Geochemical characteristics of crude oil in the eastern of Qinnan sag, Bohai Bay Basin, Eastern China

The oil and gas exploration degree of Qinnan sag is low. In this paper, all the 17 crude oil samples from the four reservoirs in the eastern Qinnan sag were taken as the research object. Research methods include GC-MS, stable carbon isotopic of crude oil and group components and other analytical methods. The geochemical characteristics of crude oil were studied by the main parameters, Pr/ph, γ/C₃₁ hopane, Ts/Tm, 4MSI and C₂₉ sterane isomerization. The results show that most of the hydrocarbon generating parent materials of all the 17 crude oils are reductive lacustrine sedimentary organic matter with higher salinity. The crude oil of Es₁₂ is mainly derived from low aquatic algae, and the crude oil of Ed₃, Ek and Mz shows the characteristics of a mixed source. The maturation characteristics of crude oils are low mature-mature. The stable carbon isotope of oil and the components is in the range of −30‰ to −26‰, which shows the distribution characteristics of δ¹³C_ST <δ¹³C_ASP <δ¹³C_NOS <δ¹³C_AR. The characteristics in B-6, C-1 and C-3 of crude oils show the heavy δ¹³C_ASP and high abundance of hopane, indicating that there is more contribution from bacterial sources. The crude oil from Es₁₂ is mainly the contribution of Es₁ source rocks. The crude oil of Mz, Ek and Ed₃ is mainly the mixed source of Es₁ and Es₃ source rocks, and Es₃ is the main source, and the oil source may mainly come from the southeast subsag. The carbon isotope of the component in Block D can be divided into two kinds of crude oil, which may contribute to the oil source of the Bozhong sag. The eastern of Qinnan sag has great hydrocarbon potential.

Introduction

The Qinnan Sag is located northwest of the Bohai Sea, with a low degree of exploration and huge potential for oil and gas exploration. Since the discovery of the QHD29-2/E oilfield in the Southeast sub-sag in 2009, there has been no major exploration breakthrough. The predecessors have systematically studied the geological conditions of hydrocarbon accumulation in the region. From the current knowledge, the Paleogene source rocks in the Qinnan Sag include the Paleogene Sha three Member (Es₃), Sha one Member (Es₁) and Dong three Member (Ed₃). There are good-quality source rocks in each member, especially in the Southeast subsag where high-quality Es₃ and Es₁ source rocks are developed, and some of them have reached the mature stage and have the material basis for the formation of large oil and gas fields. For example, Yang et al. (2019) believed that the Es₃ source rocks had higher organic matter abundance overall, and the source rocks in the Southeast Sag had the highest organic carbon content, followed by the East Sag and the West Sag. Regarding the source of oil and gas, Niu et al. (2018) discussed the reasons for the distribution of extra-heavy oil, light oil, condensate oil and heavy oil in the QHD29-2 oilfield from the perspective of evaporative fractionation and biodegradation. Zhuang et al. (2011) concluded that the oil and gas in the QHD29-2 oilfield were mixed sources of Es₁ and Es₃ source rocks, and the non-hydrocarbon natural gas (CO₂) was inorganic mantle-derived gas from natural gas composition and isotopic analysis combined with fluid inclusion analysis. Samra et al. (2022) studied the source and deposition environment of the samples by parameters such as Pr/Ph, Pr/nC₁₇, Ph/nC₁₈, C₃₁/C₁₉ and so on. Nazir et al. (2020) use biomarker compounds to predict the source of organic matter, sedimentary environment and lithology. Wang et al. (2015) analyzed the crude oil geochemical properties and thermal maturity of the QHD29-2 oilfield, and using ten parameters proposed by Hao et al. (2009), such as C₁₉/C₂₃, C₂₀/C₂₃, C₂₃/H, C₂₄/C₂₆, S/H, 4MSI, C₃₅/C₃₄, G/H, ETR and C₂₇/C₂₉, were analyzed by Hierarchical Cluster Analysis (HCA) to analyze the oil source of the crude oil in QHD29-2 oilfield. Relatively speaking, previous studies on the geochemical characteristics of crude oil in other oil reservoirs in the eastern Qinnan Sag are relatively few. Based on previous studies, this paper further samples crude oil from four major oil reservoirs in the eastern Qinnan sag. Through comparative analysis, the differences in the geochemical characteristics of crude oil in each reservoir are pointed out, which is helpful for a more reasonable understanding of the formation of oil and gas resources in the entire Qinnan sag Distribution characteristics are important.

Geological overview

The Qinnan Sag is located northwest of the Bohai Sea. It is a faulted depression surrounded by four peripheral uplifts, namely Qinnan Uplift, Matouying Uplift, Shijiutuo Uplift, and Liaoxi Low Uplift, with an area of about 2,300 km². The sag is nearly east-west, controlled by the Qinnan I and II faults, and is divided into four sub-sags: the West Sag, the Middle Sag, the East Sag, and the Southeast Sag (Shi et al., 2014). Overall, it shows the characteristics of deep in the east and shallow in the west, big in the east and small in the west, new in the east and old in the west. The sag deposited the Mesozoic (Mz) on the Archaean granite basement, and deposited the Kongdian Formation (Ek), Shahejie Formation (Es) and Dongying Formation (Ed) in the Paleogene in the rifting period, mainly delta and lacustrine facies, in the Neogene in the depression stage, the Guantao Formation (Ng), Minghuazhen Formation (Nm) and other strata were deposited, mainly fluvial facies (Shi et al., 2014; Zhao et al., 2021; Sun et al., 2022).

During the depositional period of Es3, Ed3 and Ed2, the fault activity was strong, and they were the main source rocks and caprock development period. The stronger the fault activity, the thicker the source rock and the higher the maturity; from Es12 and Ed1 to the present sedimentary period, the fault activity is weak, which is the main reservoir development period, and the reservoirs in the position with weak fault activity and the adjustment zone are more developed. The position of weak fault activity is easy to form lithological traps; the position of strong fault activity is easy to form fault-lithological traps. The vertical strong and weak evolution of faults in the Paleogene provided high-quality source rocks and reservoir and caprock conditions for hydrocarbon accumulation.

So far, oil and gas fields A and B have been discovered at the 426 subsidence on the south side of the Southeast Sag, and oil fields C and D have been discovered in the fault terrace area on the east side of the Southeast Sag.

Samples and experimental methods

This time, a total of 17 crude oil samples were collected from four oil reservoirs on the east side of the Qinnan Sag. The distribution of sample wells is shown in Figure 1. Among the 17 crude oil samples, there are two oil samples from reservoir A (two samples of Ed₃), six samples from reservoir B (six samples of Es₁₂), three samples from reservoir C (one sample of Es₁₂, one sample of Ek, one sample of Mz), and six samples from reservoir D (one sample of Ed₃, two samples of Es₁₂, one sample of Ek, two samples of Mz), of which the D-6 sample is a high water content oil sample.

FIGURE 1

FIGURE 1. Structural geological map of Qinnan sag.

All experiments were completed in the Hubei Provincial Key Laboratory of Oil and Gas Geochemistry and Environment. First, take about 20 mg of crude oil sample, add n-hexane and let stand for 12 h to precipitate asphaltenes. After filtering asphaltenes, the filtrate is separated by a solid-phase chromatography column filled with silica gel and alumina, followed by adding n-hexane, n-hexane+ Dichloromethane mixed solvent (volume ratio of 2:1) and a mixed solvent of dichloromethane + methanol (volume ratio of 93:7) were used to separate saturated hydrocarbons, aromatic hydrocarbons and non-hydrocarbon components, respectively. The separated saturated hydrocarbons were tested by gas chromatography-mass spectrometry, and the instrument model was Agilent 7890B-5977B. The chromatographic column used HP-5MS quartz elastic capillary column (30 m × 0.25 mm × 0.25 μm). The temperature of the injector was 300°C, the carrier gas was pure He, and the flow rate was 1 ml/min. The heating program was as follows: the initial temperature was 50°C, after a constant temperature of 2 min, the temperature was increased to 310°C at a rate of 3°C·min-1, and the constant temperature was maintained for 18 min, EI ionization mode, ionization energy 70 eV, atomic mass full scan range 50–550 amu. The isolated group components and the whole oil were subjected to carbon isotope detection by IsoPrime isotope mass spectrometer from GV Instruments, United Kingdom.

Crude oil geochemical characteristics

Through the GC-MS detection of saturated hydrocarbons in all of the 17 crude oil samples, it can be seen from the total ion chromatogram (TIC) analysis that the crude oil samples of this batch can be divided into three categories according to the peak shape: front peak type, back peak type The peak patterns and homohopane abundance patterns are shown in Figure 2.

FIGURE 2

FIGURE 2. Typical spectrum of crude oil in Qinnan sag.

N-alkanes and isoprenoid alkanes.

The pristane/phytan (Pr/Ph) ratio is one of the good parameters indicating the type of source rock parent material and depositional environment (Tian et al., 2018). In the sediments of strongly reducing and high salinity environments, phytanes tend to have an absolute dominance and high abundance; in the oxidative environments of swamps, pristane often has a strong dominance (Yang and Zhang, 2005). Didyk et al. (1978) considered that Pr/Ph <1.0 is a reducing environment, and Pr/Ph >1.0 is an oxidizing environment. Peters et al. (2005) and others further pointed out that Pr/Ph >3.0 represents the input of terrigenous organic matter in an oxidative environment, and Pr/Ph <0.8 represents an anoxic and reducing depositional environment. The Pr/Ph of the samples in this paper is all less than 1 (Table 1). Among them, the range of Ed₃ value is 0.66–0.82, with an average of 0.75; the range of Es₁₂ value is 0.46–0.91, with an average of 0.70; the range of Ek value is 0.70–0.86, with an average of 0.78; the range of Mz value is 0.47–0.89, with an average of 0.72, indicating that the crude oil parent materials were all formed in a relatively reduced lacustrine sedimentary environment.

TABLE 1

TABLE 1. Geochemical parameters of crude oil in the study area.

Isoprenoids are used both to indicate the depositional environment and to reflect the source of organic matter (Xie et al., 2017). Pr/nC17 and Ph/nC18 are often used to study the type of hydrocarbon parent material, source rock depositional environment and crude oil maturity (Tian et al., 2018). The Pr/nC17 value of the crude oil samples from Ed₃ of the Qinnan Sag is 0.24–0.39, with an average value of 0.33, and the Ph/nC18 value is 0.28–0.58, with an average value of 0.47; The Pr/nC17 value of Es₁₂ crude oil sample is 0.23–0.49, the average value is 0.36, the Ph/nC18 value is 0.24–1.28, the average value is 0.65; The Pr/nC17 value of Ek crude oil sample is 0.32–2.90, the average value is 1.61, the Ph/nC18 value is 0.43–4.00, the average value is 2.21; The Pr/nC17 value of Mz crude oil sample is 0.19–0.63, the average value is 0.35, the Ph/nC18 value is 0.20–1.21, the average value is 0.57. From the intersection diagram of Pr/nC17 and Ph/nC18 (Figure 3), it can be seen that the crude oil of Ed₃, Es₁₂, Ek and Mz all fell into the marine and salt lake sedimentary areas, reflecting the organic. The parent material is mainly contributed by algae and lower aquatic organisms. Among them, the D-6 sample shows an abnormality that deviates significantly from other oil samples, which may be caused by the high water content of the oil samples.

FIGURE 3

FIGURE 3. Cross plot of Pr/nC₁₇ and Ph/nC₁₈ of crude oil in Qinnan sag.

Terpenoids

Terpenes are widely present in biomarker compounds, among which tricyclic terpenes have strong characteristics and are often used to identify the source of organic matter, while tetracyclic terpenes have a good relationship with the input of terrestrial organic matter (Jin et al., 2019). The content of tricyclic terpenes (TT) and tetracyclic terpenes (Tet) can reflect the oil parent material (Huang et al., 2020), C₁₉ tricyclic terpenes/C₂₃ tricyclic terpenes (C₁₉TT/C₂₃TT) and C24 tetracyclic terpenes/The ratio of C₂₆ tricyclic terpenes (C₂₄Tet/C₂₆TT) is also regarded as an important indicator of terrigenous organic matter input. The higher the ratio, the greater the contribution of terrestrial organic matter (Chen, 2012).

The ratio of Ts/Tm is generally used to assist in judging the maturity of crude oil (Peters and Moldowan, 1993). It is generally believed that with the increase of thermal evolution degree, Tm will transform to Ts, and Ts/Tm will increase (Li et al., 2018). The Ts/Tm values ​​of Ed₃ of Qinnan Crude Oil are 0.93–1.14, with an average value of 1.05; the Ts/Tm values ​​of Es₁₂ are 0.6–1.68, with an average value of 1.08; the Ts/Tm values ​​of Ek are 0.58–1.08, with an average value of 1.08.0.83; Mz Ts/Tm values ​​range from 0.81 to 1.28, with an average value of 1.11. Figure 4 comprehensively reflects the high maturity of crude oil samples in this area. The terpene parameters of each oil sample are relatively concentrated, and the C₂₄Tet/C₂₆TT contributed by higher plants is relatively low, which is quite different from that of Ed₃, which is also consistent with the immature geological background of the source rocks of Ed₃.

FIGURE 4

FIGURE 4. Comparison of maturity of crude oil and source rock in Qinnan sag.

Gammacerane is a pentacyclic triterpenoid, mainly derived from tetrahymenin of protozoa and photosynthetic bacteria. It is formed by reduction. The enrichment of gamma wax is often related to the sedimentary environment. The high content of gamma wax indicates the sedimentary environment with strong reduction and high salinity, and when the ratio is greater than 0.2, it is a brackish or saline environment, and can be used as a sign of water stratification. In the crude oil samples, the ratio of gammacerane/C₃₁ hopane ranged from 0.21 to 0.59, with an average value of 0.38, indicating that the original organic matter was a lacustrine depositional environment between brackish water and saltwater (Figure 5).

FIGURE 5

FIGURE 5. Correlation between gammacerane/C₃₁ hopane and Pr/Ph of crude oil in Qinnan sag.

Sterane compounds

Steroids are tetracyclic compounds with alkyl side chains. Conventional steranes have a carbon number of C₂₇–C₂₉, but they also have extremely complex carbon number changes (Zhang et al., 2022). It is generally believed that C₂₇ steranes are mainly derived from lower aquatic organisms and algae, and C₂₉ steranes are mainly derived from terrestrial higher plants, but there are also cases of C₂₉ sterane parent sources such as diatoms and brown algae (Zhou and Zhang, 2021). The steranes do not undergo mutual transformation after being formed, so they can be used as important compounds for analyzing the source of organic matter. C₂₇, C₂₈, and C₂₉ regular steranes can be used to determine the biogenesis of organic matter. It is generally believed that organic matter derived from lower organisms algae has the dominant characteristics of C₂₇ steranes, and organic matter derived from higher plants has the dominant characteristics of C₂₉ steranes. When both are high, it indicates that there is a dual contribution from the mixed source of higher plants and lower aquatic organisms (Bao et al., 2006). The C₂₇, C₂₈, and C₂₉ regular steranes of 17 pieces of crude oil (Figure 6) show that the crude oil in Es₁₂ is mainly derived from algae, and the rest of the layers contribute to mixed biogenesis.

FIGURE 6

FIGURE 6. Regular sterane distribution triangle of crude oil in Qinnan sag.

C₂₉ sterane isomerization parameters C₂₉-ααα20S/(20S+20R) and C₂₉ββ/(ββ+αα) are commonly used maturity indicators, the biological configuration R is transformed into the geological configuration S, which is manifested as an increase in the ratio (Li et al., 2018; Zhou et al., 2021). Figure 7 shows the cross plot of the ratios of C₂₉-ααα20S/(20S+20R) and C₂₉ββ/(ββ+αα) for all the 17 crude oil samples. From this, it can be seen that the crude oil of Ed₃ and Mz have relatively high maturity, and the crude oil of Es₁₂ has a great change in maturity, ranging from low maturity to mature. The low maturity crude oil is mainly B5, D4, C3, and C2, Mature crude oil is mainly distributed in A and D oil fields.

FIGURE 7

FIGURE 7. Correlation between C₂₉-ααα20S/(20S+20R) and C₂₉ββ/(ββ+αα) sterane of crude oil in Qinnan sag.

C₃₀4-methylesterase (4MSI) is generally derived from 4α-methyl sterol in dinoflagellate organisms and can also be produced by some bacteria. The 4-methyl sterane in freshwater lakes is mainly the biogenic source of dinoflagellates, while the fossils of dinoflagellates containing 4-methyl sterane in the marine and saline lake environments are rare. The enzymatic reduction products became the main source of 4MIS in marine and saline lake environments. The 4MSI content of crude oil formed in a high salinity environment is lower than that of crude oil formed in a fresh or brackish water environment (Bao et al., 2007; Zan et al., 2012; Chen et al., 2016; Bo, 2018).

4MSI, as an important biogenesis index in southern Qinan, can better distinguish crude oil source information. By comparing the characteristics of Qinnan crude oil and source rocks by 4MSI and γ/C30 hopane index, it can be seen from Figure 8 that Es₁₂ of the crude oil samples analyzed in this paper are mainly contributed by Es₁ source rocks, and the rest of the layers exist Contribution of mixed source rocks of Es₁ and Es₃ source rocks.

FIGURE 8

FIGURE 8. Correlation between 4MSI and γ/C₃₀ hopane of crude oil in Qinnan sag; quote date from Wang et al. (2015).

Component carbon isotopes

It is generally believed that the carbon isotope of organic matter in sedimentary rocks mainly depends on the carbon isotopic composition of its precursors. The carbon isotopic composition of organic matter in terrigenous higher plants is heavier, generally greater than −26.0‰ (Chen et al., 2010). Generally speaking, with the increase of the polarity of saturated hydrocarbons, aromatic hydrocarbons, non-hydrocarbons and asphaltene chemical groups in sedimentary organic matter, its carbon isotope δ¹³C value will gradually become heavier, that is, δ¹³C saturated hydrocarbons < δ¹³C aromatic hydrocarbons < δ¹³C non-hydrocarbons <δ¹³C asphaltenes.

Table 2 shows the distribution range of δ¹³C crude oil value, saturated hydrocarbon, aromatic hydrocarbon, non-hydrocarbon and asphaltene in the Qinnan area. As a whole, there is a “reversal” phenomenon of δ¹³C saturated hydrocarbons < δ¹³C asphaltenes < δ¹³C non-hydrocarbons < δ¹³C aromatic hydrocarbons (Wan et al., 2020; Wang et al., 2020) (Figure 9), which all reflect the high contribution of algae and other lower aquatic organisms to hydrocarbon generation.

TABLE 2

TABLE 2. Carbon isotope distribution of crude oil and group components in Qinnan sag.

FIGURE 9

FIGURE 9. Characteristics of carbon isotopes of crude oil components in Qinnan sag.

By analyzing the stable carbon isotope composition of crude oil family components by the reservoir, it can be seen that the carbon isotope values of each sample in reservoirs A, B, and C are quite different (Figure 9), while reservoir D shows highly consistent characteristics of the two types of crude oil. The specific performance is that the difference between the saturated hydrocarbons and aromatic hydrocarbons of the A-2 sample is the largest, reaching 2.81‰, and the maximum difference of the other samples does not exceed 2‰. B-6, C-1, and C-3 show anomalous characteristics of asphaltenes, and their TICs (Figure 2) also show anomalous characteristics of a high abundance of hopanes. The TIC of samples C-1 and C-3 towards other good positions C and D gradually showed the characteristics of a sharp decrease in hopane content, and the normal peak shape changed from the back peak shape to the front peak shape.

Results and discussion

Based on the above analysis, the sedimentary environment, biological origin, maturity and carbon isotopic composition characteristics of the crude oil samples taken this time generally show the following characteristics: Pr/Ph in the crude oil samples are all less than 1, and the γ/C31 hopane ratio is between 0.21 and 0.59, reflecting that the original organic matter is the reduced lacustrine sedimentary organic matter between brackish water and saltwater; Pr/nC₁₇ and Ph/nC₁₈ cross diagram, C₁₉TT/C₂₃TT value, C₂₄Tet/C₂₆TT value and C₂₇, C₂₈, C₂₉ regular sterane are all It reflects that the organic parent material of crude oil is mainly contributed by algae and lower aquatic organisms; Ts/Tm and C₂₉-ααα20S/(20S+20R) and C₂₉ββ/(ββ+αα) parameters show that the crude oil in this area is generally in the low-mature-mature stage, the maturity of Es₁₂ varies widely, and the Ed₃ and the Mz have higher maturity.

According to the previous research results (Nazir et al., 2020; Nazir and Fazeelat, 2017a there are obvious differences in the high-quality source rock intervals in the Bohai Bay Basin: The Es₁ source rock was formed in a strongly reducing saline lake environment, and the Es₃ source rock was formed in a weakly reducing freshwater-brackish water environment, and the Ed₃ source rock was partially oxidized, and the contribution of terrigenous organic matter was obvious. In this paper, the information of this batch of oil samples is compared with the analysis of the geochemical characteristics of the previous source rocks. Based on the crude oil maturity information and 4MSI and other parameter characteristics, it can be found that the crude oil in the Mz and Ek has high maturity, medium γ and high 4MSI, mainly due to the mixed source of the Es₁ and Es₃ source rocks; the oil maturity of the Es₁₂ oil varies greatly, with high γ and low 4MSI, which are mainly self-generated and self-storage, and a small part of the Es3 source rocks cannot be ruled out contribution; the crude oil of the Ed₃ has high maturity, medium γ, and low 4MSI, which contributes to the mixed source of the Es1 and Es3 source rocks, and mainly contributes to the migration of the Es3 source rocks with higher maturity; the Ng and the Nm has high γ and low 4MSI, and is mainly contributed by the Es₁ source rocks.

The carbon isotopes of the components are generally in the range of −30‰ to −26‰, which comprehensively reflects that the organic matter is mainly contributed by low-level aquatic organisms such as algae to generate hydrocarbons, which is consistent with the biogenic characteristics. Zhang et al. (1992); Fu and Qin, (1995); Chen and Xu, (1992) and other researchers believe that various types of organisms have special chemical compositions, and their carbon isotopic compositions are also different. The average value of δ¹³C is lower than that of organic matter from higher plants. If there is more input of higher plants in the parent source of sedimentary organic matter, δ¹³C in sedimentary organic matter will often increase, so that the less polar components have relatively heavier carbon. isotopic composition. Meyers and Simoneit, (1999) and other studies have shown that thermal action can also deplete light carbon isotopes in sedimentary organic matter, thereby changing the carbon isotopic composition of sedimentary organic matter, resulting in δ¹³C saturated hydrocarbons > δ¹³C aromatic hydrocarbons > δ¹³C non-hydrocarbons > δ¹³C asphaltenes. Wang (2000) and others believe that biodegradation can also change the carbon isotopic composition of saturated hydrocarbons, aromatic hydrocarbons, non-hydrocarbons and asphaltenes, and the δ¹³C saturated hydrocarbon value of crude oil subjected to biodegradation tends to be positive, and the δ¹³C aromatic hydrocarbon value is almost No change occurred, and the non-hydrocarbon components and asphaltene components tended to be enriched in ¹²C. If it is thermal action or biodegradation, the δ¹³C saturated hydrocarbon value will not be lower than the δ₁₃C aromatic hydrocarbon value. Therefore, the carbon isotopic reversal of aromatic components in crude oil is mainly related to the source of organic matter and the depositional environment.

The carbon isotopic composition of crude oil components has the reversal characteristics of δ¹³C saturated hydrocarbons < δ¹³C asphaltenes < δ¹³C non-hydrocarbons < δ¹³C aromatics. It is speculated that the carbon isotopic reversal of aromatic components in crude oil is mainly related to the source of organic matter and the depositional environment. A-2 sample has the largest difference between saturated hydrocarbons and aromatic hydrocarbons, reaching 2.81‰. B-6, C-1, and C-3 show anomalous characteristics of asphaltene anomalies, and their TICs also show anomalous characteristics of high abundance hopanes. Based on the characteristics of crude oil samples, it is preliminarily speculated that these crude oils have strong bacterial biogenic contributions. The carbon isotopic composition of the D reservoir component can be divided into two categories. The D1 and D6 samples have similar characteristics, and the remaining samples are highly overlapping, indicating that there are two types of crude oil from different sources, and there may be oil source contributions from the Bozhong sag.

Conclusion

1) The Paleogene crude oil on the eastern side of the Qinnan Sag has the characteristics of low Pr/Ph and high γ/C₃₁, reflecting that the brackish water-salt water partially reduces the biogenic organic matter of lacustrine sediments, and the crude oil of the Es₁₂ is mainly derived from algae, and the Ed₃, Ek and Mz are mixed biogenic sources. The carbon isotopic composition of crude oil and its components ranges from −30‰ to −26‰, indicating that lower aquatic organisms are the main contribution. Some of the crude oils have obvious bacterial biogenic contributions.

2) Judging from the characteristics of biomarker combinations and differences in maturity, no crude oil contributed by the Dongying Formation source rocks has been found in this area, and the crude oil of the Es₁₂ is mainly contributed by the Es₁ and Es₃ source rocks. The crude oil in the Mz, Ek and Ed₃ is mainly the mixed source of the Es1 and Es3 source rocks. Reservoir D includes two types of crude oil, which may include the contribution of oil sources in the Bozhong sag.

3) Crude oil has been found on the eastern side of Qinnan Sag, mainly located around the Southeast Sag, while the source rocks in the East Sag have a large distribution area, deep depth, high thermal evolution maturity and great hydrocarbon generation potential, but relatively few oil and gas reservoirs have been discovered. It has great potential for oil and gas exploration.

Data availability statement

The raw data supporting the conclusion of this article will be made available by the authors, without undue reservation.

Author contributions

XS: manuscript writing. YL, ZW, XX, and YW: manuscript discussion. DW, FL, FW: assistance in sample collection. ZL: assistance in sample analysis. All authors contributed to the article and approved the submitted version.

Funding

This research was jointly funded by the National Natural Science Foundation of China (41503034, 41972122, and 42172139); Major national science and technology projects (2017ZX0500105); Project of China Geological Survey (DD20190085); Three foreign projects of CNOOC Tianjin Branch (CCl2021TJTONST0365).

Conflict of interest

Authors DW, FL, and FW were employed by CNOOC China Limited.

The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

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