample is lower than � 10‰), and δ13C varies between � 7.5‰ and 1.98‰ (average � 1.70‰). In addition, the square of the correlation coefficient (R2) between carbon and oxygen isotopes can reflect whether samples have been affected by diagenetic alteration (Talbot and Kelts, 1990). As Fig. 3d displays, the weak correlation (R2 ¼ 0.1778 < 0.5) of C–O isotopes indicates that the samples are not have been affected by

diagenetic alteration. Combining with the above three methods, the C–O isotope compositions of these samples from Es3 Formation in the Dongpu Depression were not affected by the diagenetic process or late diagenetic alteration.

4. Results

4.1. TOC content and carbon-oxygen isotopes

Stratigraphically, from Es3L to Es3M and then to Es3U, the average TOC decreases from 1.47% to 1.36% and then to 0.77% (Fig. 4a3). Areally in the SWR, BWR and FWR, the average TOC contents are 1.30%, 0.82% and 0.75%, respectively (Fig. 4b3), showing a decreasing tendency as salinity decreases. The OM abundance is the highest in the SWR and in the Es3L and Es3M submembers. Stratigraphically, from Es3L to Es3M and then to Es3U, the average value of δ13C decreases from 0.70‰ to � 2.24‰ and then to � 3.15‰ (Fig. 4a1). The average values of δ13C are � 1.65‰, � 1.4‰ and � 2.08‰ areally in the SWR, BWR and FWR, respectively (Fig. 4b1). Stratigraphically, the δ18O value increases from � 7.38‰ to � 4.77‰ and then decreases to � 6.67‰ from Es3L to Es3M then to Es3U

(Fig. 4a2). The average values of δ18O are � 5.12‰, � 5.38‰ and � 6.95‰ areally in the SWR, BWR and FWR, respectively (Fig. 4b2). Both δ13C and δ18O values are highest in the SWR (Fig. 4). Overall, strati-graphically, average values of TOC content and δ13C decrease while δ18O increases from Es3L to Es3U. Areally, the average values of TOC, δ13C and δ18O increase from the FWR to the SWR.

Fig. 4. (a1,a2,a3) maximum, average and minimum values of δ13C, δ18O and TOC content in Es3L, Es3M and Es3U submembers in the Dongpu Depression. (b1, b2, b3) the maximum, average and minimum values of δ13C, δ18O and TOC content in the FWR, BWR and SWR of the Dongpu Depression.

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4.2. Saline minerals and paleosalinity calculation

Table 1 presents several saline minerals, including gypsum, anhy-dride and halite. The content of gypsum varies from 0.1% to 11.9% (average 2.98%); anhydride ranges from 0.2% to 19.0% (average 2.26%); and halite ranges from 0.3% to 3.9% (average 2.01%). In Table 1 and Fig. 5a, the total content of saline minerals ranges from 0.1% to 18.2% with an average of 4.23%. The averages of total saline minerals display an increasing tendency from the FWR (0.83%) to the BWR (2.03%) and to the SWR (5.40%).

C–O isotopes can be used to recover paleosalinity. Previous research has confirmed that there is a positive correlation between salinity and C–O isotopes (Epstein and Mayeda, 1953; You et al., 2002). Keith and Weber (1964) combined δ13C and δ18O to investigate paleosalinity, and the formula is as follows:

Z¼ 2:048ðδ13Cþ 50Þþ 0:498ðδ18Oþ 50Þ (1)

Where Z represents paleo salinity. From Table 1 and Fig. 5b, the Z value varies between 111 and 127 with an average of 120.3 in the Dongpu Depression, and the averages of the Z value show a slight increases southward from the FWR (120.46) to the BWR (121.75) and then to the SWR (122.45), indicating that the salinity increases northward rather than southward.

Boron (B) has a linear relationship with salinity in water (Walker and Price, 1963; Walker, 1968; Coach, 1971). The Coach formula is selected to calculate salinity because this formula considers the adsorption ca-pacities of various clay minerals for boron and has a wide salinity applicability ranging from 1‰ to 35‰, the formula is as follows:

LogSp¼ðLogBc � 0:11Þ = 1:28 (2)

Bc¼B = ð4Xiþ 2XmþXkÞ (3)

Where Sp is paleosalinity, (‰); Bc denotes the corrected boron content, (ug/g); and Xi, Xm, and Xk represent illite, montmorillonite and kaolinite content, (%), respectively; the front coefficients represent adsorption capacities of boron to various clay minerals, the larger the coefficient is, the stronger the adsorption capacity. All the data are from trace element experiments and X-ray diffraction experiments. The paleosalinity varies between 2.7‰ and 23.9‰ with an average of 12.7‰ in the Es3 For-mation in the Dongpu Depression. The averages of salinity show an increasing tendency from the FWR (8.99‰) to the BWR (12.33‰), and then to the SWR (14.56‰) (Table 2 and Fig. 5c).

The Sr/Ba ratios range from 0.28 to 28 with an average of 2.34 (Fig. 5d). Among all 618 samples, the Sr/Ba ratios of 96 samples (approximately 15.5%) are lower than 0.6, and the Sr/Ba ratios of 220 samples (approximately 35.6%) range from 0.6 to 1.0; the rest are all larger than 1.0. Areally in the BWR and SWR, 89.5% and 83.2% samples of Sr/Ba ratios are larger than 0.6, respectively (Fig. 5d). The B/Ga ratios range from 0.46 to 14.8 with an average of 4.76. In the FWR, the B/Ga ratios of all the samples are lower than 7. In the BWR and SWR, the B/Ga ratios of 70 samples (56%) and 279 samples (58.9%) are larger than 4, respectively (Fig. 5e).

4.3. Major and trace elements

The major elements Si, Al, Ca and Fe are the dominant constituents with averages of 42.87%, 13.21%, 12.12% and 6.13%, respectively (Fig. 6a). Si is the predominant constituent among the major elements in the samples. In the SWR, P, Na, Mn and TOC content are more enriched than that of in the BWR and FWR. The trace elements Sr, Ba, Zr, Rb, Cr and Zn are the dominant constituents with averages of 1109 μg/g, 535

Fig. 5. (a, b, c) Maximum, average and minimum values of total saline minerals, Z value and paleosalinity in the FWR, BWR and SWR of the Dongpu Depression. (d, e) Frequency histogram of Sr/Ba and B/Ga ratios of the Es3 Formation in the Dongpu Depression.

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μg/g, 98 μg/g, 92 μg/g, 82 μg/g and 60 μg/g, respectively (Fig. 6b).

5. Discussion

5.1. Lacustrine closure

In continental lacustrine sediments, the primary C–O isotope com-positions in carbonates are very important indicators for studying the

paleo sedimentary environment and paleo climate conditions (Leng and Marshall, 2004). In an open fresh water lacustrine system, as the water column has a very short residence time, the characteristics of C–O isotope compositions in carbonates document information about injec-ted water. Both δ13C and δ18O values are negative and there is no obvious correlation between them. However, in a closed saline water lacustrine system, lake evaporation plays a decisive role in the charac-teristics of isotope compositions (Leng and Marshall, 2004). With

Table 2 The clay minerals, Boron content and paleosalinity of Es3 Formation in the Dongpu Depression.

Sample NO. Depth (m) Formation Lithology Region TOC (%) B (ug/g) I (%) K (%) C (%) I/S (%) I–S (%) M (%) Salinity (‰)

1-Hu96 3883.56 Es3M Oil shale SWR 0.74 242 38 8 5 49 25 12.25 23.43 2-Hu96 3885.80 Es3M Oil shale SWR 2.09 77.6 37 14 6 43 25 10.75 10.09 3-Hu96 3886.69 Es3M Oil shale SWR 2.16 113 40 3 3 54 25 13.50 12.33 4-Hu96 3888.01 Es3M Oil shale SWR 2.40 120 48 3 2 47 25 11.75 12.71 5-Hu96 3885.55 Es3M Oil shale SWR 2.09 104 34 9 5 52 25 13.00 12.24 6-Hu96 3888.39 Es3M Oil shale SWR 3.42 222 52 4 4 40 15 6.00 20.54 7-Hu96 4040.30 Es3M Oil shale SWR 0.39 54.9 46 6 5 43 15 6.45 7.06 8-PS18-8 3165.55 Es3U Shale SWR 0.78 62.9 48 9 3 40 20 8.00 7.93 9-PS18-8 3170.20 Es3U Shale SWR 0.88 113 83 1 3 13 10 1.30 11.56 10-PS18-8 3174.65 Es3U Shale SWR 0.46 192 85 1 5 9 10 0.90 17.75 11-PS18-8 3179.80 Es3U Shale SWR 0.54 153 85 1 1 13 10 1.30 14.41 12-PS18-8 3182.54 Es3U Oil shale SWR 1.20 43 82 1 1 16 10 1.60 5.35 13-PS18-8 3187.10 Es3U Oil shale SWR 5.70 83.4 85 1 1 13 10 1.30 8.97 14-PS18-8 3193.00 Es3U Oil shale SWR 1.64 95.4 54 1 1 44 15 6.60 10.18 15-PS4 3894.80 Es3U Mudshale BWR 0.47 110 47 4 8 41 20 8.20 12.42 16-PS4 4188.05 Es3M Mudstone BWR 0.27 101 48 5 8 39 15 5.85 11.57 17-PS4 4188.70 Es3M Mudstone BWR 1.85 59.3 41 20 12 27 15 4.05 8.82 18-PS4 4999.70 Es3M Shale BWR 0.80 58 51 3 8 38 15 5.70 7.40 19-PS4 5002.77 Es3M Shale BWR 0.70 53.7 55 2 6 37 15 5.55 6.79 20-PS8 4269.49 Es3M Shale FWR 0.25 65.4 41 9 18 32 20 6.40 9.40 21-PS8 4271.39 Es3M Shale FWR 0.44 44.2 36 7 11 46 20 9.20 6.43 22-PS8 4557.22 Es3M Shale FWR 0.61 87 51 8 8 33 20 6.60 10.55 23-PS8 4561.68 Es3M Mudshale FWR 0.83 63.8 42 9 7 42 20 8.40 8.33 24-PS8 4585.34 Es3M Mudshale FWR 1.15 72.6 26 17 9 48 25 12.00 10.18 25-PS8 4588.12 Es3M Shale FWR 1.22 62.6 32 19 8 41 25 10.25 9.03 26-Wen201 3672.75 Es3L Mudshale SWR 2.04 57.4 69 9 6 16 15 2.40 7.39 27-Wen201 3675.05 Es3L Mudstone SWR 1.24 38.7 49 4 5 42 25 10.50 5.40 28-Wen201 3676.95 Es3L Mudstone SWR 2.06 75.8 45 11 5 39 25 9.75 9.55 29-Wen201 3678.25 Es3L Oil shale SWR 1.39 81.8 56 13 6 25 25 6.25 10.21 30-Wen201 3680.00 Es3L Mudshale SWR 2.08 57.4 56 8 5 31 25 7.75 7.46 31-Wen250 3604.80 Es3M Mudstone SWR 0.12 136 60 3 12 25 15 3.75 14.80 32-Wen250 3671.30 Es3M Mudstone SWR 0.22 102 56 4 7 33 15 4.95 11.43 33-Wen250 3675.50 Es3L Mudstone SWR 1.32 93.9 52 8 5 35 20 7.00 10.91 34-Wen250 3678.50 Es3L Mudshale SWR 0.83 116 49 11 6 34 20 6.80 13.26 35-Wen250 3680.60 Es3L Mudstone SWR 1.26 26.2 60 4 6 30 20 6.00 3.94 36-Wen250 3682.00 Es3L Mudstone SWR 1.54 53.8 62 1 3 34 20 6.80 6.62 37*-Pu6-33 3403.20 Es3M Mudstone SWR 1.76 116 13 8 6 72 20 14.48 13.48 47*-Pu6-33 3533.00 Es3M Mudstone SWR 1.00 84 20 5 4 71 15 10.68 9.87 48*-Pu6-65 3348.88 Es3M Mudstone SWR 2.58 78 24 6 3 68 25 16.88 9.54 68*-Pu6-65 3205.61 Es3M Mudstone SWR 0.46 133 18 3 3 76 15 11.46 13.81 69*-Wei69 3541.39 Es3M Mudstone SWR 1.10 108 32 2 4 63 15 9.45 11.67 70*-Wei69 3547.50 Es3M Mudstone SWR 3.52 128 46 3 4 48 15 7.25 13.26 71*-Wei69 3555.58 Es3M Mudstone SWR 1.71 125 33 2 2 64 15 9.62 12.85 72*-Wei69 3560.01 Es3M Mudstone SWR 2.70 60 26 3 2 69 15 10.35 7.37 73*-Wen128 3664.60 Es3M Mudstone SWR 0.52 155 27 0 14 59 15 8.85 16.75 74*-Wen15-1 2094.20 Es3M Mudstone SWR 0.13 172 21 8 7 64 25 16.00 18.63 75*-Wen15-1 2150.96 Es3M Mudstone SWR 0.16 64 11 12 7 70 25 17.50 8.94 76*-Wen210 3717.92 Es3M Mudstone BWR 0.13 134 28 3 4 65 15 9.81 13.94 88*-Wen210 3935.15 Es3M Mudstone BWR 0.15 175 37 2 4 57 10 5.71 16.75 89*-Wen248 3316.72 Es3L Mudstone SWR 0.19 212 33 0 19 48 15 7.20 22.29 94*-Wen248 3382.34 Es3L Mudstone SWR 2.43 123 38 6 6 50 20 10.00 13.60 95*-Wen260 3637.91 Es3M Mudstone SWR 0.34 169 15 5 6 74 15 11.15 17.30 99*-Wen260 3703.37 Es3M Mudstone SWR 0.32 69 33 5 7 55 10 5.45 8.53 100*-Wen33-105 2881.80 Es3U Mudstone SWR 0.67 162 11 5 5 80 30 23.94 17.61 103*-Wen33-105 2907.02 Es3U Mudstone SWR 0.09 103 18 6 6 70 30 21.00 12.46 104*-Wen88-1 3570.66 Es3M Mudstone SWR 0.19 160 27 2 3 68 15 10.20 15.90 112*-Wen88-1 3703.10 Es3M Mudstone SWR 0.16 152 32 3 3 62 10 6.21 15.00 113*-Wen92-33 2797.80 Es3M Mudstone SWR 0.14 183 15 6 7 72 20 14.40 19.12 114*-Wen92-33 2808.22 Es3M Mudstone SWR 0.14 125 14 7 8 71 20 14.20 14.42 115*-Wen92-33 2855.30 Es3M Mudstone SWR 0.10 168 14 6 7 73 15 10.95 17.59

Note: Only part of data is listed here, * provided by Zhongyuan Oil Field Company of SINOPEC; B-boron, I-illite, K-kaolinite, C-chlorite, M-montmorillonite, I/S:illite/ smectite formation, I–S: illite/smectite ratio; LogSp ¼ ðLogBc � 0:11Þ=1:28; Sp: paleo salinity, ‰; Bc: corrected Boron content, ug/g; Xi, Xm, Xk: represent illite, montmorillonite and kaolinite content, respectively, % (Walker and Price, 1963; Walker, 1968; Coach, 1971);

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increasing lake evaporation intensity increasing, the relatively lighter δ16O and δ12C preferentially escape from water, resulting in increasingly more δ18O and δ13C remaining in the lake. The correlation between δ13C and δ18O values is obviously synchronous, and the stronger the closure is, the higher the correlation coefficient (Talbot, 1990; Li and Ku, 1997).

From Fig. 7, the casting points of C–O isotopes in open lake systems are generally in the third quadrant, such as those for Greifensee Lake, Henderson Lake and Israel Lake; the casting points of C–O isotopes in closed lake system are in the first and second quadrants, such as those for the Great Salt Lake, Turkana Lake and Natron-Magadi Lake (Talbot and Kelts, 1990). In the Es3 Formation of the Dongpu Depression, most C–O isotope casting points are in the second and third quadrants. That is, the casting points plot in the open lake and closed lake fields (Fig. 7). In the Es3L Formation, most samples are plot in the second quadrant (except for two samples in the third quadrant), indicating that the Es3L basically

represents a closed lake system. The sample distribution in Es3M are more dispersed. Samples from the FWR are in the third quadrant indicating that it was an open sedimentary environment; samples from the SWR and FWR are in the third and fourth quadrants (mainly in the third quadrant), showing that these areas had an alternating open-closed sedimentary environment. In the Es3U, all the samples are in the third quadrant, indicating that it was an open sedimentary environment. Previous studies (Ji et al., 2005; Sun et al., 2014) also supported the lake closure hypothesis during the Es3 deposition. The thickness of salt beds in Es3L is the largest at nearly 600 m, indicating a closed saline system; meanwhile, the thicknesses of salt beds in Es3M and Es3U are relatively small in a semiclosed and semiopen depositional environment (Sun et al., 2014). Ji et al. (2005) determined that the lake level changed frequently during Es3 sedimentation, resulting in the frequent alterna-tion of closed and open depositional environment.

5.2. Paleo sedimentary environment

5.2.1. Paleosalinity conditions Paleosalinity is a very important proxy indicating sedimentary

environment variations during geological history (Walker and Price, 1963; Adams et al., 1965; Coach, 1971). As paleosalinity controls the growth and reproduction of organisms, it is of great significance for OM enrichment and preservation conditions. Many quantitative methods have been proposed to recover paleosalinity (Walker and Price, 1963; Adams et al., 1965; Coach, 1971; You et al., 2002; Yang et al., 2015).

In a saline diagenetic environment, diagenetic minerals are mainly gypsum, anhydride, halite and potassium salt (Wang et al., 2003). The formation sequence of the saline minerals is carbonates (calcite and dolomite), gypsum minerals (anhydride and gypsum), mirabilite and halite, and potassium. When the lake salinity ranges from 12‰ to 13‰, gypsum begins to depositing; when salinity reaches 27.5‰, halite begins to precipitate, when salinity is 33‰ or more, potassium begins to pre-cipitate (Zheng et al., 2012). Therefore, the saline mineral content directly reflects lake salinity, indicating an arid evaporation sedimen-tary environment. The higher content of saline minerals represents higher salinity of the lake water and a more arid climate (Ji et al., 2005). From the curves of saline minerals (Fig. 8), in the FWR and BWR, the

Fig. 6. Average values of some elements in the FWR, BWR and SWR of the Dongpu Depression. (a) TOC content and major elements in the FWR, BWR and SWR of the Dongpu Depression. (b) Trace elements in the FWR, BWR and SWR of the Dongpu Depression.

Fig. 7. Cross plots of δ13C and δ18O values in the Es3 Formation of the Dongpu Depression (after Talbot and Kelts, 1990).

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saline mineral content are relatively low and almost lack halite deposits. However, in the SWR, the saline mineral content is as high as 18.2%, especially gypsum and anhydride, for which the highest contents can reach 11.9% and 10.7% (Samples 13 and 14, respectively), individual samples (Samples 14 to sample 18) also contain halite with the maximum value reaching 3.9% (Table 1). The TOC content and total saline mineral content display a certain regularity. When the content of saline mineral is suitable, the paleosalinity is constructive for OM accumulation and preservation. When the paleosalinity is extremely high, even when it is not suitable for living organisms, it inhibits OM enrichment. For samples 13, 14 and 29, the total saline minerals are very high, while the TOC contents are very low.

The average Z value is 120.3, indicating a saline lacustrine

depositional environment during the Es3 deposition in the Dongpu Depression (Hu et al., 2018; Xu et al., 2019). The average Z values are ordered as SWR > BWR > FWR (Table 1). The Z value has an obvious positive correlation with TOC content; with increasing Z value, OM is more abundant (Fig. 9a). OM is more abundant in the SWR than BWR and FWR, showing that the saline environment is more conductive for OM enrichment. The average of paleosalinity is 12.7‰ during the Es3 deposition in the Dongpu Depression (Table 2). The relationship be-tween paleosalinity and TOC content is not linear (Fig. 9b). When the salinity is lower than 7‰, the TOC content increases gradually with increasing salinity; when the paleosalinity rises from 7‰ to 15‰, the TOC content remains stable with the maximum value in this salinity interval, and the salinity interval is the most favorable for OM

Fig. 8. Variations in saline mineral compositions and TOC content in the FWR, BWR and SWR. Total Gypsum-salt Contents ¼ Gypsum þ Anhydrite þ Halite.

Fig. 9. Relationship between TOC content and paleosalinity indices. (a) Relationship between TOC content and Z value. (b) Relationship between TOC content and paleo-salinity. (c) Relationship between TOC content and the value of Sr/Ba. (d) Relationship between TOC content and the value of B/Ga.

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Table 3 The ratios of major and trace elements of Es3 Formation in the Dongpu Depression.

Sample NO. Depth (m) TOC (%) Sr/Ba B/Ga V/Sc V/Cr δU U/Th U/Mo V/Mo Re/Mo Eu/Eu* Zr/Rb Climate Index Fe/Mn Cr/Cu Al/Mg P/Al P (%) P/Ti

1-PS8 4269.49 0.25 0.69 2.97 6.46 1.16 0.80 0.22 1.30 42.45 0.0008 0.78 0.94 0.57 83.77 2.97 7.66 0.01 0.14 0.20 2-PS8 4271.39 0.44 1.29 2.99 6.30 0.72 0.97 0.31 0.66 14.94 0.0004 0.76 0.92 0.92 167.18 1.74 7.30 0.01 0.12 0.24 3-PS8 4557.22 0.61 0.42 4.60 5.75 0.41 0.81 0.23 1.70 36.31 0.0021 0.73 1.71 0.46 128.49 8.65 12.47 0.02 0.25 0.39 4-PS8 4561.68 0.83 3.42 4.49 7.25 1.27 1.28 0.59 0.58 7.38 0.0008 0.76 1.12 0.28 72.61 3.09 5.70 0.01 0.13 0.29 5-PS8 4585.34 1.15 2.14 4.13 6.05 0.91 0.91 0.28 2.29 47.52 0.0027 0.74 1.45 0.47 93.36 4.83 14.32 0.01 0.15 0.24 6-PS8 4588.12 1.22 4.13 4.89 5.64 0.73 1.14 0.44 1.80 25.08 0.0038 0.72 1.38 0.27 70.25 5.83 5.22 0.01 0.12 0.26 7-PS18-8 3165.55 0.78 1.40 4.91 7.38 1.43 1.25 0.56 1.07 16.69 0.0007 0.77 1.01 0.24 46.58 2.00 8.60 0.01 0.12 0.37 8-PS18-8 3170.20 0.88 2.52 8.43 6.29 1.00 0.91 0.28 0.76 19.41 0.0015 0.71 0.88 0.21 89.60 4.21 1.35 0.01 0.12 0.31 9-PS18-8 3174.65 0.46 1.24 8.35 6.41 1.18 0.83 0.24 1.12 32.79 0.0007 0.76 0.77 0.42 135.80 2.71 0.58 0.01 0.18 0.29 10-PS18-8 3179.80 0.54 0.92 6.65 7.34 1.23 0.95 0.30 0.88 19.34 0.0006 0.73 1.05 0.38 165.67 2.79 9.86 0.01 0.17 0.24 11-PS18-8 3182.54 1.20 4.10 7.08 6.53 0.25 1.13 0.43 0.55 8.18 0.0006 0.67 1.45 0.99 780.74 4.30 7.94 0.01 0.07 0.32 12-PS18-8 3187.10 5.70 1.91 5.25 10.87 1.55 1.53 1.09 0.39 4.66 0.0003 0.67 1.07 0.35 129.09 1.98 4.95 0.02 0.24 0.55 13-PS18-8 3193.00 1.64 1.78 7.07 6.93 1.58 1.25 0.56 0.80 12.03 0.0013 0.79 1.07 0.15 55.80 2.41 4.06 0.02 0.19 0.45 14-Hu96 3883.56 0.74 0.88 9.88 5.29 1.09 0.89 0.27 3.02 58.32 0.0029 0.73 1.22 0.56 99.12 3.56 7.17 0.00 0.08 0.12 15-Hu96 3885.80 2.09 2.17 9.18 11.20 1.86 1.75 2.37 0.41 2.03 0.0005 0.59 1.12 0.10 23.81 1.28 10.66 0.03 0.17 0.73 16-Hu96 3886.69 2.16 5.20 10.09 8.82 1.44 1.63 1.47 0.90 6.17 0.0009 0.76 1.03 0.18 45.77 2.41 14.04 0.01 0.13 0.35 17-Hu96 3888.01 2.40 1.70 13.20 9.68 1.35 1.89 5.49 3.46 5.07 0.0009 0.74 1.15 0.15 30.56 2.57 10.27 0.03 0.19 0.72 18-Hu96 3885.55 2.09 2.74 8.97 8.19 0.74 1.41 0.80 0.43 4.17 0.0005 0.72 1.11 0.14 48.95 4.25 10.76 0.01 0.10 0.28 19-Hu96 3888.39 3.42 1.86 12.07 8.87 1.43 1.30 0.63 0.75 12.57 0.0009 0.73 0.92 0.41 80.96 3.32 17.55 0.02 0.24 0.46 20-Hu96 4040.30 0.39 4.90 4.90 6.49 1.14 0.92 0.28 1.81 56.73 0.0035 0.82 0.79 0.15 48.45 3.53 2.11 0.03 0.17 0.66 21-Wen250 3604.80 0.12 0.78 5.64 6.23 0.74 0.81 0.23 4.62 116.90 0.0046 0.77 1.13 0.57 192.20 9.58 4.21 0.01 0.17 0.22 22-Wen250 3671.30 0.22 0.73 4.45 6.77 0.76 0.72 0.19 3.63 136.65 0.0051 0.83 0.90 0.46 110.75 3.30 1.12 0.01 0.13 0.19 23-Wen250 3675.50 1.32 1.39 5.46 6.64 0.95 1.35 0.69 0.63 6.37 0.0009 0.70 1.02 0.41 85.60 3.09 5.62 0.01 0.18 0.37 24-Wen250 3678.50 0.83 1.55 6.24 6.23 1.31 1.07 0.39 0.70 11.89 0.0006 0.77 0.85 0.54 120.63 2.11 5.63 0.01 0.13 0.24 25-Wen250 3680.60 1.26 2.91 3.25 5.98 0.75 1.68 1.74 0.51 2.32 0.0007 0.67 1.54 0.12 44.86 3.33 0.92 0.02 0.12 0.53 26-Wen250 3682.00 1.54 1.51 3.79 7.03 0.94 1.07 0.38 0.50 9.20 0.0012 0.74 1.06 0.32 57.36 2.81 9.84 0.01 0.10 0.24 27-Wen201 3672.75 2.04 3.26 5.13 9.43 1.03 1.82 3.42 1.97 5.62 0.0012 0.70 1.18 0.17 40.78 3.34 10.07 0.01 0.10 0.32 28-Wen201 3675.05 1.24 2.01 4.10 7.52 0.79 1.28 0.59 0.44 6.17 0.0006 0.89 1.49 0.16 27.57 4.61 9.16 0.01 0.09 0.34 29-Wen201 3676.95 2.06 2.34 4.54 6.08 0.52 0.96 0.31 0.55 12.07 0.0007 0.75 1.08 0.32 122.64 7.26 9.95 0.01 0.13 0.28 30-Wen201 3678.25 1.39 1.95 5.31 6.39 0.84 1.06 0.38 0.44 8.23 0.0011 0.88 1.05 0.41 130.20 2.96 2.77 0.02 0.21 0.45 31-Wen201 3680.00 2.08 3.24 4.56 8.20 1.51 1.52 1.04 0.48 3.89 0.0008 0.75 1.08 0.17 91.46 1.96 8.67 0.02 0.16 0.44 32-PS4 3894.80 0.47 1.56 5.31 6.50 0.79 1.05 0.37 0.94 18.36 0.0006 0.80 0.79 0.53 89.63 4.01 3.97 0.01 0.13 0.21 33-PS4 4188.05 0.27 5.24 8.63 5.29 0.94 0.90 0.28 1.86 49.55 0.0036 0.84 0.83 0.29 31.67 3.85 4.50 0.02 0.15 0.46 34-PS4 4188.70 1.85 1.72 8.77 7.62 0.39 1.15 0.45 0.88 18.94 0.0044 0.69 1.07 0.12 42.21 14.57 7.21 0.03 0.16 0.79 35-PS4 4999.70 0.80 5.62 4.68 8.13 1.15 1.14 0.45 1.33 25.96 0.0022 0.79 1.12 0.12 42.18 7.87 9.50 0.02 0.20 0.52 36-PS4 5002.77 0.70 1.45 4.84 7.89 1.35 1.23 0.54 1.37 22.55 0.0018 0.86 0.98 0.17 32.90 2.74 8.70 0.02 0.15 0.49

Note: Sr-strontium, Ba-barium, B-boron, Ga-gallium, V-vanadium, Sc-scandium, Cr-chromium, U-uranium, Th-thorium, Mo-molybdenum, Re-rhenium, Eu-europium, Zr- zirconium, Rb-rubidium, Fe-iron, Mn-manganese, Cu-copper, Al-aluminum, Mg-magnesium, P-phosphorus, Ti-titanium. δU ¼ U/[1/2(U þ Th/3)]; Eu/Eu* ¼ EuN/(SmN � GdN)1/2; Climate Index¼(Fe þ Mn þ Cr þ V þ Co þ Ni)/(Ca þ Mg þ Sr þ Ba þ K þ Na).

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accumulation and preservation. When the salinity exceeds 15‰, the TOC content drops rapidly and then remains at a stable low value (Fig. 9b).

Sr/Ba (strontium/barium) and B/Ga (boron/gallium) ratios are common indices for evaluating paleo salinity (Walker and Price, 1963; Adams et al., 1965). When the salinity of lake water grows higher and its mineralization increases, Ba2þ and SO42� are preferentially combined to precipitate BaSO4, which then settles, thus increasing the Sr/Ba ratio. When the lake evaporates, the freely soluble boron can migrate and separate into the water, but gallium is much easier to precipitate because of its weak activity. Thus, the higher the salinity is, the larger the Sr/Ba and B/Ga ratios (Walker and Price, 1963; Adams et al., 1965). According to Wang et al. (2003), Sr/Ba≦0.6 and B/Ga≦4 indicate fresh water; 0.6 ≦ Sr/Ba≦1.0 and 4 ≦ B/Ga≦7 indicate brackish water; Sr/Ba≧1.0 and B/Ga≧7 indicate saline water. The relationships between TOC content and Sr/Ba and B/Ga ratios (Fig. 9c and d) are similar to that between TOC and paleosalinity in Fig. 9b. With increasing Sr/Ba and B/Ga ratios, the TOC content first increases, then remains steady and finally decreases. When the Sr/Ba ratio ranges from 0 to 2 and the B/Ga ratio ranges from 0 to 3, the TOC content presents an increasing ten-dency with salinity indices; when the Sr/Ba ranges from 2 to 5 and the B/Ga ranges from 3 to 9, the TOC content remains maximum and steady; when the Sr/Ba ratio is larger than 5 and the B/Ga ratio is larger than 9, the TOC content decreases with increasing Sr/Ba and B/Ga ratios. At the same time, the TOC content in the SWR is larger than those in the FWR and BWR.

5.2.2. Redox conditions The depositional environment plays a very important role in the

development of source rocks with high OM abundance, especially redox conditions, which are the main controlling factors for OM preservation. It is generally believed that an anoxic reducing environment is beneficial to OM preservation (Jones and Manning, 1994; Tenger et al., 2005; Chang et al., 2009). The redox-sensitive elements such as vanadium (V), (chromium) Cr, (scandium) Sc, (thorium) Th, (rhenium) Re, (molybde-num) Mo, (europium) Eu and uranium (U) are all important indicators of

the depositional environment. The ratios of V/Cr, V/Sc, V/Mo, U/Mo, Re/Mo, U/Th, Eu/*Eu and δU can be used to determine paleo redox conditions (Kimura and Watanabe, 2001; Rimmer et al., 2004; Tenger et al., 2005).

V is preferentially combined with OM in general under reducing conditions, and Cr is usually deposited in sedimentary detritus, so V/Cr can be used as an oxygen content indicator (Rimmer et al., 2004; Tenger et al., 2005). Both V and Sc are insoluble, and V is positively correlated with Sc, so V/Sc is higher in anoxic environment and lower in oxidation conditions (Jones and Manning, 1994; Kimura and Watanabe, 2001; Rimmer et al., 2004). Insoluble U4þ causes U accumulation under strong reducing conditions, but U exists as insoluble U6þ under oxidizing conditions; Th is not affected by water redox conditions, so U/Th can reflect redox conditions (Rimmer et al., 2004). U/Th > 1.25 indicates an anoxic environment; 0.75 1 indicates an anoxic envi-ronment and δU


13

those in the BWR and FWR, and the OM abundance in the SWR is more enriched than that of in the BWR and FWR. During Es3L sedimentation, the closed lake was in an arid evaporative environment with highly saline water, and the bottom water produced a stagnant and strong anoxic environment. Under these conditions, OM was mostly preserved. During Es3M sedimentation, the lake had a semiopen and semiclosed sedimentary environment, and the bottom water was still stagnant and anoxic so that OM could still be well preserved. During Es3U sedimenta-tion, the open lake had a weak anoxic environment, and OM was not well preserved at this time. Compared with the BWR and FWR, the SWR had much higher lake salinity and much stronger anoxic reduction conditions, so the SWR was more conducive to OM preservation and enrichment.

In a reducing environment, U is deposited in the bottom water, and Mo is only enriched in free H2S sediments (Crusius et al., 1996; Wilkin et al., 1997; Zhou et al., 2011); thus, U/Mo can be used as an index for redox environment (Taylor and McLennan, 1985). Re/Mo can be used to

distinguish hypoxic and oxidizing depositional environment (Crusius et al., 1996). Re/Mo > 9 � 10� 3 indicates an oxidizing environment, Re/Mo < 9 � 10� 3 indicates a hypoxic or sulfide environment (Crusius et al., 1996). Eu/Eu* can be used to distinguish the oxidation or reduction environment; a negative anomaly indicates reducing condi-tions, while a positive anomaly indicates oxidizing conditions (Crusius et al., 1996). The TOC content exhibits significant negative correlation with the redox indices, such as U/Mo, V/Mo, Re/Mo and Eu/*Eu (Fig. 11). As the oxidizing environment grows stronger, the OM abun-dance decreases, indicating that an oxygen-enriched environment is destructive for OM preservation. The oxidative indicators in the SWR are obviously lower than those in the BWR and FWR, and the OM enrich-ment in the SWR is also higher those in the BWR and FWR.

5.2.3. Hydrodynamic conditions Hydrodynamic conditions are a comprehensive reflection of water

depth and wave base. Fine-grained sediments accompanied by

Fig. 11. Relationships between TOC content and some negative redox indicators. (a) Relationship between TOC content and U/Mo. (b) Relationship between TOC content and V/Mo. (c) Relationship between TOC content and Re/Mo. (d) Relationship between TOC content and Eu/Eu*, where, Eu/Eu* ¼ EuN/(SmN � GdN)1/2.

Fig. 12. Relationships between TOC content and hydrodynamic condition indicators. (a) Relationship between TOC content and Zr/Rb. (b) Relationship between TOC content and δ18O.

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particulate OM are usually transported from high-energy water to low- energy water, thus influencing the OM enrichment degree (Ibach, 1982; Hunt, 1996). Zirconium (Zr) is often deposited in shore-shallow sandstones with high energy, and little in mudstone. Rubidium (Rb) mainly occurs in clays, micas, fine-grained sediments and light minerals in seawater in low-energy environment (Tenger et al., 2005). Therefore, the Zr/Rb ratio can quantitatively reflect hydrodynamic variations, a high Zr/Rb ratio indicates a high-energy depositional environment, and a low Zr/Rb ratio indicates a low-energy depositional environment (Ibach, 1982; Dypvik and Harris, 2001; Tenger et al., 2005). The Zr/Rb ratio varies from 0.76 to 1.71 (average 1.09) in the Es3 Formation of the Dongpu depression (Table 3), implying weak hydrodynamic conditions overall. During Es3L sedimentation, the Zr/Rb ratios were less than 1.1, indicating the weakest hydrodynamic conditions; during Es3U sedimen-tation, hydrodynamic conditions were intermediate with Zr/Rb values less than 1.2; during Es3M sedimentation, the hydrodynamic conditions were the strongest with the Zr/Rb values reaching 1.7. The TOC content shows a tendency of increasing first and then decreasing with the Zr/Rb ratio (Fig. 12a). During Es3L sedimentation, the closed and saline lake had a weak hydrodynamic condition with a strong reducing anoxic envi-ronment. The better OM preservation conditions were conducive to OM enrichment. In the semiopen and brackish water sedimentary environ-ment during Es3M sedimentation, the hydrodynamic conditions were stronger with a large OM supply; the anoxic depositional environment effectively preserved and enriched OM. During Es3U sedimentation, the water was an open environment with strong hydrodynamic conditions and rich OM sources, but the OM preservation conditions were poor, leading to ineffective OM enrichment.

5.2.4. Paleoclimate conditions Oxygen isotopes can reflect lake hydrological equilibrium, which is

the water volume variation due to evaporation and injection, and indi-rectly reflect the paleoclimate conditions (Epstein and Mayeda, 1953). In a dry climate, the evaporation process allows δ16O to preferentially escape from the water, resulting in δ18O relatively increasing and oxy-gen isotopes becoming heavier; when the climate is relatively moist, the

oxygen isotopes of the lake become lighter because of the continuous supply of the lighter oxygen isotope from rivers and rain (Liu, 1998). Therefore, oxygen isotope are much heavier in a closed arid saline lake, while the oxygen isotopes are lighter and close to the isotopic compo-sition of atmospheric precipitation in an open lake with a humid climate (Talbot, 1990; Liu, 1998). The δ18O values in the Es3L and Es3U sub-members are obviously lighter with averages of � 7.39‰ and � 6.67‰, respectively; However, the δ18O values in Es3M are relatively higher, ranging from � 7.5‰ to 0.6‰ (average � 2.24‰) (Table 3 and Fig. 12b). The TOC content tends to increase first and then decrease with δ18O from Es3L to Es3M and then to Es3U (Fig. 12b). In a humid climate, abundant organisms provide a good material basis for OM accumulation, and OM is more enriched in the SWR with good preservation conditions. In a dry and extremely high-salinity water environment, organisms are not well developed, the basin area is reduced and source input is lacking. Even in the SWR with excellent OM preservation conditions, the OM is not enriched because of a lack of OM resources.

Under the effects of climate, the migration and enrichment capability of elements vary under different environmental conditions (Leng and Marshall, 2004; Makeen et al., 2015). Iron (Fe), manganese (Mn), chromium (Cr), vanadium (V), cobalt (Co) and nickel (Ni) are humid climate elements that have higher contents in a warm and humid climate (Guan, 1992). Calcium (Ca), magnesium (Mg), potassium (K), sodium (Na), strontium (Sr) and barium (Ba) are arid climate elements. These elements precipitate and are deposited at the bottom of the water in a dry climate and have higher contents (Guan, 1992). The climate index (CI) (Eq. (4)) can imply the paleoclimate conditions (Lerman, 1989). A higher CI reflects a more humid and warmer climate, and a lower CI shows a more arid climate (Lerman, 1989). Fe/Mn is the classic parameter reflecting the paleoclimate, where a high Fe/Mn ratio in-dicates a humid climate and a low Fe/Mn ratio implies a dry and hot climate. Lerman (1989) proposed that the Sr/Cu ratio could indicate the paleoclimate in a lake basin: 1 < Sr/Cu < 10 indicates a humid climate, and Sr/Cu > 10 suggests a hot and dry climate. Similarly, a high Al/Mg ratio indicates a hot and dry climate, and a low value represents a warm and wet climate (Makeen et al., 2015).

Fig. 13. Relationships between TOC content and paleoclimate indicators. (a) Relationship between TOC content and climate index. (b) Relationship between TOC content and Fe/Mn value. (c) Relationship between TOC content and Sr/Cu value. (d) Relationship between TOC content and Al/Mg value.

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Climate index ¼ ðFeþMnþ Crþ Vþ Coþ NiÞ=ðCaþMgþ Srþ Ba

þ Kþ NaÞ(4)

The average CI values in Es3L, Es3M and Es3U are 0.22, 0.33 and 0.47, respectively; the average Fe/Mn ratios in Es3L, Es3M and Es3U are 69.7, 75.9 and 183.8, respectively; the average values of Sr/Cu in Es3L, Es3M and Es3U

are 73.2, 69.1 and 44.0, respectively; and the average Al/Mg values in Es3L, Es3M and Es3U are 8.54, 7.38 and 6.79, respectively (Table 3). From Es3L to Es3U, as the average values of CI and Fe/Mn increase, the average values of Sr/Cu, Al/Mg and TOC content decrease. These changes indicate that the climate during Es3 deposition gradually changed from hot and dry to warm and humid (Fig. 13). With increasing CI and Fe/Mn, TOC gradually decreases (Fig. 13a and b). When the climate is warm and humid, blooms of high plants can provide abundant terrigenous clastic material and OM sources for the lake. However, when the OM is deposited, it is easily decomposed because of the lack of good OM preservation conditions, so OM is not enriched. With increasing Sr/Cu and Al/Mg, TOC gradually decreases (Fig. 13c and d). When the climate is extremely hot and dry, it is not suitable for the growth and develop-ment of higher plants, so the lake lacks abundant terrigenous OM input, which can lead to ineffective OM enrichment even though the lake has a fairly good OM preservation conditions. Only when the climate is moderately humid and moderately dry, can it provide a large amount of OM for the lake; meanwhile, the lake has good preservation conditions for OM that cause OM enrichment (Leng and Marshall, 2004; Makeen et al., 2015).

5.2.5. Paleo productivity conditions Carbon isotopes have also been applied to study lake productivity

(Hodell and Schelske, 1998). When the productivity in a lake is high, blooming phytoplankton can absorb more δ12C through photosynthesis, which leads to relatively high δ13C in the water, so the variations in carbon isotopes in lacustrine carbonates can reflect paleo productivity in the lake (Müller and Suess, 1979; Pedersen and Calvert, 1990). TOC

content is an important indicator of OM abundance, reflecting the level of paleo productivity to some extent (Peters et al., 2005). Liu (1998) found a significant positive correlation between TOC and δ13C. The nutrient element phosphorus (P) is necessary for plant growth, and plays a significant role in plant growth, development and reproduction (Tyson, 2005). P is considered a determinant of primary productivity and is an important indicator of biotic productivity in sedimentary re-cords (Gallego et al., 2007). To eliminate the dilution effect of authi-genic minerals and terrigenous clasts on the absolute P content, P/Al and P/Ti are commonly used to characterize paleo productivity, because Al and Ti are considered terrigenous inputs (Peters and Moldowan, 1993; Sageman et al., 2003). With increasing paleo productivity indicators (δ13C, P/Al, P/Ti and P content) (Table 3), the TOC content presents positive correlations (Fig. 14), thus illustrating that primary produc-tivity is very important for OM enrichment.

5.3. Mechanisms and models of OM enrichment

OM accumulation and preservation are two basic conditions dis-cussed with regard to OM enrichment mechanisms (Pedersen and Cal-vert, 1990; Gallego et al., 2007). The OM accumulation is closely related to paleo productivity, terrigenous supply and climatic conditions in an SLRB. The OM preservation conditions are often related to lake strati-fication, salinity and redox conditions in the SLRB (Peters and Moldo-wan, 1993; Peters et al., 2005).

Biological productivity directly determines OM sources and abun-dance (Tyson, 2005). High lacustrine productivity leads to water body eutrophication, and eutrophic water is beneficial for the rapid propa-gation of algae and other aquatic organisms (Hu et al., 2018). The highly abundant OM sources and input are the most essential conditions and can provide a good material basis for source rocks. After OM production and deposition, the most essential factors for OM are the burial and preservation conditions. A high yield of original OM is not equal to a high abundance of preserved OM (Rimmer et al., 2004) due to many constraints on the depositional process that lead to OM destruction and

Fig. 14. Relationships between TOC content and paleo productivity indicators. (a) Relationship between TOC content and δ13C value. (b) Relationship between TOC content and P/Al value. (c) Relationship between TOC content and P/Ti value. (d) Relationship between TOC content and P content.

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Fig. 15. Sketches illustrating the sedimentary envi-ronment evolution and OM enrichment models in the Es3 member of the Dongpu Depression (Gao et al., 2012). (a) During Es3L sedimentation, the closed lake had high salinity and strong anoxic reduction under an extremely hot and dry climate in the Dongpu Depression. (b) During Es3M sedimentation, the sem-iclosed and semiopen lake with high water levels maintained salinity and anoxic reducing conditions under a moderate hot and humid climate in the Dongpu Depression, which favored OM enrichment. (c) During Es3U sedimentation, the open lake with higher lake levels had low salinity and weak anoxic reduction under a warm and humid climate in the Dongpu Depression (Lu et al., 2013; Zhang et al., 2017a,b; Hu et al., 2018).

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dilution. A stable salinity stratification produces an anoxic environment in the bottom water, which is conducive to OM preservation (Müller and Suess, 1979; Sageman et al., 2003; Makeen et al., 2015). Therefore, high lacustrine productivity and a stable bottom water anoxic environment are the main controlling factors for OM accumulation and preservation.

In this paper, OM enrichment models in the different saline regions (FWR, BWR and SWR) during different periods (Es3U, Es3M, Es3L) are established, as shown in Fig. 15. During Es3L sedimentation, the lacus-trine basin had just begun to rift and formed a relatively closed lake (Su et al., 2006; Zhang et al., 2017a,b) (Fig. 7). The climate was extremely hot and dry with strong evaporation (Figs. 12b and 13), thus resulting in high-salinity water (Figs. 8 and 9). The northern area of the depression was a saline water region (SWR) with high salinity, the central area was a brackish-water region (BWR), and the southern area was a fresh-water region (FWR). Due to the extremely hot and dry climate, terrestrial higher plants did not develop very well, resulting in a lack of terrestrial OM input into the SLRB. Simultaneously, aquatic plankton also did not develop very well because of high-salinity water. Thus, the SLRB lacked abundant exogenous OM input and endophytic OM supply. Although the salinity stratification of the lake water caused the bottom water to be stagnant and anoxic, which is best for OM preservation, OM enrichment was difficult due to the absence of OM supply.

During Es3M sedimentation, the lacustrine basin had begun strongly rifting, and the whole basin was in alternating semiclosed and semiopen alternating depositional environment (Fig. 7). The climate was moder-ate humid and dry with relatively strong evaporation (Figs. 12b and 13), and the salinity was slightly higher (Figs. 8 and 9). Due to the humid and dry climate, terrestrial higher plants flourished, resulting in a large amount of terrestrial OM input into the lacustrine basin. At the same time, aquatic plankton flourished, which ensured a large amount of exogenous OM and endophytic OM supply to the SLRB. The salinity stratification of the lake water caused the bottom water to be stagnant and anoxic, providing the best conditions for OM accumulation and preservation and leading to easy enrichment of OM during the Es3M

sedimentary period. During Es3U sedimentation, the lacustrine basin had been formed, and

the whole basin was an open depositional environment (Fig. 7). The climate was warm and humid (Figs. 12b and 13), and the salinity was lower (Figs. 8 and 9), but the northern area still had high salinity. Due to the warm and humid climate, the terrestrial higher plants flourished, resulting in a large amount of terrestrial OM input into the SLRB. At the same time, aquatic plankton flourished due to the appropriately saline water, ensuring a large amount of exogenous OM and endophytic OM input and supply to the lacustrine basin. However, the salinity of lake water was relatively lower in the southern and central areas, and the bottom water was rich in oxygen, so the preservation conditions for OM were poor, making the enrichment of OM difficult. Conversely, in the northern area, the preservation conditions for OM were good, and OM was enriched.

6. Conclusions

The reconstruction of the paleo sedimentary environment, including lake closure, paleosalinity, redox conditions, hydrodynamic conditions, paleoclimate and paleo productivity, of the Es3 member in the Dongpu Depression was based on the investigation of geochemical and miner-alogical data and C–O isotopes to infer OM enrichment models in rela-tion to paleo depositional conditions.

(1) The lacustrine closure indices (C–O isotope) indicate that during Es3L sedimentation, the whole lake had a closed environment; during Es3M, it had an alternately semiopen and semiclosed sedi-mentary environment; during Es3U, it had an open sedimentary environment.

(2) The Z value has a positive correlation with TOC, and the paleo-salinity indices are higher in the SWR than in the BWR and FWR.

The paleosalinity, Sr/Ba and B/Ga ratios tend to increase first and then decrease with TOC. All the above paleosalinity indices indicate that the OM enrichment is positively correlated with salinity to some extent.

(3) The redox indices (V/Cr, V/Sc, U/Th, and δU) all have positive relationships with TOC: the greater the values are, the better the reducibility and the better the OM preservation conditions. The values of V/Mo, U/Mo, Re/Mo and Eu/*Eu have negative re-lationships with TOC: the greater the values are, the better the oxidizing conditions and the worse the OM preservation condi-tions. The redox condition is the most important indicator for OM preservation.

(4) The paleo hydrodynamic indicator (Zr/Rb) implies that the stronger the hydrodynamic conditions are, the lower the TOC content. The paleoclimate indices (δ18O, climate index, Fe/Mn, Sr/Cu and Al/Mg) indicate that humid and dry climate can pro-vide abundant OM sources and good OM preservation conditions.

(5) The values of paleo productivity indices (δ13C, P/Al, P/Ti and P content) have positive relationships with TOC: the greater the values are, the better the primary productivity and the richer the OM.

Acknowledgments

This work was supported by the Major Scientific and Technological Projects of SINOPEC [grant number: P15022]. We are grateful to the Analysis and Testing Center of the State Key Laboratory of the China University of Petroleum (Beijing) and the Analytical Laboratory of BRIUG, which provided the instruments and helped to test and analyze samples. We also thank the Zhongyuan Oil Field Company for providing the samples and data to support the studies. We thank the colleagues who have significantly contributed to this study.

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