Characteristics and origin of the Lower Oligocene marine source rocks controlled by terrigenous organic matter supply in the Baiyun Sag, northern South China Sea

Journal of Petroleum Science and Engineering 187 (2020) 106821

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Characteristics and origin of the Lower Oligocene marine source rocks controlled by terrigenous organic matter supply in the Baiyun Sag, northern South China Sea Rui Sun a, b, c, d, Zhong Li a, b, c, *, Zhigang Zhao d, Haizhang Yang d, Xiayang Wang e, Zhao Zhao d a

State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China Innovation Academy for Earth Science, CAS, Beijing 100029, China University of Chinese Academy of Sciences, Beijing 100049, China d CNOOC Research Institute Co., Ltd., Beijing 100028, China e Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083, China b c

A R T I C L E I N F O

A B S T R A C T

Keywords: Marine source rocks Terrigenous organic matter Redox conditions Sediment flux Baiyun Sag

To analyze the dominant factors of the origin of marine source rocks of the northern South China Sea (SCS), this paper focuses on the Lower Oligocene neritic source rocks in the Baiyun Sag of the Pearl River Mouth Basin. This study assesses the source rocks based on the principles and methods of sedimentology, geochemistry, and palynology and data on pyrolysis parameters, biomarker compounds, major and trace elements, and sedimentary organic debris. The paleoproductivity, input of terrigenous organic matter (TOM), redox conditions and sediment flux are characterized by the above parameters. The dominant effects of organic matter (OM) supply, preser­ vation, and dilution on the origin of the source rocks are studied, and a depositional model of the source rocks is established. Finally, the origin of the Lower Oligocene marine source rocks in the northern SCS is clarified. The results show that the main source of the marine source rocks was terrestrial higher plants, while the contribution of planktonic algae was small. The origin of the marine source rocks was strongly controlled by the input of TOM during the depositional period. The source rocks developed in an oxic environment, and the correlation between the redox indices and OM abundance is poor, which indicates that the redox conditions were not the main factor dominating the origin of the source rocks. With an increase in the sediment flux and the barrier formed by low uplifts, the dilution of the OM decreased, and the contact time between OM and oxygen was markedly reduced; therefore, some OM in the oxic bottom water was not oxidized but instead became enriched, thereby forming the source rocks.

1. Introduction Source rocks are the material basis of petroleum generation. Research on source rocks has important significance in petroleum exploration and resource evaluation. The formation mechanism of source rocks has always been a hot topic for scholars in China and around the world. Previous studies have suggested that organic matter (OM) supply (Ramanampisoa and Disnar, 1994; Rimmer et al., 2004; Ding et al., 2016), OM preservation (Demaison and Moore, 1980; Arthur and Dean, 1998; Luo et al., 2013) and OM dilution (Tyson, 2001; Ding et al., 2015a) are the main factors that affect OM concentration and influence OM abundance. Among these factors, the roles of paleo­ productivity in OM supply and redox conditions in OM preservation

have received the most attention and are the most discussed (Tyson and Pearson, 1991; Sageman et al., 2003; Algeo et al., 2011; Xiong et al., 2012; Adegoke et al., 2015; Ding et al., 2019). Large rivers have both advantages and disadvantages for the origin of marine source rocks. The advantage is that the nutrients brought by rivers will result in large-scale reproduction of aquatic organisms. Meanwhile, the terrigenous organic matter (TOM) brought by rivers can also increase the supply of OM (Schubert and Stein, 1996; Li et al., 2016). These factors eventually lead to the concentration of OM. The disadvantage is that the terrestrial inorganic debris brought about by rivers will dilute the abundance of OM in the source rocks and eventu­ ally lead to a decrease in the abundance of OM (Hu et al., 2006). However, in many basins developed on the continental margin, the

* Corresponding author. Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China. E-mail address: [email protected] (Z. Li). https://doi.org/10.1016/j.petrol.2019.106821 Received 20 June 2019; Received in revised form 31 October 2019; Accepted 16 December 2019 Available online 20 December 2019 0920-4105/© 2019 Elsevier B.V. All rights reserved.

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development of high-quality marine source rocks is affected by the input ~i of TOM due to the large amount of TOM provided by large rivers (Gon et al., 1997; Holtvoeth et al., 2001; Waterson and Canuel, 2008; Samuel et al., 2009; Akinlua and Torto, 2011). During the Early Oligocene in the northern South China Sea (SCS), large braided river deltas developed along the gentle slope zone in the northwestern Baiyun Sag, and a large number of terrestrial higher plants flourished on the delta plains. After passing through the delta plains, the rivers delivered a great quantity of TOM and terrestrial inorganic debris to the shallow sea (Zhang et al., 2015, 2016; Zeng et al., 2019). What are the effects of TOM and terrestrial inorganic debris on the origin of ma­ rine source rocks? Further systematic research is needed. This paper focuses on the neritic source rocks of the Lower Oligocene Enping Formation in the Baiyun Sag of the Pearl River Mouth Basin (PRMB) and uses organic and inorganic geochemistry data, including pyrolysis parameters, biomarkers, and major and trace elements of source rocks, as well as the palynological data, of sedimentary organic debris to analyze the main factors that affect the origin of marine source rocks in the northern SCS. The paleoproductivity and TOM input, redox conditions, and OM dilution associated with the origin of source rocks

are studied, and a depositional model of the source rocks is established. Finally, the origin of marine source rocks in the northern SCS is clarified. The results of the study are useful for providing a scientific basis for the prediction of marine source rock distribution and petroleum exploration in the northern SCS. 2. Geological setting The PRMB belongs to an underwater extension of the South China continent (Fig. 1). The PRMB is located in a special tectonic location at the intersection of three plates, namely, the Pacific Ocean, Eurasia and India-Australia, and features the superimposed influences of the Tethyan and Pacific tectonic domains (Hall, 2002; Hutchison, 2004; Zhang et al., 2014; Mi et al., 2018). The Baiyun Sag is situated in the northeastern PRMB and is con­ nected with low uplifts (Fig. 1). It is the largest and deepest sedimentary sag in the PRMB, and it is also the sedimentary and subsidence center of the whole basin. The area of the sag is more than 1.4 � 104 km2, and the Cenozoic sedimentary thickness is approximately 12 km. The water depth ranges from approximately 200–2800 m, and approximately 70%

Fig. 1. Map displaying the position and tectonic units of the PRMB and the Baiyun Sag. 3D topographic map in (a), modified after Yang et al. (2015). Well BY2 in (b) is currently in the uplift area; however, during the Early Oligocene, the sedimentary facies of the well were neritic facies and developed marine deposits. The stratum thickness in (b) includes the Wenchang Formation and Enping Formation. 2

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of the area is deeper than 500 m. Because rifting occurred in the continental slope area, the litho­ spheric heat flow value was high, and ductile extension of the crust occurred (Zhang et al., 2016). The structure of the Baiyun Sag has been characterized as a broad fault depression; the northern side of the main sag is a gentle slope zone, and the southern side is a steep fault zone (Fig. 2a). Because the continental slope is relatively deep, the burial depth of the same set of strata in the continental slope area is relatively deep. Owing to well BY2 is located in the continental slope area, for the same set of strata, its buried depth is deeper than that of well BY1 (Fig. 2b), which is the reason for the considerable differences in depth of samples in two studied wells (BY1 and BY2). The Baiyun Sag has undergone three tectonic evolution stages: the Paleocene-Early Oligocene that corresponded to a continental rifting stage, the Late Oligocene-Early Miocene that corresponded to a

transitional stage, and the Middle Miocene to present that corresponded to a postrift subsiding stage (Fig. 3) (Zhang et al., 2016). In the early stage of rifting, the Shenhu Formation was deposited with a limited distribution and mainly consisted of a set of coarse clastic sediments. The Wenchang Formation developed the majority of its lacustrine facies as the faulting activity increased. In the late stage of rifting, the Enping Formation was deposited, the faulting activity decreased, and the thermal subsidence gradually increased. During the period of the Zhuhai Formation, faulting activity was weak, and the Baiyun Sag and adjacent areas began to merge under the effects of thermal subsidence. From the end of the Miocene to the Pliocene, tec­ tonic subsidence accelerated, sea level rose and sediment supply decreased, which led to the development of a deep-water continental slope environment (Fig. 3) (Mi et al., 2018). The marine source rocks of the Upper Enping Formation are one of

Fig. 2. Seismic profiles displaying the tectonic framework of the Baiyun Sag. The position of the profiles are shown in Fig. 1, and the significance of seismic horizons is shown in Fig. 3. (a) The northern side of the Baiyun Sag is a gentle slope zone, and the southern side is a steep fault zone; (b) Owing to well BY2 is located in the continental slope area, for the same set of strata, its buried depth is deeper than that of well BY1. The red rectangle is the strata studied in this paper. (For inter­ pretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 3

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Fig. 3. Comprehensive stratigraphic columns of the Baiyun Sag. (a) The stratigraphic column, as well as petroleum system elements, sea-level changes, tectonic evolution stage, and sedimentary environment of the Baiyun Sag (modified after Ma et al., 2018; Zhao et al., 2018); (b) The stratigraphic column of the target horizon of the Enping Formation.

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the main proven source rocks in the Baiyun Sag (Zhang et al., 2015, 2016; Wang et al., 2018). During this depositional period, the northern topography of the sag became lower and the sedimentary range expanded. The southeastern part of the sag gradually connected with the Liwan Sag, and the sedimentary environment gradually transformed into a semienclosed bay (Fig. 4). Multistage large-scale foreset seismic reflections are identified in the northwestern gentle slope (Fig. 5), indicating that the paleo-Pearl River crossed the Panyu Low Uplift and delivered a large amount of debris to the Baiyun Sag along the NW-SE direction, which formed a large delta (Zeng et al., 2019). The depos­ ited sediments from the river advanced towards the center of the basin over a large distance and have typical river-controlled delta distribution characteristics. No Lower Oligocene strata or sediments are present on the low uplifts, indicating that part of the Baiyun Sag was encircled by low uplifts, which formed a semienclosed basin (Fig. 6). Therefore, the neritic facies includes a large amount of terrigenous organic and inor­ ganic debris from the rivers.

chromatographic column, and eluted by n-hexane to obtain saturated hydrocarbon components. The test instrument used was Agilent 78905975c, and the chromatographic column was an HP-5MS elastic quartz capillary column (60 m � 0.25 mm � 0.25 μm). The test condi­ tions were as follows: the carrier gas was 99.999% helium gas; the temperature of the inlet and the transmission line were 300 � C; in the heating program, the temperature was maintained at 50 � C for 1 min, then increased to 120 � C at 20 � C/min, then increased to 250 � C at 4 � C/ min, and finally increased to 310 � C at 3 � C/min, where it was held for 30 min; the carrier gas velocity was constant, 1 ml/min. For the mass spectrometry, the electron impact (EI) source was 70 eV, the filament current was 100 A, and the multiplier voltage was 1200 V. The experi­ mental results are shown in Table 1, Fig. 7, and Fig. 8. A total of 26 samples were used for sporopollen and algae assem­ blage analysis. The samples were processed with HCl to eliminate cal­ cium carbonate and then with HF to eliminate silica. The acid-treated products were washed and screened in an ultrasonic cleaner. The diameter of the screen silk was 10 μm, and the wash time was no more than 2 min. The obtained substances (including fossils of the organic wall of microorganisms such as sporopollen and phytoplankton, as well as various organic debris) were washed and then combined with a marker for observation and identification. Statistical work was carried out under a Leica DMR biomicroscope, and statistically significant data on the sporopollen and algae assemblages (requiring more than 100 sporopollen fossils in a single sample) were obtained. The experimental results are shown in Table 2, Fig. 9, Figs. 10 and 11. A total of 10 samples were tested for major and trace elements. The samples were dissolved by HF and HNO3, and the international standard samples (GSR-5, GSR-6, and GSD-9) and blank samples were used to correct the dissolution process. The main elements were tested by inductively coupled plasma optical emission spectrometry (ICP-OES). In the process of testing, the working curves were established by the in­ ternational unit element standard, and the correlation of the working curves of each element was greater than 0.99999. The relative standard deviation (RSD) of the element P was less than 1%, and the RSD of other major elements was less than 0.5%. In addition, inductively coupled plasma mass spectrometry (ICP-MS) was used to test the trace elements. The working curves of each element were established according to the international multielement standard, and the correlation of the working curves of each element was greater than 0.99999. During the testing process, the stability of the instrument was monitored using a 1 ppb

3. Samples and methods The samples used in this paper were all cuttings samples of source rocks, which were taken from gray to dark gray neritic mudstones of the Lower Oligocene Enping Formation in the Baiyun Sag. In this paper, pyrolysis, analysis of biomarker compounds, identification of organic debris components, and determination of major and trace elements were carried out. The sample conditions and experimental methods are described as follows. A total of 43 samples were used for pyrolysis analysis. The samples used for pyrolysis in this paper were all cuttings samples of source rocks, so it is necessary to select rock debris one by one before pyrolysis experiment to eliminate the influence of drilling fluid. After that, the cuttings samples were washed with clear water to remove the drilling fluid, dried naturally at room temperature, crushed to 80 mesh for py­ rolysis analysis. The detection temperature was 22 � C, and the relative humidity was 46%. The detection instrument was a Rock-Eval II, and the operational guidelines are based on Barker (1974) and Espitalie et al. (1977). The total organic carbon (TOC) values of the pyrolysis results are displayed in Table 1, Table 3, Fig. 10, Fig. 12 and Fig. 13. A total of 19 samples were used for gas chromatography mass spectrometry (GC-MS) analysis. Samples of source rocks were crushed to 80 mesh, extracted by chloroform, separated on a silica gel-alumina

Fig. 4. Sediment thickness map of the Upper Enping Formation (T71-T70), which shows the palaeogeomorphological characteristics of the depositional period. As shown in the figure, the Baiyun Sag was connected with the Liwan Sag and the Enping Sag, and the main provenance direction was NW-SE. The red line B–B0 represents the seismic profile of the progradational structure shown in Fig. 5, and the C–C0 line represents the seismic profile of the stratigraphic development characteristics on both sides of the low uplift shown in Fig. 6. (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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Fig. 5. Seismic profile displaying the typical seismic reflection features of the Upper Enping Formation (T71-T70) along the main provenance direction. The multistage foreset seismic reflections are shown on the northwestern side of the Baiyun Sag; the profile position is shown in Figs. 1 and 4.

Fig. 6. Seismic profile that shows the stratigraphic development of the Upper Enping Formation (T71-T70) on both sides of the Yunli Low Uplift. There are no Lower Oligocene strata or sediments on the low uplift, indicating that part of the Baiyun Sag was encircled by low uplifts, which formed a semienclosed basin; the profile position is shown in Figs. 1 and 4.

internal standard solution of Rh and Re. The RSD of the measured ele­ ments was less than 3%. The experimental results are shown in Table 4 and Table 5.

Previous studies have shown that the steranes component can be used to distinguish the source differences of OM (Huang and Mein­ schein, 1979). The C27 steranes mainly come from algae, while C29 steranes mainly come from terrestrial plants. The ternary diagram of regular steranes indicates that the proportion of C29 steranes is relatively high (26.33%~61.15%, Fig. 8), reflecting the larger contribution of the TOM and the smaller contribution of aquatic algae OM (Seifert and Moldowan, 1978; Peters and Moldowan, 1993).

4. Results 4.1. Terpanes and steranes The results of biomarker analysis (Fig. 7, Table 1) show that on the mass chromatogram of saturated hydrocarbon (m/z 191), the Lower Oligocene marine source rocks have high oleanane (in some samples, the oleanane peak is higher than the C30 hopane peak), reflecting a high input of terrestrial higher plants (Ekweozor and Telnaes, 1990; Moldo­ wan et al., 1994). Furthermore, high gammacerane values are present in the samples, reflecting a high degree of saltwater in some areas. The “V”-shaped distribution characteristics of the regular steranes ααα20RC27, ααα20RC28 and ααα20RC29 on the mass chromatogram of saturated hydrocarbons (m/z 217) are similar in the samples, reflecting the contributions of both lower aquatic organisms and higher terrestrial plants (Li and Zhang, 2017).

4.2. Palynofacies The main objects of palynofacies analysis are the organic walls of microfossils, such as sporopollen and planktonic algae, and various sedimentary organic debris in sediments (Fig. 9). In sedimentary organic debris, coaly and woody plant debris originates from terrestrial flora. Coaly debris is charcoal or oxidized woody tissue, and woody debris is the undegraded conducting tissue of higher plants and degraded plant tissue. Cutin, chitin, and spores, major types of organic debris, are relatively stable and are mainly from higher plants. Amorphous organic matter (AOM) is mostly the result of the complete decomposition of 6

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Table 1 Biomarker parameters and TOC of marine mudstone samples in the Baiyun Sag, PRMB. Wells

Depth (m)

TOC (%)

C27 steranes (%)

C28 steranes (%)

C29 steranes (%)

C27/C29

OL/C30H

C35-homoh. Index

Gam/C30H

BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY2 BY2 BY2 BY2 BY2 BY2

2955.50 2988.50 3009.50 3036.50 3054.50 3075.50 3087.50 3120.50 3144.50 3174.50 3192.50 3216.50 3228.50 3593.50 3614.50 3671.50 3701.50 3725.50 3752.50

1.50 1.25 1.37 1.37 1.49 1.41 1.49 1.27 1.40 1.26 1.37 1.53 1.62 1.19 1.01 0.88 0.56 0.78 0.98

18.96 31.48 32.80 34.97 33.84 36.28 38.60 38.23 38.86 41.34 45.56 38.76 43.04 23.00 24.43 49.66 34.83 40.53 33.51

19.89 22.82 23.32 24.07 31.10 27.17 25.21 23.84 24.78 24.39 22.71 26.22 25.55 26.38 32.10 24.01 27.84 28.04 24.40

61.15 45.71 43.89 40.97 35.06 36.55 36.20 37.93 36.36 34.27 31.73 35.03 31.41 50.62 43.47 26.33 37.33 31.42 42.09

0.31 0.69 0.75 0.85 0.97 0.99 1.07 1.01 1.07 1.21 1.44 1.11 1.37 0.45 0.56 1.89 0.93 1.29 0.80

0.93 1.00 1.05 1.11 2.02 2.00 2.04 1.24 1.17 1.24 1.05 1.49 1.23 0.43 0.45 0.41 0.34 0.34 0.26

0.02 0.02 0.02 0.01 0.07 0.04 0.03 0.02 0.02 0.02 0.03 0.04 0.02 0.05 0.05 0.05 0.06 0.06 0.06

0.06 0.04 0.10 0.10 0.08 0.10 0.09 0.10 0.05 0.11 0.11 0.12 0.11 0.06 0.07 0.17 0.16 0.15 0.16

Note: C27 steranes ¼ C27/(C27 þ C28 þ C29) regular steranes; C28 steranes ¼ C28/(C27 þ C28 þ C29) regular steranes; C29 steranes ¼ C29/(C27 þ C28 þ C29) regular steranes; C27/C29 ¼ ααα20RC27/ααα20RC29 steranes; OL/C30H ¼ oleanane/C30 hopane; C35-homoh. Index ¼ C35 homohopane/(C31–C35) homohopane [17α(H), 21β (H), 22S þ 22R]; Gam/C30H ¼ gammacerane/C30 hopane.

Fig. 7. Mass chromatograms of the terpanes (m/z 191) and steranes (m/z 217) in the marine mudstone samples. OL ¼ oleanane; Gam ¼ gammacerane; C27, C28 and C29 represent ααα20RC27, ααα20RC28 and ααα20RC29, respectively.

aquatic organisms and algae and appears cottony or cloudy, without a definite outline (Batten, 1982; Tyson, 1987, 1995). The above sedi­ mentary organic debris can be classified into four types: coaly debris, woody debris (degraded and undegraded), herbaceous debris (cutin, chitin, and spores) and AOM. The types of organic debris are visually presented in an area diagram based on the content ratio, and the TOC of the source rocks is shown vertically at the same depth (Fig. 10). The content of sporopollen in the Lower Oligocene source rocks in well BY1 is 46.95%–90.18% (average 57.49%). The content of plank­ tonic algae, primarily marine dinoflagellates, is 9.82%–53.05% (average 43.81%) (Fig. 10, Table 2). The variations in the component amounts of the sporopollen and algae assemblages show that the source rocks received contributions from both terrestrial higher plants and plank­ tonic algae but that terrestrial higher plants made a greater contribution. In the sedimentary organic debris components of well BY1, coaly debris ranges from 22.55% to 58.41% (average 32.00%), woody debris ranges from 7.18% to 37.01% (average 18.43%), herbaceous debris ranges from 10.62% to 52.31% (average 38.48%), and AOM ranges from 0 to 18.14% (average 11.09%). In the sedimentary organic debris

components of well BY2, coaly debris ranges from 45.76% to 97.73% (average 60.81%), woody debris ranges from 2.27% to 31.21% (average 22.55%), herbaceous debris ranges from 0 to 5.09% (average 1.46%), and AOM ranges from 0 to 30.00% (average 15.18%). The characteris­ tics of the sedimentary organic debris components indicate that coaly debris, woody debris, and herbaceous debris, which represent the input of terrestrial higher plant OM, are dominant, while the AOM, which represents the input of planktonic algae, is low. The percentages of sedimentary organic debris can be read at depths with the measured TOC data in Fig. 10, and the same analysis method is used for well BY2. Finally, the statistical data are listed in Table 3. These data are used in the analysis of the relationship between sedimentary organic debris and TOC below. In the ternary diagram of A-P-E (Fig. 11), the three corners represent the highest values (100%) of the corresponding three types of organic debris components. The A-P-E pattern of organic debris components not only reflects the contents of the organic debris components but also represents the environmental characteristics of the terrestrial higher plants or phytoplankton, the accumulation and storage conditions, and 7

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element U is usually preserved in anoxic environments, so the value of U/Th can reflect the degree of redox conditions (Tribovillard et al., 2006). It is generally believed that the higher the U/Th value is, the stronger the reducibility of the sedimentary environment (Barnes and Cochran, 1990; McManus et al., 2005). Previous studies have suggested that the U/Th value is less than 0.75 in an oxygen-rich environment, ranges from 0.75 to 1.25 in an oxygen-poor environment, and is greater than 1.25 in an anaerobic environment (Jones and Manning, 1994; Pattan et al., 2005). The U/Th values of the samples mainly range from 0.146 to 0.252 (with an average of 0.193), and the values are all less than 0.75, which reflects an oxygen-rich environment (Table 4). The element Ni exists mostly in anoxic environments, whereas the element Co is present mainly in oxic environments; thus, the Ni/Co ratio has been used to reflect the redox degree of sedimentary environments (Calvert and Pedersen, 1993). It is generally believed that the larger the value is, the stronger the reducibility of the sedimentary environment (Algeo and Maynard, 2004). Previous studies have suggested that the Ni/Co value is less than 5 in an oxygen-rich environment, ranges from 5 to 7 in an oxygen-poor environment, and is greater than 7 in an anaer­ obic environment (Jones and Manning, 1994). The Ni/Co values of the samples mainly range from 2.463 to 4.136 (with an average of 3.041), and the values are all less than 5, reflecting an oxygen-rich environment (Table 4). The main element analysis results are shown in Table 5 and are mainly used to evaluate the chemical index of alteration (CIA) and the sediment flux. The purpose is to study the dominating effect of dilution of OM in the origin of the source rocks, which is discussed in detail below.

Fig. 8. Ternary diagram of C27 steranes, C28 steranes, and C29 steranes. The proportions of the steranes components can be used to distinguish the source differences of OM (modified after Adegoke et al., 2014; Ding et al., 2015b).

the hydrodynamic conditions. The P-terminal region reflects a terrestrial higher plant source and oxic preservation conditions, the A-terminal region reflects a phytoplankton source and anoxic preservation condi­ tions, and the E-terminal region reflects distal deposition with strong transport and sorting along with oxic preservation conditions (Tyson, 1993). The marine source rock samples plot in the P-terminal region or fall along the P-E edge, indicating that the OM sources were terrestrial higher plants and that the depositional environment was oxic.

5. Discussion The origin of source rocks is a process of concentration of OM. OM from planktonic algae and terrestrial higher plants slowly settles to the seafloor and is partially degraded in the process of underwater burial. Residual OM enters the sediments and is preserved in the rock through diagenesis to form source rocks. Previous studies have suggested that the OM supply (Ramanampisoa and Disnar, 1994; Rimmer et al., 2004; Ding et al., 2016), OM preservation (Demaison and Moore, 1980; Arthur and Dean, 1998; Luo et al., 2013) and OM dilution (Tyson, 2001; Ding et al., 2015a) are the main factors dominating OM concentration and affecting OM abundance in source rocks. OM supply refers to the process in which OM continuously settles to the seafloor and reflects the original total amount of OM before it is degraded. OM preservation refers to the process in which OM at the bottom of the water enters the sediments and avoids being degraded. OM dilution refers to the input of inorganic terrestrial debris, which reduces the concentration of OM.

4.3. Major oxides and trace elements The distribution, cycling and differentiation of redox-sensitive ele­ ments in marine waters and sediments are affected by the chemical properties of the elements and the redox conditions of the environment. Therefore, the redox conditions of the paleomarine sedimentary envi­ ronment can be reconstructed by the content characteristics of redoxsensitive trace elements in the sediments (Morse and Luther, 1999; Whitfield, 2001; Nameroff et al., 2002). In this paper, the mass ratios of certain trace elements, i.e., m(U)/m (Th) and m(Co)/m(Ni), are used to analyze the redox conditions (Table 4). The element Th is a relatively inert element at seawater temperatures and has a high content in oxic environments, while the

Table 2 Components of sporopollen and planktonic algae in the source rocks in well BY1 in the Baiyun Sag, PRMB. Depth (m)

Sporopollen (%)

Fern spores (%)

Gymnosperm pollen (%)

Angiosperm pollen (%)

Planktonic algae (%)

Nonmarine planktonic algae (%)

Marine dinoflagellate (%)

2960 2981 2999 3020 3041 3054 3059 3080 3101 3119 3140 3154 3161 3179 3200 3221 3227

90.18 90.18 62.11 66.23 49.07 47.42 47.39 46.95 50.00 49.50 52.78 46.98 51.46 48.60 52.44 50.73 53.15

16.96 16.07 13.04 15.23 5.61 5.16 11.37 8.92 10.68 8.42 7.41 6.05 16.50 12.15 12.44 8.29 13.51

28.57 25.89 9.94 25.17 11.21 12.68 9.95 12.21 8.74 12.38 12.50 17.67 11.65 10.75 7.11 6.34 6.76

44.64 48.21 39.13 25.83 32.24 29.58 26.07 25.82 30.58 28.71 32.87 23.26 23.30 25.70 32.89 36.10 32.88

9.82 9.82 37.89 33.77 50.93 52.58 52.61 53.05 50.00 50.50 47.22 53.02 48.54 51.40 47.56 49.27 46.85

6.25 4.46 3.11 5.30 4.21 8.45 3.79 3.76 1.94 4.46 1.39 0.47 2.43 3.74 3.11 2.93 2.25

3.57 5.36 34.78 28.48 46.73 44.13 48.82 49.30 48.06 46.04 45.83 52.56 46.12 47.66 44.44 46.34 44.59

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Table 3 Compositions of sedimentary organic debris of the Lower Oligocene marine source rocks in the Baiyun Sag, PRMB. Wells

Depth (m)

TOC (%)

Coaly (%)

Woody (%)

Herbaceous (%)

Amorphous (%)

Degraded (%)

Undegraded (%)

Cutin (%)

Chitin (%)

Spores (%)

BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY2 BY2 BY2 BY2 BY2 BY2 BY2 BY2 BY2 BY2 BY2 BY2 BY2 BY2

2961.5 2982.5 2988.5 3002.5 3004.5 3009.5 3024.5 3036.5 3045.5 3054.5 3066.5 3075.5 3084.5 3087.5 3099.5 3102.5 3120.5 3123.5 3141.5 3144.5 3162.5 3174.5 3183.5 3192.5 3201.5 3216.5 3222.5 3593.5 3602.5 3614.5 3626.5 3632.0 3647.5 3671.5 3679.5 3688.5 3701.5 3713.5 3725.5 3740.5 3752.5

1.39 1.51 1.25 1.31 1.34 1.37 1.56 1.37 1.54 1.49 1.76 1.41 1.75 1.49 1.37 1.72 1.27 1.54 1.51 1.40 1.47 1.26 1.60 1.37 1.81 1.53 1.83 1.19 0.72 1.01 1.13 0.88 0.50 0.88 0.89 0.55 0.56 0.68 0.78 1.03 0.98

42.79 49.61 52.85 56.15 54.87 51.58 40.43 29.08 24.46 24.75 31.07 29.48 27.57 26.83 23.80 23.83 28.21 27.65 25.50 27.48 30.40 26.02 23.99 23.20 22.91 26.59 26.83 45.76 53.74 64.31 55.91 51.98 55.25 54.85 52.79 51.90 52.09 56.95 61.61 67.07 84.60

34.06 34.16 32.37 28.30 27.77 26.51 22.10 17.29 13.74 10.40 16.64 23.11 23.32 21.22 13.03 12.01 12.06 11.96 11.05 10.35 13.94 16.22 15.76 13.03 10.48 7.55 7.35 26.20 19.28 10.10 21.04 26.09 30.40 19.74 16.47 22.90 30.16 30.38 29.33 26.42 12.61

0.81 2.32 2.10 1.48 1.31 0.89 0.49 1.78 4.01 6.57 1.65 1.03 0.83 0.86 1.03 1.15 2.28 2.03 0.68 0.84 0.74 0.91 0.98 0.99 0.97 0.62 0.63 1.11 0.63 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.24 0.81 0.35

1.96 0.00 0.00 0.79 1.25 2.38 5.73 8.33 8.11 5.99 6.92 5.93 4.47 3.86 1.35 1.05 1.30 1.46 2.16 1.87 1.48 1.82 1.95 1.94 1.93 1.63 1.52 0.37 0.79 1.35 1.12 1.00 0.40 0.24 0.39 0.64 0.80 0.26 0.00 0.00 0.00

11.57 8.59 7.97 9.08 10.22 13.06 21.12 26.62 30.16 32.53 25.49 21.44 23.69 26.60 38.15 39.04 33.36 33.38 33.75 34.03 33.74 32.97 33.44 35.00 36.58 39.60 39.93 0.00 0.29 0.67 0.29 0.11 0.00 1.19 1.90 2.97 3.57 1.19 0.47 1.63 0.70

8.81 5.32 4.71 4.20 4.58 5.58 7.27 6.55 4.88 2.97 7.12 8.07 8.37 8.28 7.85 8.16 11.46 11.36 10.20 8.98 6.38 5.65 6.22 7.91 9.38 10.05 10.16 0.74 0.42 0.00 0.00 0.00 0.20 0.11 0.00 0.00 0.00 0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00 0.00 0.00 2.86 10.35 14.64 16.79 11.11 10.94 11.75 12.35 14.79 14.76 11.33 12.16 16.66 16.45 13.32 16.41 17.66 17.93 17.75 13.96 13.58 25.83 24.85 23.57 21.64 20.82 13.75 23.87 28.45 21.59 13.38 11.22 8.35 4.07 1.74

Fig. 9. The cuttings samples of mudstones (a–b), selected sporopollen (c–e) and planktonic algae (f), and sedimentary organic debris (g–h) of the Lower Oligocene marine source rocks in the Baiyun Sag, PRMB. Scale bar in palynofacies plate (c–h) of well BY2 is 10 μm. (a) The cuttings samples of mudstones in well BY1, 31603161 m; (b) The cuttings samples of mudstones in well BY2, 3679-3680 m. (c) Fern spore, Polypodiisporites sp., 3602.5 m. (d) Gymnosperm pollen, Pinuspollenites sp., 3647.5 m. (e) Angiosperm pollen, Gothanipollis bassensis, 3593.5 m. (f) Marine dinoflagellate, Impletosphaeridium polypes subsp. densum, 3602.5 m. (g)–(h) Sedi­ mentary organic debris; (g) Undegraded woody, 3614.5 m; (h) Coaly, 3602.5 m.

9

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Journal of Petroleum Science and Engineering 187 (2020) 106821

Fig. 10. Comparative diagram of palynofacies and TOC of the source rocks in well BY1 in the Baiyun Sag, PRMB.

steranes C27 and C29 can reflect the source of OM. A high mass fraction of C27 steranes often represents a dominant contribution by planktonic algae, while a high mass fraction of C29 steranes reflects a dominant contribution by terrestrial higher plants (Huang and Meinschein, 1979). The ω(C27)/ω(C29) ratio reflects the contribution of planktonic algae; the larger the value is, the greater the contribution of planktonic algae. The relationship between ω(C27)/ω(C29) and ω(TOC) of source rocks is shown in Fig. 12a. The correlation between ω(C27)/ω(C29) and ω(TOC) is poor. As ω(C27)/ω(C29) increases, there is almost no change in ω(TOC). In palynofacies, AOM is mostly the product of the complete decom­ position of aquatic organisms or algae. The higher the content of AOM is, the greater the paleoproductivity (Tyson, 1987). The relationship be­ tween AOM and ω(TOC) in source rocks is shown in Fig. 12b. The cor­ relation between AOM and ω(TOC) is poor. As AOM increases, there is almost no change in ω(TOC). The paleoproductivity index [ω(C27)/ω(C29) ratio and AOM per­ centage content] of source rocks has a poor fitting relationship with TOC, and the correlation coefficient is low. The TOC of the source rocks does not increase with an increase in the paleoproductivity index, indicating that paleoproductivity was not the main factor dominating the origin of the source rocks. Among the biomarkers, oleanane (OL) is considered to be a marker of higher plants since the Cretaceous. The OL/αβ C30 hopane ratio can reflect the input of OM from terrestrial higher plants. The higher the ratio is, the greater the contribution of terrestrial higher plant OM (Moldowan et al., 1994). The relationship between the oleanane index and ω(TOC) of source rocks is shown in Fig. 12c. There is a good cor­ relation between the oleanane index and ω(TOC). As the oleanane index increases, ω(TOC) significantly increases. In palynofacies, the source of herbaceous debris is mostly higher plant cell tissue, which contains high levels of fatty acids, alcohols, and lipids. Hydrocarbons can be generated by hydrolysis or reduction of herbaceous debris. The higher the content of herbaceous debris in source rocks is, the greater the contribution of terrestrial higher plants to the source rocks (Tyson, 1987). The relationship between herbaceous debris and ω(TOC) of source rocks is shown in Fig. 12d. The correlation be­ tween the percentage content of herbaceous debris and ω(TOC) is good. As the herbaceous debris increases, ω(TOC) significantly increases. In summary, the supply index of TOM (oleanane index and

Fig. 11. A-P-E comprehensive diagram of sedimentary organic debris in source rocks (plate modified after Tyson, 1993). A represents the abundant AOM of phytoplankton; P represents the coaly debris, woody debris, and cuticular organic debris from near-source higher plants; and E represents the chitin and spore organic debris.

5.1. Organic matter supply conditions The OM supply of marine source rocks consists of two parts: the paleoproductivity of planktonic algae and the input of TOM. To study the dominating effect of OM supply in the origin of the source rocks, the relationships between paleoproductivity and TOC and between the input of TOM and TOC are fitted separately. Paleoproductivity is char­ acterized by the ratio of regular steranes C27/C29 and the percentage of AOM in palynofacies. The input of TOM is characterized by the oleanane index and the percentage of herbaceous debris in palynofacies. Among the biomarker compounds, the mass fractions of the regular 10

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Journal of Petroleum Science and Engineering 187 (2020) 106821

Fig. 12. Relationship between OM supply index and TOC content of source rocks. (a) The relationship between ω(TOC) and the regular steranes C27/C29; (b) The relationship between ω(TOC) and AOM. The paleoproductivity index [ω(C27)/ω(C29) ratio and AOM percentage content] of source rocks has a poor fitting rela­ tionship with TOC, and the correlation coefficient is low, indicating that paleoproductivity was not the main factor dominating the origin of the source rocks. (c) The relationship between ω(TOC) and oleanane/C30 hopane; (d) The relationship between ω(TOC) and herbaceous debris. The supply index of TOM (oleanane index and herbaceous debris percentage content) has a good relationship with TOC, and the correlation coefficient is high, indicating that the supply of TOM was the main factor dominating the origin of the source rocks. ω is the mass fraction symbol.

Fig. 13. Relationship between redox conditions and TOC. (a) The relationship between the C35-homohopane index and TOC; (b) The relationship between the gammacerane index and TOC. The relationship between the redox index (C35-homohopane index and gammacerane index) and the TOC is poor and weakly negative, indicating that an anoxic environment was not the main factor dominating the origin of the studied source rocks.

herbaceous debris percentage content) has a good relationship with TOC, and the correlation coefficient is high. The TOC of the source rocks increases markedly with the increase in the supply index of TOM, indicating that the supply of TOM was the main factor dominating the origin of the source rocks.

Table 4 Trace element data and ratios of U/Th and Ni/Co of the samples in the Baiyun Sag, PRMB. Wells

Depth (m)

U (ppm)

Th (ppm)

Ni (ppm)

Co (ppm)

U/Th

Ni/ Co

BY1 BY1 BY1 BY1 BY2 BY2 BY2 BY2 BY2 BY2

3062.0 3099.5 3146.0 3198.5 3599.5 3644.5 3674.5 3686.5 3713.5 3758.5

2.705 3.002 3.587 4.245 3.582 3.354 3.355 4.214 3.513 3.485

10.750 14.541 16.935 18.318 22.290 16.812 22.924 22.946 20.467 20.323

35.464 42.261 41.034 39.182 30.631 40.244 27.523 33.121 33.569 30.979

8.574 11.178 13.207 13.773 9.369 13.038 10.870 12.472 13.631 12.214

0.252 0.206 0.212 0.232 0.161 0.200 0.146 0.184 0.172 0.171

4.136 3.781 3.107 2.845 3.269 3.087 2.532 2.656 2.463 2.536

5.2. Redox conditions The redox degree of the sedimentary environment is an important factor affecting the preservation of OM (Hedges and Keil, 1995). Reducing environments are generally believed to be conducive to the preservation of OM, and the degradation of OM in oxic environments is not conducive to the origin of source rocks (Richards and Vaccaro, 1956). However, under certain conditions, source rocks can also be formed in oxic environments (Nazir and Fazeelat, 2014). To study the dominating effect of redox conditions on the origin of 11

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Journal of Petroleum Science and Engineering 187 (2020) 106821

Table 5 Comparison of sediment flux and TOC between two wells in the Baiyun Sag, PRMB. Wells

Depth (m)

Al2O3 (%)

CaO (%)

K2O (%)

Na2O (%)

P2O5 (%)

CIA

Sediment flux (t/km2/yr)

BY1 BY1 BY1 BY1 BY2 BY2 BY2 BY2 BY2 BY2

3062.0 3099.5 3146.0 3198.5 3599.5 3644.5 3674.5 3686.5 3713.5 3758.5

11.845 14.453 16.365 18.314 19.222 16.181 20.602 21.284 19.501 19.560

9.787 8.060 4.840 5.726 3.966 3.740 3.085 0.652 1.178 2.434

1.426 1.815 2.148 2.337 3.184 2.753 3.275 3.356 2.833 2.524

0.935 0.983 1.216 1.146 0.418 0.462 0.822 0.963 1.170 1.281

0.063 0.084 0.069 0.111 0.074 0.074 0.068 0.061 0.059 0.073

72 74 72 74 80 78 77 77 74 74

184 150 166 136 82 91 111 101 150 150

source rocks in sedimentary environments, this paper has demonstrated that the sedimentary environment is oxic based on the mass ratios of certain trace elements [m(U)/m(Th) and m(Co)/m(Ni)]. The relation­ ship between redox condition parameters and TOC is established below. Biomarker compounds have chemical stability and relatively strong thermal stability during deposition and early burial, and they can be used as a criterion to distinguish redox conditions of the paleosedi­ mentary environment. In this paper, the redox conditions of the sedi­ mentary environment are discriminated using the C35-homohopane index and the gammacerane index. Homohopane is a product of the formation of low-carbon homolo­ gous compounds modified by bacteria in the earliest stage of diagenesis and is common in sulfide-rich anoxic deposits. The C35-homohopane index is defined as C35 homohopane/(C31–C35) homohopane [17α(H), 21β(H), 22S þ 22R]. A value less than 0.06 indicates an oxic environ­ ment in the bottom water or sediments, and a value greater than 0.1 indicates an anoxic environment (Peters and Moldowan, 1991). The C35-homohopane index of source rocks ranges from 0.01 to 0.07 (with an average of 0.04), and 84% of the samples are less than 0.06, indi­ cating that these deposits formed in an oxic sedimentary environment (Table 1). To study the dominating effect of redox degree on the origin of the source rocks, the relationship between the C35-homohopane index and the TOC of the source rocks was fitted. As shown in Fig. 13a, the fitting relationship between the C35-homohopane index and TOC is poor, and there is a faint negative correlation between them. As the C35-homohopane index increases, the TOC tends to decrease to a certain extent, indicating that an anoxic environment was not the main factor dominating the origin of the source rocks. Gammacerane is a type of biomarker compound formed by pyrolysis of tetrahymena. The gammacerane index is defined as gammacerane/ [17α(H), 21β(H) C30 hopane]. A high gammacerane index often in­ dicates a high-salinity environment and water stratification, and water stratification is favorable to the formation of an anoxic environment (Sinninghe Damst�e et al., 1995; Pancost et al., 1998; Li et al., 2015). The gammacerane index of the source rocks ranges from 0.04 to 0.17 (with an average of 0.10), with low overall contents (Table 1). This result indicates that the salinity of the water body was low and that the water body was not stratified during the sedimentary period, which made it difficult to form a reducing environment. At the same time, the corre­ lation between the gammacerane index and the TOC is poor. As shown in Fig. 13b, there is a faint negative correlation between them. As the gammacerane index increases, the TOC tends to decrease to a certain extent, indicating that an anoxic environment was not the main factor dominating the origin of the source rocks. In summary, the trace element parameters and biomarker parame­ ters of the redox conditions in the Baiyun Sag indicate that the Lower Oligocene strata formed in an oxic environment. At the same time, the relationship between the redox index (C35-homohopane index and gammacerane index) and the TOC is poor and weakly negative, indi­ cating that good source rocks can form in oxic environments and that an anoxic environment was not the main factor dominating the origin of the studied source rocks. This phenomenon can be explained by the fact that

when the supply of TOM is large, the water body will be turbulent and the environment will be oxic. Furthermore, the original OM content in the basin was high. Thus, even in an oxic environment, after the degradation of the OM, a large amount of OM was deposited and led to the origin of the source rocks. 5.3. Dilution of organic matter The supply and preservation of OM affect the total content of OM in the basin. However, the relative abundance of OM also depends on the content of inorganic matter. The inorganic matter delivered by rivers dilutes the concentration of OM. The sedimentation rate is usually used to reflect the dilution of OM. Previous researchers have studied the relationship between TOC and sedimentation rate in marine sediments (Betts and Holland, 1991). Researchers have concluded that a deposition rate that is too low leads to an increase in the duration of exposure to oxygen, which is unfavorable to the preservation of OM (Henrichs and Reeburgh, 1987). In contrast, a high sedimentation rate shortens the duration of OM degradation, and the preservation of OM is good, which is conducive to the development of source rocks (Müller and Suess, 1979; Hartnett et al., 1998). In this paper, the dilution of OM is studied using the sediment flux to characterize the degree of the source input. Sediment flux refers to the mass of sediment deposited per unit area per unit time, and the larger the value is, the greater the sedimentation rate. The formula is as follows: sediment flux (t/km2/yr) ¼ 2.25 � power [10, 5-0.0435 � (CIA)], where CIA is the chemical index of alteration (McLennan, 1993; Young and Nesbitt, 1999). The most important metal elements in the upper crust are Si, Al, Na, K and Ca. The elements Na, K and Ca are relatively easily lost, while the element Al is relatively stable. Therefore, in the process of weathering, the molar number of Al in­ creases as weathering intensity increases. Based on this, the CIA was proposed by predecessors (Nesbitt and Young, 1982). The formula is as follows: CIA ¼ molar [(Al2O3)/(Al2O3 þ CaO* þ Na2O þ K2O)] � 100, where CaO* refers to CaO in silicates, that is, CaO in the whole rock minus the molar fraction of CaO formed by chemical deposition (Nesbitt and Young, 1989). When this parameter cannot be obtained indepen­ dently, the lower value between [moles (CaO) – moles (P2O5) � 10/3] and moles (Na2O) is taken (McLennan, 1993; Zeng et al., 2015). The variation in the sediment flux in the Lower Oligocene strata in the two wells is shown in Table 5. The sediment flux in well BY1 ranges from 136 to 184 t/km2/yr (with an average of 159 t/km2/yr), with an average TOC value of 1.49%, and the sediment flux in well BY2 ranges from 82 to 150 t/km2/yr (with an average of 114 t/km2/yr), with an average TOC value of 0.84%. The sediment flux in well BY1 is 1.4 times that of well BY2, and the average TOC value of the former is 1.8 times higher than that of the latter (Table 5). Therefore, through comparative analysis, a high sediment flux was conducive to the concentration of OM and an increase in TOC. The degradation of OM is intense in oxic en­ vironments; however, an increase in the sediment flux obviously reduces the duration of the exposure of OM to oxygen, which is conducive to the concentration of OM. Additionally, no Lower Oligocene strata or sediments are present on 12

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Journal of Petroleum Science and Engineering 187 (2020) 106821

the low uplifts, indicating that part of the Baiyun Sag was encircled by low uplifts, which formed a semienclosed basin. With the barrier formed by these low uplifts, the dilution of OM was reduced, which is favorable to the concentration of OM. Therefore, the dilution of OM played a role in the origin of the studied marine source rocks.

in dominating the origin of marine source rocks. The OM in the marine source rocks mainly originated from terrestrial higher plants, and the contribution of planktonic algae was small. A large amount of terrestrial debris entered the water body, causing water body turbulence, and the dissolved oxygen content increased. Thus, the degree of oxygenation of the depositional environment increased, which was unfavorable to the preservation of OM. Although the oxic environment was not conducive to the preservation of OM, the total OM in the water body was high due to the large input of TOM. At the same time, with the high sediment flux and the barrier formed by the low uplifts, the dilution of the OM was weak, and the duration of exposure of the OM to oxygen was low. Thus, a great quantity of OM reached the bottom of the oxic water body and became enriched, leading to the origin of the source rocks. The model of the factors dominating the origin of the marine source rocks via the input of TOM is shown in Fig. 14.

5.4. Depositional model of marine source rocks Climate characteristics affect the intensity of weathering, especially chemical weathering, which is intense in warm and humid climates but weak in dry and cold climates. Therefore, chemical weathering intensity can also be used to distinguish paleoclimate characteristics. Research suggests that CIA values between 50 and 60 correspond to a low chemical weathering intensity and represent arid climates; CIA values between 60 and 80 correspond to medium chemical weathering in­ tensity and generally represent warm and humid climates; and CIA values greater than 80 correspond to a high chemical weathering in­ tensity and represent hot and humid climates (Fedo et al., 1995; Niu et al., 2019). Table 5 shows that the CIA values in the Lower Oligocene strata of the Baiyun Sag range from 72 to 80 (average 75), indicating a medium intensity of chemical weathering. These values correspond to warm and humid climatic conditions, which are conducive to the growth of terrestrial higher plants. Therefore, the climatic conditions were favorable for the production of a large amount of TOM. During the Early Oligocene, large braided river deltas were distrib­ uted along the NW-SE direction in the northwestern gentle slope of the Baiyun Sag, and a large number of terrestrial higher plants were present on the delta plain. When the rivers passed through the delta plains, they entrained a large amount of TOM and delivered it to the shallow sea, thus forming the main component of the OM supply. The origin of the marine source rocks in the Baiyun Sag was obvi­ ously affected by the supply of TOM, which was minimally affected by the preservation conditions, and the dilution effect played a definite role

6. Conclusions This study assesses the source rocks based on the data of pyrolysis parameters, biomarker compounds, major and trace elements, and sedimentary organic debris. The dominant effects of OM supply, pres­ ervation, and dilution are studied, and a depositional model of the source rocks is established. The following conclusions could be drawn: (1) Biomarker characteristics of marine source rocks in the Lower Oligocene of the Baiyun Sag in the northern SCS show that the source rocks have a high content of oleanane, which represents terrestrial higher plants. Most of the sedimentary organic debris in the palynofacies is terrestrial plant debris. Therefore, the OM in the marine source rocks mainly originated from terrestrial higher plants, and the contribution of planktonic algae was small. (2) The analysis and test results of the sedimentary organic debris, the mass ratios of certain trace elements[m(U)/m(Th) and m

Fig. 14. Depositional model of the marine source rocks. The origin of the marine source rocks was mainly affected by the supply of TOM, while the contribution of planktonic algae was small. The formation process was minimally affected by the preservation conditions, and the high sediment flux and low uplift barrier reduced the OM dilution. 13

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Journal of Petroleum Science and Engineering 187 (2020) 106821

(Co)/m(Ni)], and biomarker parameters (C35-homohopane index and gammacerane index) show that the Lower Oligocene source rocks in the Baiyun Sag developed in an oxic environment. (3) The origin of marine source rocks in the Lower Oligocene of the Baiyun Sag was mainly controlled by the supply of TOM. As the supply of TOM increased, the abundance of OM markedly increased, and the OM was generally minimally affected by the preservation conditions. The input of TOM was high, and the total amount of OM reaching the seafloor was large. Because of the increased sediment flux and the barrier formed by the low uplifts, the dilution of OM was weak, and the duration of exposure of the OM to oxygen was reduced. Therefore, some OM in the oxic bottom water was not oxidized and became enriched, forming the studied source rocks.

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