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WITHDRAWN: A preliminary study of geological and geochemical characterizations of organic rich Lower Jurassic Beipiao Formation in the Jinyang Basin, northeast China
Geochemistry xxx (xxxx) xxxx
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A preliminary study of geological and geochemical characterizations of organic rich Lower Jurassic Beipiao Formation in the Jinyang Basin, northeast China Tao Zhanga,b,*, Yongfei Lia, Shouliang Suna, Shuwang Chena, Qiushi Suna, Wenming Zonga a b
Shenyang Center of Geological Survey, China Geological Survey, Shenyang 110032, Liaoning, China State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Beipiao formation Jinyang Basin Hydrocarbon potential Biomarker Coal measure strata
The Jinyang Basin located in NE China is a NNE elongated, Mesozoic intracontinental volcano-sedimentary basin. The black shale in Lower Jurassic Beipiao Formation is an important source rock in the Jinyang Basin. Here, we address the hydrocarbon potential based on the results of integrated organic geochemistry and petrology of representative outcrop sections and geological survey wells in and near the Jinyang Basin. The results show a great heterogeneities inside the Beipiao Formation shales. The results suggest that the shale of the Beipiao Formation contains mainly Type II-III organic matter (OM) and is mainly gas-prone and also have some potential to generate oil. Vitrinite reflectance (Ro), Rock-Eval pyrolysis, and biomarker proxies suggest that most of the samples analyzed are in the mature stage, with some in the late mature stages. Samples from different areas have distinctly different biomarker assemblages, suggest that the high heterogeneities in the samples are from both the OM sources, sedimentary conditions as well as thermal maturities. These results and understandings provide insights for expanding new hydrocarbon exploration targets in northeastern China and may also have implications for understanding the environmental changes corresponded to changes in paleoclimate during early to middle Jurassic time.
1. Introduction The Jinyang Basin is located at the southern area of the Songliao basin in Northeast China (Fig. 1), which is the world’s largest lacustrine petroliferous basin and have produced oil and gas since the 1950s (Gong et al., 2015; Liu et al., 2009; Yang, 1985). Preliminary stratigraphic correlations and other studies in this area indicate that Beipiao Formation black shale is widely distributed, suggesting possibly significant hydrocarbon resource potential (He et al., 2006, 2008). However, previous works, especially works performed by Liaohe Oilfield of CNPC, were confined to the surface outcrops and few scattered coal wells, and lack more detailed study of the black shales and depositional environment. The potential of hydrocarbon resources remains rather ambiguous. Empowered by a new campaign of oil and gas exploration in the new area and the new layer in the peripheral area of oil and gas producing basins, much new regional petroleum geologic exploration works were implemented by China Geological Survey (CGS) in the Jinyang Basin and its neighboring area since 2010s. Preliminary
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outcomes indicated that the Jinyang Basin is a petroliferous basin with great potential and the prime source rock in this area is the Lower Jurassic Beipiao Formation shale (Sun et al., 2017; Zhang et al., 2015, 2017; Zhen et al., 2016). However, due to the high risk of exploration and lack of commercial oilfields, the Beipiao Formation in the Jinyang Basin and its adjacent areas had not received much attention. Here we present organic geochemical analyses to evaluate the hydrocarbon resource potential and provide new data and understanding for regional hydrocarbon exploration. The sediments of Beipiao Formation were mainly deposited in a shallow to semi-deep lake during a period without significant volcanisms in the early Jurassic time. The Beipiao Formation is a coal-bearing unit and consists of alternating fluvial lacustrine shale, sandy shale with sandstone, conglomerate and coal-bearing layers. Numerous studies had revealed that coal-bearing strata can have the potential source rocks to generate oil and have been reported in the many areas inside and outside of China. These examples including the Junggar Basin, Turpan Basin and Qaidam Basin in China (Huang et al., 1991; Huang and Palacky, 2006; Li et al., 2016; Xu et al., 2016), the Gippsland Basin
Corresponding author at: Shenyang Center of Geological Survey, China Geological Survey, Shenyang 110032, Liaoning, China. E-mail address: [email protected] (T. Zhang).
https://doi.org/10.1016/j.chemer.2020.125604 Received 23 December 2019; Accepted 30 December 2019 0009-2819/ © 2020 Published by Elsevier GmbH.
Please cite this article as: Tao Zhang, et al., Geochemistry, https://doi.org/10.1016/j.chemer.2020.125604
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Fig. 1. Location and structural elements of the Jinyang Basin and adjacent areas. Area number,1: Sanbao, 2: Nanyao, 3: Baiyao, 4: Kuntou, 5: Shimen, 6: Wolong.
discovered petroliferous basin in the peripheral area of the world famous Songliao Basin, Northeast China (Fig. 1). Mesozoic basins occur widely in the northern part of north China craton, and Jinyang Basin is one of these Mesozoic volcano-sedimentary basins (Li et al., 2004; Yan et al., 2002; Zhou and Zhao, 1999). The Jinyang Basin is characterized by its northeastern elongated shape. However, according recent years of study, the shape of the basin during early Jurassic times is E–W trend (Liu et al., 2007, 2004). The Yanshanian orogeny reshaped the basin during the middle to late Jurassic times. The Jinyang Basin is a typical Mesozoic volcano-sedimentary basin which developed on the northeastern part of north China craton. The residual sedimentary strata in Jinyang Basin are mainly composed of Jurassic strata (Fig. 2). These sedimentary strata reach up to 5000 m thick, including the Proterozoic carbonates, Carboniferous, Permian, Triassic, Jurassic and Cretaceous from bottom to top. The lower part of Beipiao Formation is formed by alluvial and lacustrine facies and is represented by coarse grained, weakly cemented conglomerate, conglomeratic sandstone, siltstone and dark claystone interbedded with coal seams. These sediments rest directly on Lower Jurassic andesite or Precambrian carbonates. The upper part is represented by sandstone, siltstone and dark claystone interbedded with fine conglomerates. Beipiao Formation is deposited as a result of subsidence that related
in Australia and Mahakam Delta in Indonesia (Monthioux et al., 1985; Thomas, 1982). To be specific, the Junggar Basin in northwest China, coal bearing strata have been thoroughly studied due to its significance in oil and gas potential (Chen et al., 2016; Hu et al., 2018; Qiu et al., 2016; Wu et al., 2014). This work can not only provide a basic knowledge of the hydrocarbon potential of Beipiao Formation, but also critical for future indepth studies of the volcano-sedimentary successions of basins in the peripheral area of the Songliao Basin. In this study we utilized integrated methods and parameters including Rock-Eval pyrolysis, total organic carbon (TOC) content, vitrinite reflectance (%Ro), stable carbon isotopic compositions, element analysis of kerogen and also saturated hydrocarbon biomarkers to characterize the Beipiao Formation source rocks. Based on the results of organic geochemical analysis, type of OM, sedimentary paleoenvironment, level of thermal maturation and potential for hydrocarbon generation (oil and/or gas) of the Beipiao Formation source rocks were investigated. 1.1. Geological setting The Jinyang Basin, tectonically bounded by the Nantianmen thrust fault to the west and the Songlingmen uplift to the east, is a newly 2
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Fig. 2. Generalized stratigraphy of the Jinyang Basin.
3
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Fig. 3. Cross diagrams of O/C atomic ratios versus H/C atomic ratios(A) and Tmax versus HI (B) for the Beipiao Formation shale samples in the Jinyang Basin.
chromatography-mass spectrometry (GC–MS).A total of 27 selected samples were carried out by this kind of experiments. Detailed experiment procedures and conditions can be found in Tang et al. (Tang et al., 2019).
to the formation of Yanshanian orogeny during Mesozoic (Davis et al., 2001; Fu et al., 2018; Li et al., 2004). The Jurassic successions are the consequences of the Indosinian and Yanshanian tectonic regimes in northeastern China and record the events related to Mesozoic deconstruction of the North China Craton (Li et al., 2012; Yang et al., 2018; Zhu et al., 2019). Compared with other Mesozoic basins, Jinyang Basin is small and located at the eastern area of Yinshan-Yanshan Orogen. Jurassic coal-bearing clastics and continental volcano-sedimentary units unconformably overlie older units. Early to Late Jurassic mafic to intermediate volcanism was wide spread across the Yanshan area, and reached its maximum intensity in the late Jurassic and Early Cretaceous. The Jinyang Basin, with the development of black mudstones and shales, has potential for hydrocarbon generation. This potential is of significance to regional hydrocarbon exploration in peripheral areas of Songliao Basin. However, it has not been investigated in detail.
3. Results and discussions 3.1. Abundance of OM The abundance of OM is fundamental to hydrocarbon generation potential in source rocks and high contents of OM are a requisite for a good source rock (Tissot and Welte, 1984). Here we use total organic carbon (TOC) content, Rock-Eval parameters, such as petroleum generation potential (S1+S2) (PG value), and chloroform-extractable bitumen content. The TOC content of the samples ranges from 0.03 to 22.4% wt., with an average of 2.78 %. Of all the samples analyzed, 2 of the samples have abnormally high TOC content with value of 22.4% and 19.3% wt. When exclude these 2 samples, the average value is 1.98 %, indicating most of the samples are good source rock referring to the widely accepted criteria (Peters, 1986). The PG values range from 0.02∼6.33 mg/g, with an average of 1.29 mg/g, indicating the potential had been mostly exhausted. But the original PG values of these samples were apparently higher as a consequence of weathering and high thermal maturity level for some samples. For samples with high maturity level, hydrocarbons had been generated and expelled, thus we got extremely low PG value for some samples, especially samples from the Wolong section as indicated by the Ro value.
2. Samples and methods 2.1. Samples Samples were taken from 1 geological survey well named as SZK04 that drilled on the Beipiao Formation outcrops located to the north east of the Chaoyang city. Samples from five geological outcrop sections with good to fair exposure were also collected to further assess the heterogeneities of Beipiao Formation (Fig. 1). A total of 47 samples were collected in this study. To minimize the potential effects of surface weathering and contamination from sample collection and storage, samples used for analyses were freshly cut after removing weathered surface or cleaned with deionized water.
3.2. Type and origin of OM 2.2. Methods The type of OM is an important factor determining its origins and whether source rock is oil prone or gas prone (Tissot and Welte, 1984). The H/C and O/C atomic ratios of kerogen from Beipiao shale range from 0.33 to 9.87 and 0.06 to 0.99 with an average of 1.01 and 0.17, respectively. The H/C-O/C plot shows that the OM type is dominated by type II and type III, indicate that OM sourced from land plant mixed with algae. A pseudo-van Krevelen diagram can be seen in Fig. 3. The HI values vary from 1∼200 mg HC/g TOC, yielding an average of 62.5 mg HC/g TOC. These values indicate most of the samples have type II or III kerogen, suggesting that the black shales are mainly gas-prone. The extremely low HI combined with high Tmax for some samples suggest that hydrocarbon potential for these samples had been exhausted. Fig. 3 shows that the majority of the samples are distributed along the trend lines for type II/III and type III OM, which is in agreement with the
All rock samples were cleaned by deionized water prior to powdering. Total organic carbon content is measured by LECO CS230 Carbon/Sulfur analytical instrument. Rock-Eval pyrolysis analyses were analyzed with OGE-VI Rock-Eval workstation to obtain the values of Tmax, S1, S2 and S3. Subsequently, the hydrogen index (HI), oxygen index (OI), production index (PI) were calculated. Carbon isotope (δ13C) values were determined by using a EUO E3000 GV Isopime instrument with a precision better than 0.1‰. Isotopic ratios are reported in standard δ-notation relative to the Vienna Peedee Belemnite (VPDB) standard (Yang et al., 2015). The resulting extracts of the chloroform-extracted bitumen analysis were further fractioned using open silica gel column chromatography, yielding saturated hydrocarbons that were analyzed by gas 4
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Table 1 Organic geochemistry data of the shale samples from the Beipiao Formation in Jinyang Basin. No.
TOC
TS
S1
S2
S1 + S2
Tmax
PI
HI
δ13C
Ro
KT-1 KT-2 KT-3 KT-4 KT-5 KT-6 KT-7 KT-8 KT-9 KT-10 KT-11 KT-12 KT-13 KT-14 KT-15 KT-16 KT-17 KT-18 KT-19 KT-20 KT-21 KT-22 KT-23 KT-24 NY-1 NY-2 SB-1 SB-2 SHM-1 SHM-2 TH-1 TH-2 TH-3 WL-1 WL-2 WL-3 WL-4 WL-5 WL-6 WL-7 WL-8 WL-9 WL-10 WL-11 WL-12 WL-13 WL-14
0.03 2.17 3.27 1.56 1.39 2.39 1.44 1.66 0.95 0.65 0.87 0.76 3.39 3.79 3.10 2.68 3.53 4.78 1.64 2.07 2.41 3.81 0.61 2.96 22.40 1.60 0.30 0.05 19.30 3.67 0.71 0.72 2.14 0.22 2.18 3.20 2.90 2.57 1.29 1.14 1.63 1.68 0.61 4.22 1.08 1.79 3.45
0.07 0.08 0.08 0.06 0.08 0.00 0.10 0.10 0.07 0.07 0.00 0.07 0.11 0.09 0.09 0.08 0.12 0.10 0.06 0.10 0.12 0.09 0.10 0.09 0.79 0.09 0.07 0.09 0.11 0.10 0.02 0.02 0.06 0.08 0.10 0.08 0.11 0.09 0.08 0.07 0.08 0.08 0.06 0.03 0.01 0.01 0.21
0.09 0.14 0.30 0.09 0.12 0.16 0.04 0.20 0.25 0.06 0.13 0.05 0.43 0.24 0.27 0.40 0.49 0.51 0.20 0.21 0.21 0.12 0.01 0.11 0.04 0.02 0.03 0.01 0.04 0.11 0.01 0.02 0.12 0.04 0.09 0.05 0.03 0.06 0.11 0.02 0.03 0.04 0.02 0.14 0.05 0.01 0.03
0.12 1.23 2.69 0.97 0.35 1.22 0.19 1.16 1.08 0.28 0.61 0.10 3.36 1.90 2.54 2.99 4.57 4.94 1.63 2.10 2.25 6.21 0.49 3.85 0.13 0.05 0.10 0.01 0.72 0.35 0.03 0.03 1.43 0.07 0.19 0.12 0.08 0.15 0.19 0.05 0.04 0.10 0.03 3.55 0.14 0.02 0.06
0.21 1.37 2.99 1.06 0.47 1.38 0.23 1.36 1.33 0.34 0.74 0.15 3.79 2.14 2.81 3.39 5.06 5.45 1.83 2.31 2.46 6.33 0.50 3.96 0.17 0.07 0.13 0.02 0.76 0.46 0.04 0.05 1.55 0.11 0.28 0.17 0.11 0.21 0.30 0.07 0.07 0.14 0.05 3.69 0.19 0.03 0.09
312 446 448 447 444 446 470 450 447 444 444 439 460 464 459 463 459 464 463 454 451 443 442 454 500 368 452 437 558 446 515 414 441 424 573 558 531 447 471 339 427 430 472 441 370 294 408
0.43 0.10 0.10 0.08 0.26 0.12 0.17 0.15 0.19 0.18 0.18 0.33 0.11 0.11 0.10 0.12 0.10 0.09 0.11 0.09 0.09 0.02 0.02 0.03 0.24 0.29 0.23 0.50 0.05 0.24 0.25 0.40 0.08 0.36 0.32 0.29 0.27 0.29 0.37 0.29 0.43 0.29 0.40 0.04 0.26 0.33 0.33
168 91 200 135 39 110 25 103 161 40 106 20 97 95 117 96 133 139 83 92 83 117 51 88 2 1 37 20 44 10 3 3 195 40 10 4 3 6 16 5 3 7 5 60 8 1 2
−24.06 −25.24 −24.75 −24.60 −24.86 −24.99 −25.17 −24.90 −24.52 −24.57 −24.67 −24.26 −24.67 −24.91 −25.18 −24.71 −26.07 −26.58 −26.60 −25.07 −25.08 −25.37 −25.44 −25.69 −23.20 −23.00 −24.74 −23.83 −25.60 −25.30 −24.90 −24.50 −24.10 −24.87 −25.07 −25.76 −25.90 −24.85 −24.89 −25.15 −25.98 −25.72 −25.57 −24.70 −24.70 −25.40 −25.50
7.62 0.82 0.86 0.86 0.97 1.02 1.08 1.09 1.14 1.14 1.11 1.48 1.06 1.01 1.07 1.11 0.99 1.05 1.17 1.15 1.04 0.95 0.96 1.02 1.76 2.11 1.01 0.91 1.16 1.13 1.09 1.05 1.03 0.82 1.45 1.51 1.43 0.97 1.13 0.40 0.84 0.86 1.14 1.01 2.05 1.74 1.96
TOC: total organic carbon, wt%, TS: total sulfur, wt%; S1: residual hydrocarbon, mg HC/g rock; S2: pyrolysis hydrocarbon, mg HC/g rock; S1 + S2: hydrocarbon potential (PG), mg HC/g rock; Tmax: peak temperature of pyrolysis; PI: production index = S1/(S1 + S2); HI: hydrogen index = 100×S2/TOC, mg HC/g TOC; δ13C = stable carbon isotope value, ‰; Ro, = vitrinite reflectance, %.
3.3. Maturity of OM
result shown in the previous plot. This results are also supported by the δ13C value of kerogen (Table 1). The δ13C value of kerogen of the Beipiao black shales range between −23.0∼−26.0‰, with an average of −24.6‰. The type I, II, and III kerogens of source rocks from typical continental basins in China generally have δ13C values of −27‰ to −29‰, −26‰ to −27‰, and −22.5 to −26‰, respectively (Huang et al., 1984). This criteria has been widely and accurately used by Chinese researchers to study the continental sedimentary basins in China (e.g. Cao et al., 2018; Wang et al., 1997). Most of the values fall into the type III kerogen range, higher than phytoplankton and algae values, indicating source of terrigenous higher plants input and oxic environment. These values, when compared with the Lower to middle Jurassic coal-bearing strata in East Junggar Basin, Northwest China, also suggests they were mainly derived from terrestrial plants, while combined with a small contribution of planktonic algae and microbes (Peters et al., 2005; Qian et al., 2018; Yang et al., 2015).
Vitrinite reflectance value (Ro %) is the most widely used indicator of maturity in hydrocarbon source rocks. The Ro values of all the 47 samples vary from 0.4% to 2.11% with an average of 1.15%, with except that 1 sample generate the Ro value of 7.62 because of heavy graphitization of the OM. Most of the samples with middle Ro value in the oil window. But samples from Wolong area and Nanyao area have abnormally high maturity level. Previous studies have illustrated that igneous intrusions in these areas are common. It can be concluded that igneous intrusions may have thermal effects on the evolution of source rocks in the aureole area(Arora et al., 2017; Galushkin, 1997; Spacapan et al., 2018). The results is in consistent with the Tmax value of RockEval pyrolysis. The Tmax values, which is generally considered a thermal maturity indicator, vary in a wide range between 294 °C and 563 °C, with an average of 447 °C, also reflecting the samples varied significantly and suggesting the source rocks have low to high maturity OM. In the cross plot of HI vs Tmax, the majority of the shale samples
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Fig. 4. Total ion current plots for the saturated fraction of selected shale samples from the Beipiao Formation in the Jinyang Basin.
thermally mature or over mature samples because of the break of CeC bond. This pattern may be resulted from the contribution of lower organism’s sources (Bao et al., 2016). Another possible reason for this pattern may be resulted from the igneous intrusions in this section. When source rocks are heated by intrusive bodies, many strange patterns may be seen both in the saturated and aromatic hydrocarbon chromatograms. These patterns mainly resulted from cracking of long chain compounds to short chain compounds, and crack of n-alkanes can be seen during maturation interval 0.95–1.40% Ro, especially affecting the C27+-range (Leythaeuser et al., 1980; Zhu et al., 2008). However, further investigation should be conducted to make clear of this point. Pristane/phytane (Pr/Ph) ratios are commonly used as a redox indicator (Rupprecht et al., 2017). The Pr/Ph ratios of the samples vary from 0.29 to 3.24 (average = 1.10) (Table 2), showing a very wide range between the samples (Fig. 5A). The high ratios mainly occur in the Kuntou area, with an average of 2.52, suggesting that shale from this area was formed mainly in a suboxic to oxic environment. (Didyk et al., 1978). Samples from other areas show low Pr/Ph ratios with an average of 0.70, indicating anaerobic conditions. A common plot of Pr/ nC17 versus Ph/nC18 shows that strong heterogeneities between the samples (Fig. 5A). Such heterogeneities can be explained as evidence of varying redox conditions from anoxic through transitional to oxic (Hao et al., 2011). This phenomenon may be also a reflection of sedimentary environments vary from deep lake to lake margins or swamps. Another interesting trend is showing by the Pr/Ph ratios decrease as Ph/n-C18 increases (Fig. 5B), suggest contribution from eukaryotic phytoplankton may not be the main cause of the high phytane abundance (Hao et al., 2011).
are within the oil window, with some in the early oil zone and some in the over mature zone (Fig. 3B). However, some samples own extremely low HI value as discussed above, and hence this plot should be used with caution. 3.4. Biomarker geochemistry Biomarkers are of great use for its high credibility in assessing key organic geochemistry parameters, such as OM type, depositional environments, thermal maturity level (Peters et al., 2005). Abundant different kind of biomarkers, mainly including n-alkanes, isoprenoids, terpanes, and steranes, were identified in the samples analyzed from the Beipiao Formation shale in this work. 3.4.1. n-Alkanes and isoprenoids The distribution of n-alkanes can provide useful information about the OM sources and thermal maturity level. Organic matter derived from higher plants often have high abundance of high molecular weight n-alkanes n-C27 to n-C31, while marine and lacustrine algae often have abundant short chain n-alkanes in the n-C15 to n-C19 range (Rodriguez and Philip, 2015). The n-alkane carbon numbers are mainly distributed between n-C12 and n-C35 (Fig. 4). Most samples show a unimodal distribution ranging from n-C17 to n-C25 with maximum at C19 or C25 (Fig. 4), which are interpreted as remains of mixed source from algae and higher plants. This is very different from previous studies that the main source of OM is from land plants. On the other way, tissues of higher plants can constitute a significant source of the short chain nalkanes with an odd-over-even predominance found in soil OM (Kuhn et al., 2010). There were no strong odd-over-even (OEP) preference, suggesting a mature stage. This may also suggests a combined contribution of terrestrial higher plants and algae for the OM (Eglinton and Hamilton, 1963). It is very confusing that samples from the Wolong, Nanyao and Sanbao area exhibit the bimodal pattern, distinctly different from all the other samples (Fig. 4). Previous studies indicate that this pattern is common only in low maturity samples, and when the thermal evolution of source rocks increase, this pattern finally diminished, seldom saw in
3.4.2. Terpanes Abundant tricyclic terpanes and pentacyclic triterpanes (hopanes) were identified in all the samples analyzed (Fig. 6). The C20 and C19 tricyclic terpanes (C20T, C19T) are generally considered to source from higher plants (Peters et al., 2005), while the C23 tricyclic terpane (C23T) is sourced from algal and bacteria. The tricyclic terpanes of the samples exhibit normal distribution pattern with the main peak of C23T or C20T (Fig. 6). The relatively high abundance of C23T, in some samples are 6
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Table 2 Various biomarker indices for the Beipiao Formation shale samples in Jinyang Basin. No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
16
17
18
19
20
BY-1 BY-2 BY-3 KT-2 KT-7 KT-8 KT-22 KT-23 KT-24 NY-1 NY-2 SB-1 SB-2 SHM-1 SHM-2 WL-1 WL-2 WL-3 WL-4 WL-5 WL-6 WL-8 WL-9 WL-10 WL-11 WL-12 WL-13
0.86 0.29 0.70 1.93 2.57 2.95 2.07 2.39 3.24 0.31 0.40 0.68 0.57 0.46 0.67 0.46 0.60 0.57 0.75 0.63 0.59 0.59 0.85 0.77 2.60 0.54 0.73
0.59 0.58 0.40 0.11 0.37 0.43 0.88 0.83 1.09 0.64 0.66 0.35 0.30 0.34 0.56 0.47 0.29 0.31 0.07 0.29 0.35 0.34 0.07 0.12 2.14 0.89 0.13
0.86 2.82 0.94 0.05 0.12 0.13 0.23 0.24 0.26 0.97 0.98 0.50 0.42 0.49 0.92 0.60 0.21 0.36 0.09 0.29 0.55 0.47 0.11 0.15 0.89 1.09 0.19
1.17 3.81 1.33 1.19 1.25 1.15 1.34 1.37 2.24 0.22 0.25 1.35 1.24 0.94 1.40 1.23 1.17 1.13 1.20 1.26 1.17 1.25 1.10 1.25 1.43 1.24 1.20
1.20 2.86 1.47 1.01 1.05 1.01 1.22 1.21 1.29 0.78 0.83 1.04 1.15 1.00 1.14 1.12 1.12 1.12 1.09 1.12 1.12 1.23 1.12 1.16 1.24 0.77 1.02
1.27 0.82 2.87 0.82 4.14 1.02 1.77 1.47 1.60 0.45 0.47 0.54 0.55 0.57 0.76 0.55 0.49 0.67 0.70 0.55 0.52 0.62 0.50 0.87 2.17 0.50 0.49
0.22 1.77 1.35 1.02 1.65 0.95 0.70 0.66 1.16 0.07 0.07 0.09 0.07 0.00 0.77 0.10 0.08 0.05 0.22 0.09 0.07 0.12 0.10 0.16 0.64 0.09 0.11
0.76 1.23 1.29 2.14 1.74 1.46 0.77 0.77 1.14 0.40 0.37 0.33 0.21 0.46 1.15 0.26 0.23 0.16 0.43 0.21 0.24 0.29 0.32 0.17 0.88 0.56 0.54
0.91 1.15 0.82 1.51 1.10 1.36 1.54 1.24 1.24 0.93 0.86 1.10 1.21 0.74 0.77 1.17 1.21 1.17 1.03 1.07 1.20 1.06 1.21 1.14 1.11 0.84 0.80
0.19 0.31 0.04 0.22 0.04 0.06 0.07 0.04 0.04 0.25 0.18 0.13 0.25 0.21 0.13 0.12 0.17 0.16 0.13 0.18 0.19 0.15 0.18 0.14 0.10 0.29 0.30
0.65 0.13 0.06 0.89 0.40 0.69 0.20 0.12 0.09 0.48 0.46 0.53 0.53 0.52 0.33 0.53 0.54 0.52 0.55 0.54 0.52 0.54 0.51 0.51 0.37 0.44 0.44
0.48 0.32 0.50 0.51 0.58 0.57 0.59 0.59 0.59 0.55 0.56 0.60 0.61 0.58 0.54 0.58 0.60 0.59 0.61 0.59 0.59 0.59 0.60 0.62 0.59 0.58 0.59
0.18 0.29 0.22 0.09 0.13 0.12 0.31 0.28 0.29 0.15 0.15 0.12 0.14 0.14 0.19 0.15 0.13 0.14 0.15 0.13 0.13 0.13 0.13 0.13 0.26 0.15 0.17
32.33 36.23 14.06 32.29 26.91 26.55 15.53 14.72 11.71 35.11 42.26 39.04 33.83 37.27 32.30 36.20 35.64 41.89 38.16 45.26 37.82 35.55 38.89 39.83 34.47 41.41 37.92
18.00 20.38 12.56 25.12 22.27 27.30 22.50 21.63 21.80 32.04 29.82 30.79 32.58 24.89 28.73 33.74 30.55 27.74 26.58 26.34 29.60 31.60 30.38 28.71 15.77 28.80 30.16
49.68 43.39 73.38 42.59 50.82 46.15 61.97 63.65 66.49 32.85 27.92 30.17 33.59 37.84 38.98 30.06 33.81 30.37 35.26 28.41 32.58 32.85 30.74 31.46 49.75 29.79 31.92
0.22 0.14 0.42 0.52 0.48 0.50 0.46 0.45 0.45 0.43 0.43 0.48 0.48 0.38 0.47 0.49 0.49 0.45 0.43 0.47 0.46 0.48 0.50 0.45 0.45 0.44 0.43
0.08 0.04 0.38 0.49 0.43 0.52 0.38 0.40 0.40 0.41 0.41 0.41 0.40 0.38 0.38 0.43 0.43 0.40 0.36 0.43 0.42 0.42 0.43 0.40 0.21 0.39 0.39
0.13 0.09 0.43 0.53 0.52 0.54 0.50 0.47 0.48 0.47 0.47 0.53 0.50 0.40 0.51 0.56 0.53 0.50 0.46 0.52 0.52 0.54 0.57 0.50 0.45 0.46 0.46
1: Pr/Ph; 2: Pr/n-C17; 3:Pr/n-C18; 4:CPI = [(C25 + C27 + C29 + C31 + C33)/(C24 + C26 + C28 + C30 + C32) + (C25 + C27 + C29 + C31 + C33)/(C26 + C28 + C30 + C32 + C34)]/2; 5: OEP = (C23 + 6 × C25 + C27)/(4 × C24 + 4 × C26); 6: C24 tetracyclic terpane/C26 tricyclic terpane; 7: C19 tricyclic terpane/C23 tricyclic terpane; 8: C20 tricyclic terpane/C23 tricyclic terpane; 9: C26 tricyclic terpane/C25 tricyclic terpane; 10: Gammacerane/C30 hopane; 11: Ts/(Ts + Tm); 12: C31(22S)/ (22S + 22R) hopanes; 13: C30 moretane/C30 hopane; 14: C27 ααα(20R) sterane, %; 15: C28 ααα(20R) sterane, %; 16: C29 ααα(20R) sterane, %; 17: C29 ββ/(αα + ββ) sterane; 18: C29 ααα20S/(20S + 20R) sterane.
C30-hopane (C30H) is the most abundant hopanes in extracts from source rocks, C31 and higher only account minor part of hopanes (Fig. 6). The homohopanes series is dominated by C31 decreasing towards the C35 homohopane (Fig. 6). The gammacerane index (G/C30H) of the samples analyzed in this study show a high heterogeneity, ranges from 0.04 to 0.31 with an average of 0.12 (Table 2, Fig. 7). The low values mainly include the samples from Kuntou area with an average of 0.08. Abundant gammacerane is often found in sediments deposited under hypersaline conditions but is not necessarily restricted to this type of deposits, and. is usually used as evidence for the presence of a stratified water column (Peters et al., 2005; Sinninghe Damsté et al., 1995). High gammacerane index values are mostly occur in high salinity environments, which are favorable for the preservation of OM (Fu et al., 1990; Hanson et al., 2000; Summons et al., 2008). The low gammacerane index combined with the low Pr/Ph values of samples
more abundant than C30 hopanes, demonstrating algae and bacteria provide significant contribution to OM (Adekola et al., 2012; Neto et al., 1982, 1986; Simoneit et al., 1990). However the samples show high heterogeneity, with some samples have most contributions from higher plants (Fig. 6). For example, both the C24 tetracyclic terpanes/ C26 tricyclic terpane (C24Te/C26T) and C19 tricyclic terpane/C23 tricyclic terpane (C19/C23T) increase as C20T/C23T ratios increase (Fig. 7), indicate that high important contribution from terrigenous OM for some samples (George et al., 2004; Hanson et al., 2000; Hao et al., 2011; Volk et al., 2005). This is consistent with the fact that coal measures are common in Beipiao Formation, especially in the Sanbao area. In the samples from the Beipiao Formation. The C26T/C25T ratio is a useful index to differentiate plant and bacteria OM. In this study, the ratios vary from 0.74 to 1.54 (Table 2), which is also indicative of a lacustrine source.(Peters et al., 2005; Volk et al., 2005).
Fig. 5. Variation of Phytane/n-C18 versus Pristane/n-C17 and Phytane/n-C18 versus pristane/phytane for samples from the Beipiao Formation in Jinyang Basin. 7
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Fig. 6. The terpane (m/z 191) fragmentograms of the Beipiao Formation shale of the Jinyang Basin.
Fig. 7. Variation of C19 tricyclic terpane/C23 tricyclic terpanes versus C24 tetracyclic terpanes/C26 tricyclic terpanes (A), C19 tricyclic terpane/C23 tricyclic terpanes versus C20 tricyclic terpane/C23 tricyclic terpanes (B), Ts/(Ts + Tm) ratio versus Gammacerane/C30 hopane ratio(C) and Ts/(Ts + Tm) ratio versus C3122S/(22S + 22R) hopanes ratio(D) for samples from the Beipiao Formation in Jinyang Basin.
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Fig. 8. The sterane (m/z 217) fragmentograms of the Beipiao Formation shale of the Jinyang Basin.
environment, here we also present the cross-plots of Pristane/Phytane ratios versus C29/C27 regular steranes (Fig. 9C). C29/C27 regular steranes ratios are in the range of 0.63–5.68, suggests different OM sources and preserve conditions. For samples from Wolong, Nanyao and Shimen areas, the OM are mainly from algal or bacterial sources preserved under anoxic conditions. While samples from other areas show a large portion of terrigenous OM in oxic to suboxic conditions. Steranes are important biomarkers in source rock and crude oil studies also for they can provide useful information for thermal evolution of OM in source rocks. The C29 ααα20S/(20S + 20R) steranes and C29 αββ/(αββ + ααα) steranes ratios of the samples show a relatively low range of 0.40–0.57 and 0.21–0.52, respectively, with except of 2 samples from Baiyao area (Table 2, Fig. 9D). Combined with the Tmax values of these samples, it can be concluded that weathering maybe the mainly reason that yield such low values in the Baiyao area (Cao et al., 2018; Clayton and King, 1987), and such values should be use with caution. What’s more, the isomerization ratios of steranes cannot be applied to evaluate OM with high maturity level, primarily due to thermal degradation of the steranes (Peters et al., 2005). In other words, the samples in this study mainly in the high maturity level, and the thermal equilibration of steranes has been achieved and thus yield the relatively low range of the ratios.
from Kuntou area suggest OM from this area deposited in freshwater under an oxidizing environment. Samples from the other areas are deposited in stratified high salinity water under a reductive environment. The Ts/(Ts + Tm) ratio for the samples vary from 0.06 to 0.89 (average = 0.45). The Ts/(Ts + Tm) ratio exhibits an increasing trend with gammacerane/C30 hopane ratios but not with the C3122S/(22S + 22R) hopanes ratios (Fig. 7C and D). These results indicate that the relative abundance of Ts to Tm is not only controlled by the thermal maturity level, but controlled by differences in terrestrial and lacustrine OM input or water salinity (Peters et al., 2005). The C30M/C30H ratios of the samples analyzed range from 0.09 to 0.31 (average 0.17), indicating most samples are at the mature stage (Table 2). Maturities measured by the C31 22S/(22S + 22R) hopanes ratios vary from 0.50 to 0.60. The C31H 22S/ (22S + 22R) ratios equilibrate at a ratio ranging from 0.57 to 0.62, also suggesting stages of maturity. 3.4.3. Steranes Abundant steranes were identified in the samples analyzed (Fig. 8). Our results show distinct patterns for samples from different areas. The patterns also show high heterogeneities for samples from different areas. Samples from the Wolong, Sanbao and Nanyao area, the regular steranes are dominated by C27 steranes and show an asymmetrical Vshape in the samples (Fig. 8). For some samples, no predominance of C27 steranes nor C29 sterane, as displayed in the C27, C28 and C29 steranes ternary diagram (Fig. 9). The samples plot principally in the mix area of land plant and plankton/bacterial(Gao et al., 2018; Yurewicz et al., 1998). For samples from Kuntou and Baiyao area, the diagram also show an asymmetrical V-shape, but dominated by the C29 sterane. For the Beipiao Formation shale samples, an overall trend of increasing C28/C29 sterane ratio with increasing C27/C29 sterane ratio (Fig. 9B) was observed. C28/C29 sterane ratios show moderate to high values for Beipiao Formation samples (0.17–1.12, average 0.73). High values mostly occur in the samples from Wolong, Nanyao and Sanbao areas. This suggests possible contribution from chlorophyll-c containing phytoplankton for some samples in this area (Knoll et al., 2007). To better understand the OM type and redox conditions of depositional
4. Conclusions The shale from the Jurassic Beipiao Formation in the Jinyang Basin is an important source rock. The results of TOC, Rock-Eval pyrolysis, carbon isotope as well as biomarker analyses show that there is a great deal of fair to good source rock in the Beipiao Formation. The Beipiao Formation shale samples contain mainly type II-III OM, and the shale is mainly gas-prone and have some potential to generate oil. A comprehensive thermal maturity assessment indicate that most samples are in the oil window, but also with high heterogeneities for some samples have abnormally high maturity level, possibly resulted from igneous intrusions. There are also great heterogeneities in OM input for the Beipiao shale in Jinyang Basin. Even almost all the samples are of type II-III OM, 9
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Fig. 9. Ternary diagram of regular steranes (C27, C28 and C29) showing the relationship between sterane composition and OM inputs (A), cross diagram of C27ααα/ C29ααα steranes ratio values versus C28ααα/C29 ααα ratio values (B), C29ααα(20R)/C27ααα(20R) sterane ratio values versus Pr/Ph ratio values(C), C29αββ/(ααα + αββ) steranes ratio values versus C2920S/(20S + 20R) steranes ratio values (D) of Beipiao Formation shale samples in the Jinyang Basin.
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements This study was supported by China Geological Survey oil and gas survey program (Grant No. DD20190098) and State Key Laboratory of Organic Geochemistry, GIGCAS (Grant No. SKLOG-201711). We thank the team members from both Yangtze University and Guangzhou Institute of Geochemistry for their helps with organic geochemistry analyses works.
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A preliminary study of geological and geochemical characterizations of organic rich Lower Jurassic Beipiao Formation in the Jinyang Basin, northeast China Tao Zhanga,b,*, Yongfei Lia, Shouliang Suna, Shuwang Chena, Qiushi Suna, Wenming Zonga a b
Shenyang Center of Geological Survey, China Geological Survey, Shenyang 110032, Liaoning, China State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Beipiao formation Jinyang Basin Hydrocarbon potential Biomarker Coal measure strata
The Jinyang Basin located in NE China is a NNE elongated, Mesozoic intracontinental volcano-sedimentary basin. The black shale in Lower Jurassic Beipiao Formation is an important source rock in the Jinyang Basin. Here, we address the hydrocarbon potential based on the results of integrated organic geochemistry and petrology of representative outcrop sections and geological survey wells in and near the Jinyang Basin. The results show a great heterogeneities inside the Beipiao Formation shales. The results suggest that the shale of the Beipiao Formation contains mainly Type II-III organic matter (OM) and is mainly gas-prone and also have some potential to generate oil. Vitrinite reflectance (Ro), Rock-Eval pyrolysis, and biomarker proxies suggest that most of the samples analyzed are in the mature stage, with some in the late mature stages. Samples from different areas have distinctly different biomarker assemblages, suggest that the high heterogeneities in the samples are from both the OM sources, sedimentary conditions as well as thermal maturities. These results and understandings provide insights for expanding new hydrocarbon exploration targets in northeastern China and may also have implications for understanding the environmental changes corresponded to changes in paleoclimate during early to middle Jurassic time.
1. Introduction The Jinyang Basin is located at the southern area of the Songliao basin in Northeast China (Fig. 1), which is the world’s largest lacustrine petroliferous basin and have produced oil and gas since the 1950s (Gong et al., 2015; Liu et al., 2009; Yang, 1985). Preliminary stratigraphic correlations and other studies in this area indicate that Beipiao Formation black shale is widely distributed, suggesting possibly significant hydrocarbon resource potential (He et al., 2006, 2008). However, previous works, especially works performed by Liaohe Oilfield of CNPC, were confined to the surface outcrops and few scattered coal wells, and lack more detailed study of the black shales and depositional environment. The potential of hydrocarbon resources remains rather ambiguous. Empowered by a new campaign of oil and gas exploration in the new area and the new layer in the peripheral area of oil and gas producing basins, much new regional petroleum geologic exploration works were implemented by China Geological Survey (CGS) in the Jinyang Basin and its neighboring area since 2010s. Preliminary
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outcomes indicated that the Jinyang Basin is a petroliferous basin with great potential and the prime source rock in this area is the Lower Jurassic Beipiao Formation shale (Sun et al., 2017; Zhang et al., 2015, 2017; Zhen et al., 2016). However, due to the high risk of exploration and lack of commercial oilfields, the Beipiao Formation in the Jinyang Basin and its adjacent areas had not received much attention. Here we present organic geochemical analyses to evaluate the hydrocarbon resource potential and provide new data and understanding for regional hydrocarbon exploration. The sediments of Beipiao Formation were mainly deposited in a shallow to semi-deep lake during a period without significant volcanisms in the early Jurassic time. The Beipiao Formation is a coal-bearing unit and consists of alternating fluvial lacustrine shale, sandy shale with sandstone, conglomerate and coal-bearing layers. Numerous studies had revealed that coal-bearing strata can have the potential source rocks to generate oil and have been reported in the many areas inside and outside of China. These examples including the Junggar Basin, Turpan Basin and Qaidam Basin in China (Huang et al., 1991; Huang and Palacky, 2006; Li et al., 2016; Xu et al., 2016), the Gippsland Basin
Corresponding author at: Shenyang Center of Geological Survey, China Geological Survey, Shenyang 110032, Liaoning, China. E-mail address: [email protected] (T. Zhang).
https://doi.org/10.1016/j.chemer.2020.125604 Received 23 December 2019; Accepted 30 December 2019 0009-2819/ © 2020 Published by Elsevier GmbH.
Please cite this article as: Tao Zhang, et al., Geochemistry, https://doi.org/10.1016/j.chemer.2020.125604
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Fig. 1. Location and structural elements of the Jinyang Basin and adjacent areas. Area number,1: Sanbao, 2: Nanyao, 3: Baiyao, 4: Kuntou, 5: Shimen, 6: Wolong.
discovered petroliferous basin in the peripheral area of the world famous Songliao Basin, Northeast China (Fig. 1). Mesozoic basins occur widely in the northern part of north China craton, and Jinyang Basin is one of these Mesozoic volcano-sedimentary basins (Li et al., 2004; Yan et al., 2002; Zhou and Zhao, 1999). The Jinyang Basin is characterized by its northeastern elongated shape. However, according recent years of study, the shape of the basin during early Jurassic times is E–W trend (Liu et al., 2007, 2004). The Yanshanian orogeny reshaped the basin during the middle to late Jurassic times. The Jinyang Basin is a typical Mesozoic volcano-sedimentary basin which developed on the northeastern part of north China craton. The residual sedimentary strata in Jinyang Basin are mainly composed of Jurassic strata (Fig. 2). These sedimentary strata reach up to 5000 m thick, including the Proterozoic carbonates, Carboniferous, Permian, Triassic, Jurassic and Cretaceous from bottom to top. The lower part of Beipiao Formation is formed by alluvial and lacustrine facies and is represented by coarse grained, weakly cemented conglomerate, conglomeratic sandstone, siltstone and dark claystone interbedded with coal seams. These sediments rest directly on Lower Jurassic andesite or Precambrian carbonates. The upper part is represented by sandstone, siltstone and dark claystone interbedded with fine conglomerates. Beipiao Formation is deposited as a result of subsidence that related
in Australia and Mahakam Delta in Indonesia (Monthioux et al., 1985; Thomas, 1982). To be specific, the Junggar Basin in northwest China, coal bearing strata have been thoroughly studied due to its significance in oil and gas potential (Chen et al., 2016; Hu et al., 2018; Qiu et al., 2016; Wu et al., 2014). This work can not only provide a basic knowledge of the hydrocarbon potential of Beipiao Formation, but also critical for future indepth studies of the volcano-sedimentary successions of basins in the peripheral area of the Songliao Basin. In this study we utilized integrated methods and parameters including Rock-Eval pyrolysis, total organic carbon (TOC) content, vitrinite reflectance (%Ro), stable carbon isotopic compositions, element analysis of kerogen and also saturated hydrocarbon biomarkers to characterize the Beipiao Formation source rocks. Based on the results of organic geochemical analysis, type of OM, sedimentary paleoenvironment, level of thermal maturation and potential for hydrocarbon generation (oil and/or gas) of the Beipiao Formation source rocks were investigated. 1.1. Geological setting The Jinyang Basin, tectonically bounded by the Nantianmen thrust fault to the west and the Songlingmen uplift to the east, is a newly 2
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Fig. 2. Generalized stratigraphy of the Jinyang Basin.
3
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Fig. 3. Cross diagrams of O/C atomic ratios versus H/C atomic ratios(A) and Tmax versus HI (B) for the Beipiao Formation shale samples in the Jinyang Basin.
chromatography-mass spectrometry (GC–MS).A total of 27 selected samples were carried out by this kind of experiments. Detailed experiment procedures and conditions can be found in Tang et al. (Tang et al., 2019).
to the formation of Yanshanian orogeny during Mesozoic (Davis et al., 2001; Fu et al., 2018; Li et al., 2004). The Jurassic successions are the consequences of the Indosinian and Yanshanian tectonic regimes in northeastern China and record the events related to Mesozoic deconstruction of the North China Craton (Li et al., 2012; Yang et al., 2018; Zhu et al., 2019). Compared with other Mesozoic basins, Jinyang Basin is small and located at the eastern area of Yinshan-Yanshan Orogen. Jurassic coal-bearing clastics and continental volcano-sedimentary units unconformably overlie older units. Early to Late Jurassic mafic to intermediate volcanism was wide spread across the Yanshan area, and reached its maximum intensity in the late Jurassic and Early Cretaceous. The Jinyang Basin, with the development of black mudstones and shales, has potential for hydrocarbon generation. This potential is of significance to regional hydrocarbon exploration in peripheral areas of Songliao Basin. However, it has not been investigated in detail.
3. Results and discussions 3.1. Abundance of OM The abundance of OM is fundamental to hydrocarbon generation potential in source rocks and high contents of OM are a requisite for a good source rock (Tissot and Welte, 1984). Here we use total organic carbon (TOC) content, Rock-Eval parameters, such as petroleum generation potential (S1+S2) (PG value), and chloroform-extractable bitumen content. The TOC content of the samples ranges from 0.03 to 22.4% wt., with an average of 2.78 %. Of all the samples analyzed, 2 of the samples have abnormally high TOC content with value of 22.4% and 19.3% wt. When exclude these 2 samples, the average value is 1.98 %, indicating most of the samples are good source rock referring to the widely accepted criteria (Peters, 1986). The PG values range from 0.02∼6.33 mg/g, with an average of 1.29 mg/g, indicating the potential had been mostly exhausted. But the original PG values of these samples were apparently higher as a consequence of weathering and high thermal maturity level for some samples. For samples with high maturity level, hydrocarbons had been generated and expelled, thus we got extremely low PG value for some samples, especially samples from the Wolong section as indicated by the Ro value.
2. Samples and methods 2.1. Samples Samples were taken from 1 geological survey well named as SZK04 that drilled on the Beipiao Formation outcrops located to the north east of the Chaoyang city. Samples from five geological outcrop sections with good to fair exposure were also collected to further assess the heterogeneities of Beipiao Formation (Fig. 1). A total of 47 samples were collected in this study. To minimize the potential effects of surface weathering and contamination from sample collection and storage, samples used for analyses were freshly cut after removing weathered surface or cleaned with deionized water.
3.2. Type and origin of OM 2.2. Methods The type of OM is an important factor determining its origins and whether source rock is oil prone or gas prone (Tissot and Welte, 1984). The H/C and O/C atomic ratios of kerogen from Beipiao shale range from 0.33 to 9.87 and 0.06 to 0.99 with an average of 1.01 and 0.17, respectively. The H/C-O/C plot shows that the OM type is dominated by type II and type III, indicate that OM sourced from land plant mixed with algae. A pseudo-van Krevelen diagram can be seen in Fig. 3. The HI values vary from 1∼200 mg HC/g TOC, yielding an average of 62.5 mg HC/g TOC. These values indicate most of the samples have type II or III kerogen, suggesting that the black shales are mainly gas-prone. The extremely low HI combined with high Tmax for some samples suggest that hydrocarbon potential for these samples had been exhausted. Fig. 3 shows that the majority of the samples are distributed along the trend lines for type II/III and type III OM, which is in agreement with the
All rock samples were cleaned by deionized water prior to powdering. Total organic carbon content is measured by LECO CS230 Carbon/Sulfur analytical instrument. Rock-Eval pyrolysis analyses were analyzed with OGE-VI Rock-Eval workstation to obtain the values of Tmax, S1, S2 and S3. Subsequently, the hydrogen index (HI), oxygen index (OI), production index (PI) were calculated. Carbon isotope (δ13C) values were determined by using a EUO E3000 GV Isopime instrument with a precision better than 0.1‰. Isotopic ratios are reported in standard δ-notation relative to the Vienna Peedee Belemnite (VPDB) standard (Yang et al., 2015). The resulting extracts of the chloroform-extracted bitumen analysis were further fractioned using open silica gel column chromatography, yielding saturated hydrocarbons that were analyzed by gas 4
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Table 1 Organic geochemistry data of the shale samples from the Beipiao Formation in Jinyang Basin. No.
TOC
TS
S1
S2
S1 + S2
Tmax
PI
HI
δ13C
Ro
KT-1 KT-2 KT-3 KT-4 KT-5 KT-6 KT-7 KT-8 KT-9 KT-10 KT-11 KT-12 KT-13 KT-14 KT-15 KT-16 KT-17 KT-18 KT-19 KT-20 KT-21 KT-22 KT-23 KT-24 NY-1 NY-2 SB-1 SB-2 SHM-1 SHM-2 TH-1 TH-2 TH-3 WL-1 WL-2 WL-3 WL-4 WL-5 WL-6 WL-7 WL-8 WL-9 WL-10 WL-11 WL-12 WL-13 WL-14
0.03 2.17 3.27 1.56 1.39 2.39 1.44 1.66 0.95 0.65 0.87 0.76 3.39 3.79 3.10 2.68 3.53 4.78 1.64 2.07 2.41 3.81 0.61 2.96 22.40 1.60 0.30 0.05 19.30 3.67 0.71 0.72 2.14 0.22 2.18 3.20 2.90 2.57 1.29 1.14 1.63 1.68 0.61 4.22 1.08 1.79 3.45
0.07 0.08 0.08 0.06 0.08 0.00 0.10 0.10 0.07 0.07 0.00 0.07 0.11 0.09 0.09 0.08 0.12 0.10 0.06 0.10 0.12 0.09 0.10 0.09 0.79 0.09 0.07 0.09 0.11 0.10 0.02 0.02 0.06 0.08 0.10 0.08 0.11 0.09 0.08 0.07 0.08 0.08 0.06 0.03 0.01 0.01 0.21
0.09 0.14 0.30 0.09 0.12 0.16 0.04 0.20 0.25 0.06 0.13 0.05 0.43 0.24 0.27 0.40 0.49 0.51 0.20 0.21 0.21 0.12 0.01 0.11 0.04 0.02 0.03 0.01 0.04 0.11 0.01 0.02 0.12 0.04 0.09 0.05 0.03 0.06 0.11 0.02 0.03 0.04 0.02 0.14 0.05 0.01 0.03
0.12 1.23 2.69 0.97 0.35 1.22 0.19 1.16 1.08 0.28 0.61 0.10 3.36 1.90 2.54 2.99 4.57 4.94 1.63 2.10 2.25 6.21 0.49 3.85 0.13 0.05 0.10 0.01 0.72 0.35 0.03 0.03 1.43 0.07 0.19 0.12 0.08 0.15 0.19 0.05 0.04 0.10 0.03 3.55 0.14 0.02 0.06
0.21 1.37 2.99 1.06 0.47 1.38 0.23 1.36 1.33 0.34 0.74 0.15 3.79 2.14 2.81 3.39 5.06 5.45 1.83 2.31 2.46 6.33 0.50 3.96 0.17 0.07 0.13 0.02 0.76 0.46 0.04 0.05 1.55 0.11 0.28 0.17 0.11 0.21 0.30 0.07 0.07 0.14 0.05 3.69 0.19 0.03 0.09
312 446 448 447 444 446 470 450 447 444 444 439 460 464 459 463 459 464 463 454 451 443 442 454 500 368 452 437 558 446 515 414 441 424 573 558 531 447 471 339 427 430 472 441 370 294 408
0.43 0.10 0.10 0.08 0.26 0.12 0.17 0.15 0.19 0.18 0.18 0.33 0.11 0.11 0.10 0.12 0.10 0.09 0.11 0.09 0.09 0.02 0.02 0.03 0.24 0.29 0.23 0.50 0.05 0.24 0.25 0.40 0.08 0.36 0.32 0.29 0.27 0.29 0.37 0.29 0.43 0.29 0.40 0.04 0.26 0.33 0.33
168 91 200 135 39 110 25 103 161 40 106 20 97 95 117 96 133 139 83 92 83 117 51 88 2 1 37 20 44 10 3 3 195 40 10 4 3 6 16 5 3 7 5 60 8 1 2
−24.06 −25.24 −24.75 −24.60 −24.86 −24.99 −25.17 −24.90 −24.52 −24.57 −24.67 −24.26 −24.67 −24.91 −25.18 −24.71 −26.07 −26.58 −26.60 −25.07 −25.08 −25.37 −25.44 −25.69 −23.20 −23.00 −24.74 −23.83 −25.60 −25.30 −24.90 −24.50 −24.10 −24.87 −25.07 −25.76 −25.90 −24.85 −24.89 −25.15 −25.98 −25.72 −25.57 −24.70 −24.70 −25.40 −25.50
7.62 0.82 0.86 0.86 0.97 1.02 1.08 1.09 1.14 1.14 1.11 1.48 1.06 1.01 1.07 1.11 0.99 1.05 1.17 1.15 1.04 0.95 0.96 1.02 1.76 2.11 1.01 0.91 1.16 1.13 1.09 1.05 1.03 0.82 1.45 1.51 1.43 0.97 1.13 0.40 0.84 0.86 1.14 1.01 2.05 1.74 1.96
TOC: total organic carbon, wt%, TS: total sulfur, wt%; S1: residual hydrocarbon, mg HC/g rock; S2: pyrolysis hydrocarbon, mg HC/g rock; S1 + S2: hydrocarbon potential (PG), mg HC/g rock; Tmax: peak temperature of pyrolysis; PI: production index = S1/(S1 + S2); HI: hydrogen index = 100×S2/TOC, mg HC/g TOC; δ13C = stable carbon isotope value, ‰; Ro, = vitrinite reflectance, %.
3.3. Maturity of OM
result shown in the previous plot. This results are also supported by the δ13C value of kerogen (Table 1). The δ13C value of kerogen of the Beipiao black shales range between −23.0∼−26.0‰, with an average of −24.6‰. The type I, II, and III kerogens of source rocks from typical continental basins in China generally have δ13C values of −27‰ to −29‰, −26‰ to −27‰, and −22.5 to −26‰, respectively (Huang et al., 1984). This criteria has been widely and accurately used by Chinese researchers to study the continental sedimentary basins in China (e.g. Cao et al., 2018; Wang et al., 1997). Most of the values fall into the type III kerogen range, higher than phytoplankton and algae values, indicating source of terrigenous higher plants input and oxic environment. These values, when compared with the Lower to middle Jurassic coal-bearing strata in East Junggar Basin, Northwest China, also suggests they were mainly derived from terrestrial plants, while combined with a small contribution of planktonic algae and microbes (Peters et al., 2005; Qian et al., 2018; Yang et al., 2015).
Vitrinite reflectance value (Ro %) is the most widely used indicator of maturity in hydrocarbon source rocks. The Ro values of all the 47 samples vary from 0.4% to 2.11% with an average of 1.15%, with except that 1 sample generate the Ro value of 7.62 because of heavy graphitization of the OM. Most of the samples with middle Ro value in the oil window. But samples from Wolong area and Nanyao area have abnormally high maturity level. Previous studies have illustrated that igneous intrusions in these areas are common. It can be concluded that igneous intrusions may have thermal effects on the evolution of source rocks in the aureole area(Arora et al., 2017; Galushkin, 1997; Spacapan et al., 2018). The results is in consistent with the Tmax value of RockEval pyrolysis. The Tmax values, which is generally considered a thermal maturity indicator, vary in a wide range between 294 °C and 563 °C, with an average of 447 °C, also reflecting the samples varied significantly and suggesting the source rocks have low to high maturity OM. In the cross plot of HI vs Tmax, the majority of the shale samples
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Fig. 4. Total ion current plots for the saturated fraction of selected shale samples from the Beipiao Formation in the Jinyang Basin.
thermally mature or over mature samples because of the break of CeC bond. This pattern may be resulted from the contribution of lower organism’s sources (Bao et al., 2016). Another possible reason for this pattern may be resulted from the igneous intrusions in this section. When source rocks are heated by intrusive bodies, many strange patterns may be seen both in the saturated and aromatic hydrocarbon chromatograms. These patterns mainly resulted from cracking of long chain compounds to short chain compounds, and crack of n-alkanes can be seen during maturation interval 0.95–1.40% Ro, especially affecting the C27+-range (Leythaeuser et al., 1980; Zhu et al., 2008). However, further investigation should be conducted to make clear of this point. Pristane/phytane (Pr/Ph) ratios are commonly used as a redox indicator (Rupprecht et al., 2017). The Pr/Ph ratios of the samples vary from 0.29 to 3.24 (average = 1.10) (Table 2), showing a very wide range between the samples (Fig. 5A). The high ratios mainly occur in the Kuntou area, with an average of 2.52, suggesting that shale from this area was formed mainly in a suboxic to oxic environment. (Didyk et al., 1978). Samples from other areas show low Pr/Ph ratios with an average of 0.70, indicating anaerobic conditions. A common plot of Pr/ nC17 versus Ph/nC18 shows that strong heterogeneities between the samples (Fig. 5A). Such heterogeneities can be explained as evidence of varying redox conditions from anoxic through transitional to oxic (Hao et al., 2011). This phenomenon may be also a reflection of sedimentary environments vary from deep lake to lake margins or swamps. Another interesting trend is showing by the Pr/Ph ratios decrease as Ph/n-C18 increases (Fig. 5B), suggest contribution from eukaryotic phytoplankton may not be the main cause of the high phytane abundance (Hao et al., 2011).
are within the oil window, with some in the early oil zone and some in the over mature zone (Fig. 3B). However, some samples own extremely low HI value as discussed above, and hence this plot should be used with caution. 3.4. Biomarker geochemistry Biomarkers are of great use for its high credibility in assessing key organic geochemistry parameters, such as OM type, depositional environments, thermal maturity level (Peters et al., 2005). Abundant different kind of biomarkers, mainly including n-alkanes, isoprenoids, terpanes, and steranes, were identified in the samples analyzed from the Beipiao Formation shale in this work. 3.4.1. n-Alkanes and isoprenoids The distribution of n-alkanes can provide useful information about the OM sources and thermal maturity level. Organic matter derived from higher plants often have high abundance of high molecular weight n-alkanes n-C27 to n-C31, while marine and lacustrine algae often have abundant short chain n-alkanes in the n-C15 to n-C19 range (Rodriguez and Philip, 2015). The n-alkane carbon numbers are mainly distributed between n-C12 and n-C35 (Fig. 4). Most samples show a unimodal distribution ranging from n-C17 to n-C25 with maximum at C19 or C25 (Fig. 4), which are interpreted as remains of mixed source from algae and higher plants. This is very different from previous studies that the main source of OM is from land plants. On the other way, tissues of higher plants can constitute a significant source of the short chain nalkanes with an odd-over-even predominance found in soil OM (Kuhn et al., 2010). There were no strong odd-over-even (OEP) preference, suggesting a mature stage. This may also suggests a combined contribution of terrestrial higher plants and algae for the OM (Eglinton and Hamilton, 1963). It is very confusing that samples from the Wolong, Nanyao and Sanbao area exhibit the bimodal pattern, distinctly different from all the other samples (Fig. 4). Previous studies indicate that this pattern is common only in low maturity samples, and when the thermal evolution of source rocks increase, this pattern finally diminished, seldom saw in
3.4.2. Terpanes Abundant tricyclic terpanes and pentacyclic triterpanes (hopanes) were identified in all the samples analyzed (Fig. 6). The C20 and C19 tricyclic terpanes (C20T, C19T) are generally considered to source from higher plants (Peters et al., 2005), while the C23 tricyclic terpane (C23T) is sourced from algal and bacteria. The tricyclic terpanes of the samples exhibit normal distribution pattern with the main peak of C23T or C20T (Fig. 6). The relatively high abundance of C23T, in some samples are 6
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Table 2 Various biomarker indices for the Beipiao Formation shale samples in Jinyang Basin. No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
16
17
18
19
20
BY-1 BY-2 BY-3 KT-2 KT-7 KT-8 KT-22 KT-23 KT-24 NY-1 NY-2 SB-1 SB-2 SHM-1 SHM-2 WL-1 WL-2 WL-3 WL-4 WL-5 WL-6 WL-8 WL-9 WL-10 WL-11 WL-12 WL-13
0.86 0.29 0.70 1.93 2.57 2.95 2.07 2.39 3.24 0.31 0.40 0.68 0.57 0.46 0.67 0.46 0.60 0.57 0.75 0.63 0.59 0.59 0.85 0.77 2.60 0.54 0.73
0.59 0.58 0.40 0.11 0.37 0.43 0.88 0.83 1.09 0.64 0.66 0.35 0.30 0.34 0.56 0.47 0.29 0.31 0.07 0.29 0.35 0.34 0.07 0.12 2.14 0.89 0.13
0.86 2.82 0.94 0.05 0.12 0.13 0.23 0.24 0.26 0.97 0.98 0.50 0.42 0.49 0.92 0.60 0.21 0.36 0.09 0.29 0.55 0.47 0.11 0.15 0.89 1.09 0.19
1.17 3.81 1.33 1.19 1.25 1.15 1.34 1.37 2.24 0.22 0.25 1.35 1.24 0.94 1.40 1.23 1.17 1.13 1.20 1.26 1.17 1.25 1.10 1.25 1.43 1.24 1.20
1.20 2.86 1.47 1.01 1.05 1.01 1.22 1.21 1.29 0.78 0.83 1.04 1.15 1.00 1.14 1.12 1.12 1.12 1.09 1.12 1.12 1.23 1.12 1.16 1.24 0.77 1.02
1.27 0.82 2.87 0.82 4.14 1.02 1.77 1.47 1.60 0.45 0.47 0.54 0.55 0.57 0.76 0.55 0.49 0.67 0.70 0.55 0.52 0.62 0.50 0.87 2.17 0.50 0.49
0.22 1.77 1.35 1.02 1.65 0.95 0.70 0.66 1.16 0.07 0.07 0.09 0.07 0.00 0.77 0.10 0.08 0.05 0.22 0.09 0.07 0.12 0.10 0.16 0.64 0.09 0.11
0.76 1.23 1.29 2.14 1.74 1.46 0.77 0.77 1.14 0.40 0.37 0.33 0.21 0.46 1.15 0.26 0.23 0.16 0.43 0.21 0.24 0.29 0.32 0.17 0.88 0.56 0.54
0.91 1.15 0.82 1.51 1.10 1.36 1.54 1.24 1.24 0.93 0.86 1.10 1.21 0.74 0.77 1.17 1.21 1.17 1.03 1.07 1.20 1.06 1.21 1.14 1.11 0.84 0.80
0.19 0.31 0.04 0.22 0.04 0.06 0.07 0.04 0.04 0.25 0.18 0.13 0.25 0.21 0.13 0.12 0.17 0.16 0.13 0.18 0.19 0.15 0.18 0.14 0.10 0.29 0.30
0.65 0.13 0.06 0.89 0.40 0.69 0.20 0.12 0.09 0.48 0.46 0.53 0.53 0.52 0.33 0.53 0.54 0.52 0.55 0.54 0.52 0.54 0.51 0.51 0.37 0.44 0.44
0.48 0.32 0.50 0.51 0.58 0.57 0.59 0.59 0.59 0.55 0.56 0.60 0.61 0.58 0.54 0.58 0.60 0.59 0.61 0.59 0.59 0.59 0.60 0.62 0.59 0.58 0.59
0.18 0.29 0.22 0.09 0.13 0.12 0.31 0.28 0.29 0.15 0.15 0.12 0.14 0.14 0.19 0.15 0.13 0.14 0.15 0.13 0.13 0.13 0.13 0.13 0.26 0.15 0.17
32.33 36.23 14.06 32.29 26.91 26.55 15.53 14.72 11.71 35.11 42.26 39.04 33.83 37.27 32.30 36.20 35.64 41.89 38.16 45.26 37.82 35.55 38.89 39.83 34.47 41.41 37.92
18.00 20.38 12.56 25.12 22.27 27.30 22.50 21.63 21.80 32.04 29.82 30.79 32.58 24.89 28.73 33.74 30.55 27.74 26.58 26.34 29.60 31.60 30.38 28.71 15.77 28.80 30.16
49.68 43.39 73.38 42.59 50.82 46.15 61.97 63.65 66.49 32.85 27.92 30.17 33.59 37.84 38.98 30.06 33.81 30.37 35.26 28.41 32.58 32.85 30.74 31.46 49.75 29.79 31.92
0.22 0.14 0.42 0.52 0.48 0.50 0.46 0.45 0.45 0.43 0.43 0.48 0.48 0.38 0.47 0.49 0.49 0.45 0.43 0.47 0.46 0.48 0.50 0.45 0.45 0.44 0.43
0.08 0.04 0.38 0.49 0.43 0.52 0.38 0.40 0.40 0.41 0.41 0.41 0.40 0.38 0.38 0.43 0.43 0.40 0.36 0.43 0.42 0.42 0.43 0.40 0.21 0.39 0.39
0.13 0.09 0.43 0.53 0.52 0.54 0.50 0.47 0.48 0.47 0.47 0.53 0.50 0.40 0.51 0.56 0.53 0.50 0.46 0.52 0.52 0.54 0.57 0.50 0.45 0.46 0.46
1: Pr/Ph; 2: Pr/n-C17; 3:Pr/n-C18; 4:CPI = [(C25 + C27 + C29 + C31 + C33)/(C24 + C26 + C28 + C30 + C32) + (C25 + C27 + C29 + C31 + C33)/(C26 + C28 + C30 + C32 + C34)]/2; 5: OEP = (C23 + 6 × C25 + C27)/(4 × C24 + 4 × C26); 6: C24 tetracyclic terpane/C26 tricyclic terpane; 7: C19 tricyclic terpane/C23 tricyclic terpane; 8: C20 tricyclic terpane/C23 tricyclic terpane; 9: C26 tricyclic terpane/C25 tricyclic terpane; 10: Gammacerane/C30 hopane; 11: Ts/(Ts + Tm); 12: C31(22S)/ (22S + 22R) hopanes; 13: C30 moretane/C30 hopane; 14: C27 ααα(20R) sterane, %; 15: C28 ααα(20R) sterane, %; 16: C29 ααα(20R) sterane, %; 17: C29 ββ/(αα + ββ) sterane; 18: C29 ααα20S/(20S + 20R) sterane.
C30-hopane (C30H) is the most abundant hopanes in extracts from source rocks, C31 and higher only account minor part of hopanes (Fig. 6). The homohopanes series is dominated by C31 decreasing towards the C35 homohopane (Fig. 6). The gammacerane index (G/C30H) of the samples analyzed in this study show a high heterogeneity, ranges from 0.04 to 0.31 with an average of 0.12 (Table 2, Fig. 7). The low values mainly include the samples from Kuntou area with an average of 0.08. Abundant gammacerane is often found in sediments deposited under hypersaline conditions but is not necessarily restricted to this type of deposits, and. is usually used as evidence for the presence of a stratified water column (Peters et al., 2005; Sinninghe Damsté et al., 1995). High gammacerane index values are mostly occur in high salinity environments, which are favorable for the preservation of OM (Fu et al., 1990; Hanson et al., 2000; Summons et al., 2008). The low gammacerane index combined with the low Pr/Ph values of samples
more abundant than C30 hopanes, demonstrating algae and bacteria provide significant contribution to OM (Adekola et al., 2012; Neto et al., 1982, 1986; Simoneit et al., 1990). However the samples show high heterogeneity, with some samples have most contributions from higher plants (Fig. 6). For example, both the C24 tetracyclic terpanes/ C26 tricyclic terpane (C24Te/C26T) and C19 tricyclic terpane/C23 tricyclic terpane (C19/C23T) increase as C20T/C23T ratios increase (Fig. 7), indicate that high important contribution from terrigenous OM for some samples (George et al., 2004; Hanson et al., 2000; Hao et al., 2011; Volk et al., 2005). This is consistent with the fact that coal measures are common in Beipiao Formation, especially in the Sanbao area. In the samples from the Beipiao Formation. The C26T/C25T ratio is a useful index to differentiate plant and bacteria OM. In this study, the ratios vary from 0.74 to 1.54 (Table 2), which is also indicative of a lacustrine source.(Peters et al., 2005; Volk et al., 2005).
Fig. 5. Variation of Phytane/n-C18 versus Pristane/n-C17 and Phytane/n-C18 versus pristane/phytane for samples from the Beipiao Formation in Jinyang Basin. 7
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Fig. 6. The terpane (m/z 191) fragmentograms of the Beipiao Formation shale of the Jinyang Basin.
Fig. 7. Variation of C19 tricyclic terpane/C23 tricyclic terpanes versus C24 tetracyclic terpanes/C26 tricyclic terpanes (A), C19 tricyclic terpane/C23 tricyclic terpanes versus C20 tricyclic terpane/C23 tricyclic terpanes (B), Ts/(Ts + Tm) ratio versus Gammacerane/C30 hopane ratio(C) and Ts/(Ts + Tm) ratio versus C3122S/(22S + 22R) hopanes ratio(D) for samples from the Beipiao Formation in Jinyang Basin.
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Fig. 8. The sterane (m/z 217) fragmentograms of the Beipiao Formation shale of the Jinyang Basin.
environment, here we also present the cross-plots of Pristane/Phytane ratios versus C29/C27 regular steranes (Fig. 9C). C29/C27 regular steranes ratios are in the range of 0.63–5.68, suggests different OM sources and preserve conditions. For samples from Wolong, Nanyao and Shimen areas, the OM are mainly from algal or bacterial sources preserved under anoxic conditions. While samples from other areas show a large portion of terrigenous OM in oxic to suboxic conditions. Steranes are important biomarkers in source rock and crude oil studies also for they can provide useful information for thermal evolution of OM in source rocks. The C29 ααα20S/(20S + 20R) steranes and C29 αββ/(αββ + ααα) steranes ratios of the samples show a relatively low range of 0.40–0.57 and 0.21–0.52, respectively, with except of 2 samples from Baiyao area (Table 2, Fig. 9D). Combined with the Tmax values of these samples, it can be concluded that weathering maybe the mainly reason that yield such low values in the Baiyao area (Cao et al., 2018; Clayton and King, 1987), and such values should be use with caution. What’s more, the isomerization ratios of steranes cannot be applied to evaluate OM with high maturity level, primarily due to thermal degradation of the steranes (Peters et al., 2005). In other words, the samples in this study mainly in the high maturity level, and the thermal equilibration of steranes has been achieved and thus yield the relatively low range of the ratios.
from Kuntou area suggest OM from this area deposited in freshwater under an oxidizing environment. Samples from the other areas are deposited in stratified high salinity water under a reductive environment. The Ts/(Ts + Tm) ratio for the samples vary from 0.06 to 0.89 (average = 0.45). The Ts/(Ts + Tm) ratio exhibits an increasing trend with gammacerane/C30 hopane ratios but not with the C3122S/(22S + 22R) hopanes ratios (Fig. 7C and D). These results indicate that the relative abundance of Ts to Tm is not only controlled by the thermal maturity level, but controlled by differences in terrestrial and lacustrine OM input or water salinity (Peters et al., 2005). The C30M/C30H ratios of the samples analyzed range from 0.09 to 0.31 (average 0.17), indicating most samples are at the mature stage (Table 2). Maturities measured by the C31 22S/(22S + 22R) hopanes ratios vary from 0.50 to 0.60. The C31H 22S/ (22S + 22R) ratios equilibrate at a ratio ranging from 0.57 to 0.62, also suggesting stages of maturity. 3.4.3. Steranes Abundant steranes were identified in the samples analyzed (Fig. 8). Our results show distinct patterns for samples from different areas. The patterns also show high heterogeneities for samples from different areas. Samples from the Wolong, Sanbao and Nanyao area, the regular steranes are dominated by C27 steranes and show an asymmetrical Vshape in the samples (Fig. 8). For some samples, no predominance of C27 steranes nor C29 sterane, as displayed in the C27, C28 and C29 steranes ternary diagram (Fig. 9). The samples plot principally in the mix area of land plant and plankton/bacterial(Gao et al., 2018; Yurewicz et al., 1998). For samples from Kuntou and Baiyao area, the diagram also show an asymmetrical V-shape, but dominated by the C29 sterane. For the Beipiao Formation shale samples, an overall trend of increasing C28/C29 sterane ratio with increasing C27/C29 sterane ratio (Fig. 9B) was observed. C28/C29 sterane ratios show moderate to high values for Beipiao Formation samples (0.17–1.12, average 0.73). High values mostly occur in the samples from Wolong, Nanyao and Sanbao areas. This suggests possible contribution from chlorophyll-c containing phytoplankton for some samples in this area (Knoll et al., 2007). To better understand the OM type and redox conditions of depositional
4. Conclusions The shale from the Jurassic Beipiao Formation in the Jinyang Basin is an important source rock. The results of TOC, Rock-Eval pyrolysis, carbon isotope as well as biomarker analyses show that there is a great deal of fair to good source rock in the Beipiao Formation. The Beipiao Formation shale samples contain mainly type II-III OM, and the shale is mainly gas-prone and have some potential to generate oil. A comprehensive thermal maturity assessment indicate that most samples are in the oil window, but also with high heterogeneities for some samples have abnormally high maturity level, possibly resulted from igneous intrusions. There are also great heterogeneities in OM input for the Beipiao shale in Jinyang Basin. Even almost all the samples are of type II-III OM, 9
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Fig. 9. Ternary diagram of regular steranes (C27, C28 and C29) showing the relationship between sterane composition and OM inputs (A), cross diagram of C27ααα/ C29ααα steranes ratio values versus C28ααα/C29 ααα ratio values (B), C29ααα(20R)/C27ααα(20R) sterane ratio values versus Pr/Ph ratio values(C), C29αββ/(ααα + αββ) steranes ratio values versus C2920S/(20S + 20R) steranes ratio values (D) of Beipiao Formation shale samples in the Jinyang Basin.
References
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements This study was supported by China Geological Survey oil and gas survey program (Grant No. DD20190098) and State Key Laboratory of Organic Geochemistry, GIGCAS (Grant No. SKLOG-201711). We thank the team members from both Yangtze University and Guangzhou Institute of Geochemistry for their helps with organic geochemistry analyses works.
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