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Article

Geochemical Characteristics of Crude Oil and Oil–Source Correlations in the Yongfeng Sub-Sag of the Bogda Mountain Front Belt

by
Xiangcan Sun
1,2,3,
Jianwei Wu
1,2,3,
Xingui Zhou
1,2,3,
Yongjin Gao
1,2,3,
Youxing Yang
1,2,3,
Zhongkai Bai
1,2,3,
Kun Yuan
1,2,3,
Lei Wen
1,2,3,* and
Yi Chen
1,2,3,*
1
Oil & Gas Survey, China Geological Survey, Beijing 100083, China
2
State Key Laboratory of Continental Shale Oil, Beijing 100029, China
3
Key Laboratory of Unconventional Oil & Gas, China Geological Survey, Beijing 100029, China
*
Authors to whom correspondence should be addressed.
Energies 2025, 18(4), 917; https://doi.org/10.3390/en18040917
Submission received: 16 January 2025 / Revised: 11 February 2025 / Accepted: 11 February 2025 / Published: 14 February 2025
(This article belongs to the Special Issue Exploration and Development of Unconventional Oil and Gas Resources: Latest Advances and Prospects: 2nd Edition)
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Abstract

:
The exploration level of the Bogda Mountain front belt is relatively low, and the research on hydrocarbon accumulation is limited, resulting in unclear sources of discovered oil. To further investigate the geochemical characteristics and sources of crude oil in the Bogda Mountain front belt, this study conducted geochemical experimental analysis and oil–source correlations on crude oil and hydrocarbon source rock samples from the Permian Lucaogou Formation in the Yongfeng sub-sag and surrounding areas of the Bogda Mountain front belt. By using gas chromatography–mass spectrometry technology, the geochemical characteristics of saturated hydrocarbons and aromatic compounds were analyzed. Combined with stable carbon isotopes of saturated hydrocarbons and aromatic hydrocarbons, the organic matter source, maturity, and sedimentary environment were determined. The research results indicate that the crude oil from Well Xyd 1 exhibits mature characteristics, and the source material was deposited in a reducing to weakly oxidizing, weakly reducing environment. The source rocks of the Lucaogou Formation in Well Xyd 1 were formed in a reducing, semi-saline–saline sedimentary environment, while those from the Gjg and Dhs outcrops developed in a weakly oxidizing–weakly reducing, non-high-salinity, weakly stratified sedimentary environment. Carbon isotope, terpane, and isoalkane characteristics confirm a significant genetic relationship between the crude oil from Well Xyd 1 and the local Luzhaogou Formation source rocks. The source rocks of the Luzhaogou Formation in the Yongfeng sub-sag exhibit strong heterogeneity, with significant differences in sedimentary environments and parent materials in their spatial distribution. Maturity analysis indicates that the Luzhaogou Formation source rocks in Well Xyd 1 have reached a mature stage, whereas those from the Gjg and Dhs outcrops are at a relatively low maturity level.

1. Introduction

The Bogda Mountain front belt is located in the eastern part of the thrust belt in the southern margin of the Junggar Basin, with a low degree of oil and gas exploration. In the early stage, oil companies conducted extensive oil and gas exploration and research work in the western section of the thrust belt in the southern margin of the Junggar Basin, and successively discovered Dushanzi, Qigu, Hutubi, and other oil and gas fields [1,2], showing good oil and gas exploration prospects in the southern margin of the Junggar Basin. In recent years, significant progress has been made in the exploration of the eastern part of the thrust belt in the southern margin of the Junggar Basin. The exploration is mainly concentrated in the sag area in the northern margin of Bogda Mountain, and a billion-ton oil field has been discovered in Jimsar Sag [3]. The Permian Lucaogou Formation is the main source rock series and exploration target layer. The main types of oil and gas reservoirs are shale oil and gas reservoirs with source reservoir integration or tight oil and gas reservoirs adjacent to source rocks. However, due to the complex structural deformation, faults, difficult stratigraphic correlation, unclear understanding of sedimentary facies, and unclear distribution range of source rocks in the Bogda Mountain front belt, the understanding of oil and gas reservoir characteristics, distribution patterns, and resource potential in the Bogda Mountain front belt is unclear, resulting in significant delays in exploration progress. In order to solve the key geological problems of oil and gas accumulation, expand the new field of oil and gas survey, and promote the exploration process in the Bogda Mountain front belt, the China Geological Survey has conducted a series of basic research and oil and gas surveys in the Bogda Mountain front belt since 2013, deployed and implemented Well Xjc 1 in the northern edge of the Bogda Mountain front belt, and obtained industrial gas flow in the Permian Lucaogou Formation and the shallow Triassic Karamay Formation, achieving a major breakthrough in new-layer and new-type oil and gas exploration in the new area, confirming that the Bogda Mountain front belt was the sedimentary center during the sedimentary period of the Permian Lucaogou Formation [4], and developing high-quality hydrocarbon source rocks of the Permian Lucaogou Formation. In order to further expand the scope of the Permian Lucaogou Formation oil and gas system on the plane, the overall deployment strategy has been shifted towards the western edge of the Bogda Mountain front belt. In the Yongfeng sub-sag, Well Xyd 1 has been deployed and drilled [5,6], which is the first exploration well deployed in the Yongfeng sub-sag in nearly 30 years. The low yield oil and gas flow obtained from oil testing in the Lucaogou Formation confirms that the area has the basic elements for oil and gas accumulation and shows that the Bogda Mountain front belt has good exploration prospects. As mentioned above, early exploration mainly focused on the sag area in the northern margin of Bogda Mountain, represented by the Jimsar sag. A lot of research has been conducted on the characteristics of the Lucaogou Formation source rocks and the evaluation of unconventional shale oil and gas reservoirs in the Jimsar sag, and a series of geological understandings have been obtained, promoting the construction of a national shale oil demonstration zone [7,8,9,10,11,12,13]. Unlike the sag area, the exploration level of the Bogda Mountain front belt is low, and early exploration has not yet discovered oil and gas, resulting in unclear understanding of the basic oil and gas accumulation elements, accumulation processes, and resource potential of the area. The recent exploration discoveries of Wells Xjc 1 and Xyd 1 provide a good data basis for studying the oil and gas accumulation conditions in the Bogda Mountain front belt. Therefore, in order to further implement the oil and gas exploration prospects in the Bogda Mountain front belt, especially in the Yongfeng sub-sag, this study focuses on the geochemical characteristics analysis of source rocks and crude oil in the research area and surrounding areas. Combining the various biomarker parameter characteristics of source rocks and crude oil, fine oil source correlation is carried out to clarify the contribution of oil sources, in order to provide a basis for the study of oil and gas accumulation in the study area and provide direction for the next step of oil and gas exploration.

2. Regional Geological Condition

The Yongfeng sub-sag is located in the western section of the Chaiwopu sag on the southern edge of Bogda Mountain (Figure 1), with an area of about 1500 km2. The sedimentary strata in the study area are mainly Permian to Jurassic, and the Middle Permian Lucaogou Formation has developed high-quality lacustrine source rocks with a large sedimentary thickness, providing an important material basis for oil and gas accumulation in the study area [14]. The study area relies on the source rocks of the Lucaogou Formation for hydrocarbon supply, which can form multiple sets of reservoir combinations vertically and develop various types of oil and gas reservoirs, coexisting with conventional and unconventional oil and gas reservoirs. Among them, the Permian Lucaogou Formation has a well-developed source and reservoir relationship, forming interbedded shale oil and gas reservoirs as well as adjacent tight sandstone oil and gas reservoirs (Figure 2). Moreover, it is distributed continuously on the plane, not controlled by structures, with a large area and overall resource scale, making it the most important exploration target layer in the study area.

3. Sample Collection and Analysis

3.1. Sample Collection

In this study, two crude oil samples were collected from Well Xyd 1, with a depth of 2045 m to 2072 m (Figure 2); 62 samples of source rocks were collected, including 18 samples of source rocks cuttings of the Lucaogou Formation from Well Xyd 1, 24 samples of source rocks of the Lucaogou Formation from the Gjg outcrop, and 20 samples of source rocks of the Lucaogou Formation from the Dhs outcrop. The source rock samples were mainly collected from the black mudstone section of the Lucaogou Formation. Chromatographic analysis of total hydrocarbons, light hydrocarbons, and saturated hydrocarbons, biomarker compound analysis, and carbon isotope analysis were conducted for the crude oil samples. Total organic carbon content (TOC) analysis, chloroform bitumen “A” analysis, rock pyrolysis, kerogen vitrinite reflectance (Ro), group component analysis, biomarker compound analysis, and carbon isotope analysis were conducted for the source rock samples.

3.2. Sample Analysis Method

The analysis of organic carbon, chloroform bitumen “A”, rock-eval pyrolysis, and determination of kerogen vitrinite reflectance for source rocks were carried out using the same methods as in reference [6], and the analysis method will not be described in detail here.
Organic carbon isotope analysis was carried out using an American-made Delta V Advantage isotope mass spectrometer. The analysis of total hydrocarbon gas chromatography of crude oil was carried out using an American-made Agilent 7890B chromatograph, with an HP-PONA elastic quartz capillary column (50 m × 0.20 mm × 0.5 m), nitrogen gas as the carrier gas (constant flow rate 1.0 mL/min), and injection using a split-flow mode (split ratio 10:1). Set the initial temperature of the chromatography to 40 °C (hold for 10 min), raise it to 310 °C at a rate of 3 °C/min, and hold for 30 min.
The experimental process for analyzing biomarker compounds was as follows: first, Soxhlet extraction was performed on the source rock and crude oil to separate the components and obtain soluble organic matter. After the solvent evaporated, n-hexane was added, and the mixture was refrigerated overnight to precipitate asphaltene. After removing asphaltene, a chromatography column filled with alumina stationary phase was used to separate the group components. Add n-hexane, benzene, and mixed solvent of dichloromethane and methanol (9:1, v/v) in sequence to wash out saturate, aromatic, and resin. The processing method for crude oil samples was the same as that for soluble organic matter in source rocks. Chromatographic analysis and chromatography–mass spectrometry analysis were performed on the separated saturated hydrocarbon components using an American-made Agilent 7890B gas chromatograph and an American-made Agilent 7890B/5977A gas chromatography–mass spectrometer, respectively. Isotopes of saturate, aromatic, resin, asphaltene, and chloroform bitumen “A” in the extracted and separated source rocks were determined using a German-made Elementar isotope precision stable isotope ratio mass spectrometer.
Agilent 7890B/5977A gas chromatography–mass spectrometry was used for analysis. The saturated hydrocarbon chromatography column is an HP-5MS elastic quartz capillary column (60 m × 0.25 mm × 0.25 m), with constant flow mode injection and helium gas as the carrier gas (constant flow rate 1.0 mL/min). Heating program: Initial temperature of 50 °C, hold for 1 min, heat up at a rate of 15 °C/min to 100 °C, heat up at a rate of 2 °C/min to 200 °C, and then heat up at a rate of 1 °C/min to 315 °C, and hold for 20 min.

4. Physical Characteristics and Group Components of Crude Oil

4.1. Physical Characteristics of Crude Oil

The crude oil density of Well Xyd 1 in the Yongfeng sub-sag ranges from 0.8930 g/cm3 to 0.9034 g/cm3, with an average density of 0.8982 g/cm3; at 50 °C, the viscosity of crude oil ranges from 230.06 mpa.s to 618.52 mpa.s, with an average of 424.29 mpa.s; the wax content of crude oil ranges from 6.36% to 9.56%, with an average of 7.96%; the freezing point of crude oil ranges from 8.0 °C to 22.0 °C. Overall, the crude oil from Well Xyd 1 has relatively high density and viscosity, and belongs to heavy oil.

4.2. Group Components SARA (Saturate, Aromatic, Resin, and Asphaltene) of Crude Oil

The content of saturates and aromatics in the crude oil of Well Xyd 1 in the Yongfeng sub-sag is from 54.25% to 59.96% and from 15.33% to 17.68%, respectively, the ratio of saturates to aromatics (saturates/aromatics) is from 3.39 to 3.54, and the content of resins and asphaltene is from 13.82% to 15.80%. The content of saturates and aromatics of the black mudstone in the Lucaogou Formation of Well Xyd 1 is from 19.43% to 52.33% and from 8.57% to 20.71%, respectively, the saturates/aromatics value is from 1.31 to 4.49, and the content of resins and asphaltene is from 31.32% to 67.43%. The content of saturates and aromatics of the black mudstone in the Lucaogou Formation of the Gjg outcrop is from 19.88% to 28.07% and from 10.22% to 23.14%, respectively, the saturates/aromatics value is from 1.11 to 2.32, and the content of resins and asphaltene is from 48.34% to 60.76%. The content of saturates and aromatics of the black shale in the Lucaogou Formation of the Dhs outcrop is from 18.93% to 23.25% and from 8.50% to 15.38%, respectively, the saturates/aromatics value is from 1.50 to 2.24, and the content of resins and asphaltene is from 57.69% to 62.73% (Table 1). The content of SARA in the soluble organic matter of source rocks is influenced by various factors, including parent material type, maturity, and secondary transformation [15]. The crude oil from Well Xyd 1 has a low content of polar compounds and a high saturates/aromatics value, exhibiting the characteristics of mature crude oil as a whole. The source rocks of the Lucaogou Formation in Well Xyd 1 have strong heterogeneity, with relatively high saturate content and a high saturates/aromatics value greater than 3 in some sample extracts; the other part of the sample extracts is mainly composed of resins and asphaltene, with a lower content of saturates. This heterogeneity may be related to the widespread development of fractures in the area, and the strong biodegradation or water washing of the Lucaogou Formation source rocks, resulting in a significant decrease in the content of relatively light components such as saturates in the group components. In contrast, resin components dominate the source rock extracts of the Lucaogou Formation in the Gjg and Dhs outcrops, with a low saturates/aromatics value less than 2, and the content of resins and asphaltene is relatively high. This indicates that compared with the source rocks of the Lucaogou Formation in Well Xyd 1, the maturity of the source rocks in these two outcrops is lower, and the degree of organic matter transformation and evolution has not yet reached the level of the source rocks in Well Xyd 1.

5. Geochemical Characteristics of Crude Oil

The distribution characteristics and composition of biomarkers can reflect information such as the sedimentary environment, parent material source, and maturity of source rocks [16]. Based on the characteristics of biomarkers such as alkanes, terpenes, and hopane in crude oil and source rock samples, this paper systematically analyzed the sedimentary environment, water salinity, and parent material sources of crude oil in the Yongfeng sub-sag and source rocks around the Bogda Mountain front belt.

5.1. Composition and Distribution Characteristics of Alkane Series

The gas chromatography–mass spectrometry analysis results show that the carbon number distribution range of n-alkanes in the saturated hydrocarbon fractions of two crude oil samples from the Xinyingdi 1 well is nC9–nC32, and the main peak carbon number is concentrated in nC14–nC15. The oil sample Oil-1 suffered from a certain degree of mud contamination, and the baseline of the Oil-2 sample’s alkane chromatography also showed obvious “UCM” (Unsolved Complex Mixture) bulges, which is sufficient to prove its potential for biodegradation. The distribution of CPI ranges from 1.09 to 1.18, and the distribution of OEP ranges from 1.06 to 1.13. Normal alkanes have a slight odd–even advantage, indicating that the maturity of crude oil is in the early to mid-maturity stage and has not yet fully reached the peak of mature oil generation (Figure 3).
In oil and gas geochemical analysis, the ratio of pristane to phytane (Pr/Ph) is considered an important indicator reflecting the sedimentary environment [17]. Generally, higher Pr/Ph values are often associated with oxidative sedimentary environments, while hypoxic or high salt conditions are often accompanied by lower Pr/Ph values [18]. Specifically, test data indicate that a Pr/Ph value less than 1 indicates a hypoxic environment; when the Pr/Ph value is greater than 1, it represents a weakly oxidizing environment; and a Pr/Ph value greater than 3 usually means that terrestrial organic matter is deposited in oxidized water environments [19].
Taking Well Xyd 1 as an example, the crude oil contains generally high levels of pristane and phytane, with the advantage of pristane being particularly evident. The Pr/Ph values of the crude oil in this well are generally high, ranging from 1.06 to 1.66. These data reflect that the sedimentary environment of the crude oil source material is a reducing to weakly oxidizing and weakly reducing environment. In the extracts of the source rocks of the Lucaogou Formation in Well Xyd 1, the Pr/Ph values are generally low, ranging from 0.53 to 0.73, with an average value of 0.62. This indicates that the original sedimentary environment of the source rocks has strong reducibility. In contrast, the Pr/Ph values in the source rock extracts of the Gjg and Dhs outcrops are higher than that of Well Xyd 1, ranging from 0.15 to 0.94, with an average value of 0.77, which reflects that these source rocks were formed in the sedimentary environment of weak oxidation and reduction (Table 2).

5.2. Composition and Distribution Characteristics of Terpenoid Hopane Series

Among the numerous biomarker parameters in the terpene series, tricyclic and tetracyclic terpenes play extremely important roles and can provide a lot of information about the source material of oil. Tricyclic terpenes are widely distributed in petroleum and sedimentary organic matter, possibly originating from the cell membranes of protozoa, with tricyclohexaisoprenol as its precursor [20], and algae as another source [21]. C24 tetracyclic terpenes are widely distributed in crude oil and rock extracts. Many scholars believe that the abundant C24 tetracyclic terpenes are often associated with terrestrial parent materials [22,23]. Therefore, the relative content of C24 tetracyclic terpenes and tricyclic terpenes can reflect the organic matter source of crude oil. Generally speaking, in source rocks from lower organic matter and related crude oils, C23 tricyclic terpenes dominate, while C24 tetracyclic terpenes have relatively low content; however, C19 and C20 tricyclic terpenes are more abundant in source rocks and related crude oils derived from terrestrial organic matter, and the content of C24 tetracyclic terpenes is relatively high. Abundant terpenoids are detected in the crude oil and source rock extracts of Well Xyd 1, including pentacyclic triterpenes, tricyclic terpenes, tetracyclic terpenes, and gammacerane (Figure 4). Among them, the main peaks of tricyclic terpenes are C21 and C23, indicating that the contribution of lower aquatic organisms dominates in their organic matter sources.
Tetracyclic terpenes are generally believed to be formed by the cleavage of the five-membered ring, i.e., the E ring, in hopane or its precursor, hopene, due to thermal or biodegradation effects. This compound has stronger thermal stability than hopane and is sometimes associated with high-salt sedimentary environments [24]. In the crude oil and source rock extracts from Well Xyd 1, the ratio of C24 tetracyclic terpenes to C26 tricyclic terpenes (C24TeT/C26TT) ranges from 0.51 to 0.90, with an average value of 0.62; the ratio of hydrocarbon source rock extracts in the Gjg outcrop ranges from 0.63 to 1.55, with an average value of 0.96; the ratio of hydrocarbon source rock extracts in the Dhs outcrop ranges from 0.77 to 2.61, with an average value of 1.56. These data show that the contribution of low-grade bio organic matter in the source rocks of the Lucaogou Formation in the area where Well Xyd 1 is located is significantly higher than that in the Gjg and Dhs outcrops.
Among pentacyclic terpenes, C30 hopane has a high abundance, while the abundance of other hopanes is relatively low. It is particularly obvious that the ratio of Ts/Tm in the crude oil of Well Xyd 1 is greater than 1, and the ratio of Ts/Tm in source rock extracts of Well Xyd1 is generally greater than 2, with an average of 2.58, which is significantly higher than that in the source rock extracts of the Gjg and Dhs outcrops. This phenomenon not only confirms the existence of sedimentary environment differences in the source rocks of the Lucaogou Formation around the Bogda Mountain front belt, but also reflects the differences in their maturity on the plane.
Abundant gammaceranes were detected in crude oil and source rock extracts, and their relative abundance has a good correlation with the salinity of sedimentary water bodies. Therefore, they are often used to indicate the ancient salinity of water bodies during source rock deposition. The gammacerane index (Ga/C30H, the ratio of gammaceranes to C30 hopane) can reflect the relative content of gammacerane in samples. Due to its sensitivity to hyper-saline environments, it is currently the most commonly used molecular geochemical parameter to reflect paleosalinity in sedimentary environments. The gammacerane index in the crude oil and source rock extracts of Well Xyd 1 ranges from 0.22 to 0.45, indicating certain characteristics of brackish water to saline sedimentary environments [25]; the gammacerane index in source rock extracts of the Lucaogou Formation from the Gjg and Dhs outcrops range from 0.07 to 0.20, reflecting the original sedimentary environment of non-high salinity and relatively weak stratification.

5.3. Composition and Distribution Characteristics of Sterane Series

The distribution characteristics of C27-C28-C29 regular steranes are commonly used to indicate the species contributing to sedimentary organic matter. Generally, the dominance of C27 regular steranes indicates the contribution of algae and aquatic organisms, while the dominance of C29 regular steranes reflects the contribution of terrestrial higher plants [26]. The C27-C28-C29 regular sterane source identification chart (Figure 5a) shows that the hydrocarbon generating parent materials of the crude oil from Well Xyd 1 all have contributions from phytoplankton and terrestrial plants; the distribution of regular steranes shows a “V” shape distribution of C27 to C29 (Figure 5b–d), indicating that their organic matter is mainly contributed by algae, aquatic organisms, and terrestrial higher plants.

6. Oil Maturity

Biomarkers are the most common and reliable method for determining the maturity of crude oil at present, but not every parameter among the numerous biomarker maturity parameters can reflect the original maturity well. In the context of a saline lake sedimentary environment, higher salinity is not conducive to the conversion of Tm to Ts, resulting in a lower ratio of Ts to Tm (Ts/Tm) of crude oil compared to the source rock extract from Well Xyd 1.
As the degree of thermal evolution increases, the R configuration of regular steranes will transition to the S configuration, and similarly, the less thermally stable α configuration of regular steranes will transition to the β configuration [27,28]. The crude oil sample from Well Xyd 1 has weak odd–even dominance. In addition, the regular sterane isomerization parameters show that the crude oil in this area has basically not exceeded the equilibrium endpoint and is in the mature interval, so it is judged that the crude oil is from the mature source rock. The regular sterane isomerization parameters of the source rock extracts from the Gjg and Dhs outcrops show that their maturity is in the low-maturity to mature stage (Figure 6a).
As an important component of aromatics, the maturity parameters of phenanthrene series compounds are widely used in the evaluation of source rocks. The maturity parameters of the methyl phenanthrene series have a good indicative effect in the low-maturity to maturity stage, and have been compared with vitrinite reflectance in some studies [29]. A method for measuring organic matter maturity using the relative abundance of phenanthrene and four isomers of methyl phenanthrene, namely the methyl phenanthrene index MPI-1, has been proposed. Some scholars have corrected the relationship between the methyl phenanthrene index and vitrinite reflectance and established a conversion relationship between them [30]. The crude oil and source rocks in the Yongfeng sub-sag have not reached the high maturity evolution stage (Ro < 1.3%), so Formula (1) can be used to convert the equivalent maturity of crude oil and source rock extracts.
Rc = 0.6MPI-1 + 0.4
In the formula, Rc is the kerogen vitrinite reflectance value converted from MPI-1, measured in %; the results (Table 2) show that the maturity of crude oil in Well Xyd 1 ranges from 0.74 to 1.04, and the maturity of source rock extracts in Well Xyd 1 ranges from 0.72 to 0.82, with an average value of 0.76; the maturity of hydrocarbon source rock extracts in the Gjg outcrop ranges from 0.55 to 0.74, with an average of 0.61; the maturity of hydrocarbon source rock extracts in the Dhs outcrop ranges from 0.63 to 0.65, with an average of 0.64 (Figure 6b). This is consistent with the maturity characteristics of C29 regular sterane isomerization parameters.
Dibenzothiophene has a unique thiophene molecular structure, which is widely used in oil and gas tracing. The relative abundance ratio between different isomers also has a certain linear relationship with the degree of thermal evolution. As the degree of thermal evolution increases, unstable compounds will gradually transform into stable compounds, and the parameter values will also increase accordingly, making it a good maturity parameter [31]. It should be noted that the maturity parameter of dibenzothiophene is mainly applicable to the mature to high-maturity stage of the source rock. The relative contents of 1-MDBT and 4-MDBT in the source rock extracts of Well Xyd 1 change significantly, indicating that their maturity is much higher than that of the source rock of the Lucaogou Formation in the Gjg and Dhs outcrops, while the response of the crude oil of Well Xyd 1 to this parameter is not obvious.

7. Oil–Source Correlations

7.1. Geochemical Characteristics of Source Rocks

High-quality hydrocarbon source rocks from the Permian Lucaogou Formation, deposited in deep to semi-deep lacustrine environments, with high organic matter abundance, good types, and varying degrees of maturity, are widely developed in the surrounding area of Bogda Mountain. The Pr/Ph values of hydrocarbon source rock extracts of the Permian Lucaogou Formation from Well Xyd 1, as well as the Gjg and Dhs outcrops, are all less than 1, reflecting that the sedimentary environment is a weak reduction environment (Figure 7a), and the gammacerane index indicates that the paleosalinity of the water body during the deposition of the hydrocarbon source rock from the Lucaogou Formation in Well Xyd 1 is higher than that in the Gjg and Dhs outcrops (Figure 7b).
Steroids in rocks are important organic compounds that can provide rich information. Steroids include various types, among which 4-methyl steroids and dinoflagellates have important indicative significance in environmental discrimination. Abundant 4-methylsterane was detected in the hydrocarbon source rock extracts of the Lucaogou Formation in the Gjg outcrop (Figure 8). 4-methylsterane mainly comes from marine organisms such as dinoflagellates, but can also exist in lacustrine environments. The content of 4-methylsterane in the crude oil and source rock extracts of Well Xyd 1 is relatively low. The C27 regular sterane abundance in the Lucaogou Formation source rock of Well Xyd 1 is high (Figure 5), while the C28 regular sterane content in the Lucaogou Formation source rock of the Gjg outcrop is relatively high. Similar findings have been made by predecessors. Chen Jianping et al. believe that a high abundance of C28 regular steranes is a typical distribution form of hydrocarbon source rocks of the Permian Lucaogou Formation in the Junggar Basin [32]. Wang Jian et al. believe that the high abundance of C28 steranes is derived from hydrocarbon generating parent materials, mainly algae and supermicroorganisms, and may be related to various phytoplankton groups such as diatoms, coccoliths, and dinoflagellates that are increasing in the geological history of the Lucaogou Formation [33,34]. This phenomenon can reflect the differences in the sedimentary environment and source material on the plane of the hydrocarbon source rocks in the Lucaogou Formation around the Yongfeng sub-sag, and can also serve as evidence for distinguishing different types of hydrocarbon source rocks in the region.

7.2. Multi Factor Oil–Source Correlations

7.2.1. Correlations of Carbon Isotope Composition Characteristics

Figure 9a shows the correlation between stable carbon isotopes (δ13C) of saturates and aromatics of the crude oil and source rocks in the Yongfeng sub-sag. The stable carbon isotope of saturates in the crude oil and source rock extracts of Well Xyd 1 ranges from −30.6‰ to −28.2‰, that in the Gjg outcrop is from −33.7‰ to −31.7‰, and that in the Dhs outcrop is from −40.5‰ to −32.2 ‰. By comparison, it can be found that the crude oil and source rock extracts from Well Xyd 1 have relatively heavy carbon isotope values, and the sample data distribution shows high consistency. However, due to the lack of clear regularity in the stable carbon isotopes of aromatics in crude oil and source rocks in the Yongfeng sub-sag, further tracing of the origin of crude oil in Well Xyd 1 requires the use of more biomarker parameters for judgment.

7.2.2. Correlations of Distribution Characteristics of Isoprene Alkanes

Figure 9b shows the correlation between the Pr/Ph and gammacerane index in the crude oil and source rocks in the Yongfeng sub-sag. Both of these parameters can effectively reflect the sedimentary environment during the deposition of the source rock. The gammacerane index of source rock extracts from the Gjg and Dhs outcrops is less than 0.2. The gammacerane index of crude oil and source rock extracts from Well Xyd 1 is more than 0.2. This difference provides a strong basis for us to distinguish source rocks in different sedimentary environments. It is worth noting that the Pr/Ph value of the crude oil sample Oil-1 from Well Xyd 1 is relatively high, which may be related to mud contamination. However, overall, the sedimentary environment of the crude oil and source rocks in Well Xyd 1 is similar.

7.2.3. Correlations of Distribution Characteristics of Terpenoid Compounds

Figure 9c shows the correlation between the Ts/Tm value and C24TeT/C26TT value in the crude oil and source rocks in the Yongfeng sub-sag. By comparison, it can be found that the salinity of the paleowater body during the deposition of the source rocks in Well Xyd 1 is higher, and the input of algae is also more abundant; in the Gjg and Dhs outcrops, the input of terrigenous organic matter is dominant. In addition, the Ts/Tm value and C24TeT/C26TT value of the crude oil from Well Xyd 1 are more similar to their corresponding ratios in the source rocks from Well Xyd 1, further confirming their genetic relationship.
Figure 9d shows the correlation between the ratio of C20 tricyclic terpenes to C21 tricyclic terpenes (C20TT/C21TT) and the ratio of C20 tricyclic terpenes to C23 tricyclic terpenes (C20TT/C23TT) in the crude oil and source rocks in the Yongfeng sub-sag. Generally speaking, C19 to C23 tricyclic terpenes in freshwater lacustrine source rocks and crude oil samples mostly have a C21TT advantage, while in saline lacustrine source rocks and crude oil samples, C19 to C23 tricyclic terpenes mostly have a C23TT relative advantage. Through comparison, it can be found that the salinity of the paleowater body during the deposition of the source rocks of Well Xyd 1 is significantly different from that of the source rocks of the Gjg and Dhs outcrops, which is consistent with the analysis of the sedimentary environment. At the same time, the content characteristics of C19 to C23 tricyclic terpenes in crude oil and source rock extracts from Well Xyd 1 also show high similarity.
In summary, we can draw the following conclusion: the crude oil from Well Xyd 1 mainly comes from the local Permian Lucaogou Formation source rocks. At the same time, the source rocks of the Lucaogou Formation in the Yongfeng sub-sag and around the Bogda Mountain front belt have strong heterogeneity, and there are significant differences in their sedimentary environment and hydrocarbon parent materials on the plane.

8. Conclusions

(1) The source rocks of the Lucaogou Formation from the Yongfeng sub-sag exhibit significant heterogeneity on the plane. Based on the analysis of alkanes, terpenes, and hopanes in the extracts of source rock samples, the source rocks of Lucaogou Formation from Well Xyd 1 were formed in a reducing, semi-saline–saline sedimentary environment, while those from the Gjg and Dhs outcrops developed in a weakly oxidizing–weakly reducing, non-high-salinity, weakly stratified sedimentary environment.
(2) The source rocks of the Lucaogou Formation from Well Xyd 1 have strong heterogeneity, with some sample extracts showing relatively high saturated hydrocarbon content and a saturates/aromatics value greater than 3, and the other part of the samples is mainly composed of resins and asphaltene, with low saturate content. The group components extracted from the source rocks of the Lucaogou Formation from the Gjg and Dhs outcrops are mainly composed of resins, with a saturates/aromatics value less than 2, a high resins/asphaltene value, and low maturity. The crude oil from Well Xyd 1 has mature crude oil characteristics, with a high saturates/aromatics value (3.39–3.54) and low polarity components.
(3) The crude oil and source rock extracts from Well Xyd 1 have heavy carbon isotope values and exhibit high consistency in sample data distribution, with the gammacerane index greater than 0.2. The distribution characteristics of terpenes show that the Ts/Tm value and C24TeT/C26TT value of the crude oil from Well Xyd 1 are more similar to their corresponding values in the source rock. The content characteristics of C19 to C23 tricyclic terpenes in crude oil and source rock extracts from Well Xyd 1 also show high similarity. Multiple parameters confirm that the crude oil from Well Xyd 1 mainly comes from the local Middle Permian Lucaogou Formation source rock.

Author Contributions

Conceptualization and methodology, X.S.; validation, formal analysis, investigation, and resources, L.W. and Y.C.; data curation, J.W., X.Z., Y.G., Y.Y., Z.B. and K.Y.; writing—original draft preparation, L.W., Y.C. and X.S.; writing—review and editing, visualization, supervision, and project administration, X.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Geological Survey Project of China Geological Survey (DD20190090, DD20230042, DD20240044, and DD20242210).

Data Availability Statement

We state that the data are unavailable due to privacy or ethical restrictions of the company and university.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Location map of the study area [6].
Figure 1. Location map of the study area [6].
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Figure 2. Column of Well Xyd 1.
Figure 2. Column of Well Xyd 1.
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Figure 3. Distribution characteristics (a,b) of alkane series in crude oil from Well Xyd 1.
Figure 3. Distribution characteristics (a,b) of alkane series in crude oil from Well Xyd 1.
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Figure 4. (a–d) Mass chromatogram (m/z 191) of source rocks and crude oil from Well Xyd 1.
Figure 4. (a–d) Mass chromatogram (m/z 191) of source rocks and crude oil from Well Xyd 1.
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Figure 5. Identification of organic matter input types based on sterane series (a) and distribution of sterane compounds (b–d) in crude oil and source rocks from Well Xyd 1.
Figure 5. Identification of organic matter input types based on sterane series (a) and distribution of sterane compounds (b–d) in crude oil and source rocks from Well Xyd 1.
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Figure 6. Maturity characteristics of C29 regular sterane isomerization parameters (a) and aromatics maturity characteristics (b) in crude oil and source rocks around the Bogda Mountain front belt.
Figure 6. Maturity characteristics of C29 regular sterane isomerization parameters (a) and aromatics maturity characteristics (b) in crude oil and source rocks around the Bogda Mountain front belt.
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Figure 7. Integrated identification of sedimentary environment (a) and paleosalinity of water body (b) based on Ph/nC18 and Pr/nC17 ratios in crude oil and source rocks around the Bogda Mountain front belt.
Figure 7. Integrated identification of sedimentary environment (a) and paleosalinity of water body (b) based on Ph/nC18 and Pr/nC17 ratios in crude oil and source rocks around the Bogda Mountain front belt.
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Figure 8. (a–d) Distribution of sterane series compounds in crude oil and source rock extracts in the Yongfeng sub-sag.
Figure 8. (a–d) Distribution of sterane series compounds in crude oil and source rock extracts in the Yongfeng sub-sag.
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Figure 9. Geochemical analysis for oil–source correlation. (a) Correlation between stable carbon isotopes (δ13C) of saturates and aromatics; (b) correlation between Pr/Ph and the gammacerane index (Ga/C30H); (c) correlation between the Ts/Tm and C24TeT/C26TT; (d) correlation between C20TT/C21TT and C20TT/C23TT.
Figure 9. Geochemical analysis for oil–source correlation. (a) Correlation between stable carbon isotopes (δ13C) of saturates and aromatics; (b) correlation between Pr/Ph and the gammacerane index (Ga/C30H); (c) correlation between the Ts/Tm and C24TeT/C26TT; (d) correlation between C20TT/C21TT and C20TT/C23TT.
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Table 1. Group component characteristics of crude oil and source rocks around the Bogda Mountain front belt.
Table 1. Group component characteristics of crude oil and source rocks around the Bogda Mountain front belt.
Sample SourceSample No.TypeSaturate
/%		Aromatic
/%		Resin
/%		Asphaltene
/%		Resin and Asphaltene
/%		Collection Rate
/%		Saturates/Aromatics
Well Xyd 170Rock cutting52.33 11.66 18.65 15.54 34.19 98.19 4.49
9119.43 8.57 32.00 35.43 67.43 95.43 2.27
9727.22 20.71 33.14 15.98 49.12 97.04 1.31
10232.43 9.26 35.97 16.89 52.86 94.55 3.50
11240.60 12.26 26.98 14.99 41.97 94.82 3.31
11740.40 19.19 22.90 8.42 31.32 90.91 2.11
12129.90 19.28 29.74 16.99 46.73 95.92 1.55
12735.03 18.84 29.94 10.52 40.46 94.34 1.86
Oil-1 Crude oil59.96 17.68 10.57 3.25 13.82 91.46 3.39
Oil-254.25 15.33 10.85 4.95 15.80 85.38 3.54
Gjg outcrop35Rock25.62 23.14 28.51 19.83 48.34 97.11 1.11
3723.21 20.98 34.37 18.75 53.12 97.32 1.11
3928.07 17.84 33.63 14.91 48.54 94.44 1.57
4123.62 15.35 38.19 14.57 52.76 91.73 1.54
4327.40 15.75 33.33 18.95 52.28 95.43 1.74
4523.66 10.22 39.25 21.51 60.76 94.62 2.32
4719.88 16.67 30.92 26.91 57.83 94.38 1.19
Dhs outcrop12Rock23.08 15.38 50.00 7.69 57.69 96.15 1.50
1518.93 10.44 65.78 4.37 70.15 99.51 1.81
1819.00 8.50 57.25 5.00 62.25 89.75 2.24
2123.25 11.81 56.83 5.90 62.73 97.79 1.97
Table 2. Biomarker compound parameters of crude oil and source rocks around the Bogda Mountain front belt.
Table 2. Biomarker compound parameters of crude oil and source rocks around the Bogda Mountain front belt.
Sample SourceSample No.TypePr
/Ph		Pr/nC17Ph/nC18Ga/
C30H		Ts/TmC24TET/C26TTMPI-1Rc
/%		4-/1-MDBTC29-αββ/(αββ + ααα)C29-ααα20S/(20S + 20R)
Well Xyd 170Rock cutting0.71 0.42 0.58 0.22 4.43 0.90 0.51 0.72 5.26 0.44 0.40
910.58 0.48 0.89 0.31 2.28 0.64 0.51 0.72 9.68 0.51 0.40
970.57 0.49 0.95 0.27 2.48 0.70 0.54 0.74 11.83 0.49 0.39
1020.58 0.54 1.03 0.32 2.77 0.50 0.52 0.73 8.64 0.51 0.38
1120.60 0.55 1.07 0.35 2.09 0.61 0.55 0.74 10.25 0.43 0.39
1170.53 0.53 1.05 0.38 1.91 0.61 0.66 0.80 11.36 0.42 0.40
1210.65 0.54 0.90 0.35 2.39 0.56 0.67 0.81 10.44 0.51 0.41
1270.73 0.64 1.12 0.45 2.31 0.60 0.69 0.82 11.69 0.40 0.41
Oil-1Crude oil1.66 0.31 0.24 0.24 1.49 0.54 0.54 0.74 3.27 0.53 0.41
Oil-21.06 0.54 0.60 0.24 1.04 0.51 1.08 1.04 2.57 0.51 0.40
Gjg outcrop35Rock0.79 0.25 0.26 0.18 0.55 0.65 0.32 0.62 4.67 0.33 0.46
370.54 0.48 0.92 0.10 0.19 1.55 0.55 0.74 2.98 0.37 0.44
390.83 0.36 0.40 0.10 0.98 1.03 0.20 0.55 3.45 0.39 0.46
410.85 0.40 0.43 0.14 0.70 0.63 0.31 0.61 4.22 0.38 0.46
430.83 0.48 0.52 0.16 0.72 0.92 0.26 0.58 3.27 0.36 0.46
450.80 0.24 0.27 0.16 0.68 0.92 0.29 0.60 2.27 0.34 0.47
470.85 0.24 0.27 0.19 0.64 0.98 0.26 0.58 2.82 0.32 0.45
Dhs outcrop12Rock0.15 0.47 0.74 0.15 0.59 0.77 ///0.37 0.30
150.93 0.49 0.52 0.20 0.14 1.48 0.39 0.65 0.98 0.36 0.33
180.94 0.43 0.43 0.07 0.19 2.61 0.37 0.65 4.57 0.25 0.27
210.83 0.63 0.82 0.09 0.27 1.36 0.36 0.64 1.99 0.24 0.23
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Sun, X.; Wu, J.; Zhou, X.; Gao, Y.; Yang, Y.; Bai, Z.; Yuan, K.; Wen, L.; Chen, Y. Geochemical Characteristics of Crude Oil and Oil–Source Correlations in the Yongfeng Sub-Sag of the Bogda Mountain Front Belt. Energies 2025, 18, 917. https://doi.org/10.3390/en18040917

AMA Style

Sun X, Wu J, Zhou X, Gao Y, Yang Y, Bai Z, Yuan K, Wen L, Chen Y. Geochemical Characteristics of Crude Oil and Oil–Source Correlations in the Yongfeng Sub-Sag of the Bogda Mountain Front Belt. Energies. 2025; 18(4):917. https://doi.org/10.3390/en18040917

Chicago/Turabian Style

Sun, Xiangcan, Jianwei Wu, Xingui Zhou, Yongjin Gao, Youxing Yang, Zhongkai Bai, Kun Yuan, Lei Wen, and Yi Chen. 2025. "Geochemical Characteristics of Crude Oil and Oil–Source Correlations in the Yongfeng Sub-Sag of the Bogda Mountain Front Belt" Energies 18, no. 4: 917. https://doi.org/10.3390/en18040917

APA Style

Sun, X., Wu, J., Zhou, X., Gao, Y., Yang, Y., Bai, Z., Yuan, K., Wen, L., & Chen, Y. (2025). Geochemical Characteristics of Crude Oil and Oil–Source Correlations in the Yongfeng Sub-Sag of the Bogda Mountain Front Belt. Energies, 18(4), 917. https://doi.org/10.3390/en18040917

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