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Article

Molecular and Carbon Isotopic Compositions of Crude Oils from the Kekeya Area of the Southwest Depression, Tarim Basin: Implications for Oil Groups and Effective Sources

by
Xiaojie Gao
1,
Qilin Xiao
1,*,
Zhushi Ge
1,
Suyang Cai
1,
Haizhu Zhang
2,
Xiang Wang
2,
Zhenping Xu
2,
Zhanghu Wang
1,
Xiaomin Xie
1 and
Qiang Meng
1
1
Key Laboratory of Oil and Gas Resources and Exploration Technology, College of Resources and Environment, Yangtze University, Wuhan 430100, China
2
Research Institute of Exploration and Development, PetroChina Tarim Oilfield Branch, Korla 841000, China
*
Author to whom correspondence should be addressed.
Energies 2024, 17(3), 760; https://0-doi-org.brum.beds.ac.uk/10.3390/en17030760
Submission received: 7 January 2024 / Revised: 30 January 2024 / Accepted: 1 February 2024 / Published: 5 February 2024
(This article belongs to the Section H: Geo-Energy)
Browse Figures
Versions Notes

Abstract

:
Molecular and stable carbon isotopic compositions of 32 crude oils from the Kekeya area of the Southwest Depression, Tarim Basin, were analyzed comprehensively to clarify oil groups and trace oil sources. The results indicate that lacustrine shale sequences within the Upper-Middle Permian Pusige Formation (P3–2p) are the major effective oil sources; the thermal maturation effects exert the crucial impact on geochemical compositions of crude oils. In the Kekeya structural belt, crude oils produced from the Lower-Neogene, Middle-Paleogene and Middle-Cretaceous sandstone reservoirs were generated mainly from deeply buried P3–2p at the late-to-high maturity stage. These condensates are depleted in terpanes, steranes and triaromatic steranes and enriched in adamantanes and diamantanes. The evaluated thermal maturity levels of crude oils by terpanoids and steranes are generally lower than that of diamondoids, implying at least two phases of oil charging. In the Fusha structural belt, oils produced from the Lower-Jurassic reservoirs (J1s) of Well FS8 were generated from the local P3–2p at the middle to late mature stage. On the contrary, these oils are relatively rich in molecular biomarkers such as terpanes and steranes and depleted in diamondoids with only adamantanes detectable. The P3–2p-associated oils can migrate laterally from the Kekeya to Fusha structural belt, but not to the location of Well FS8. The Middle-Lower Jurassic (J1–2) lacustrine shales as the major oil sources are limited to the area around Well KS101 in the Kekeya structural belt. Crude oils originated from J1–2 and P3–2p can mix together within the Cretaceous reservoirs of Well KS101 by presenting the concurrence of high concentrations of terpane and sterane biomarkers and diamondoids as well as 2–4% 13C-enriched n-alkanes than those of P3–2p derived oils. This study provides a better understanding of hydrocarbon sources and accumulation mechanisms and hence petroleum exploration in this region.

1. Introduction

The Southwestern Depression of Tarim Basin is a typical foreland basin developed in Miocene and Pliocene [1] (Figure 1a). The Kekeya Oilfield is located at the southwestern margin of the Southwestern Depression [1] (Figure 1b), which was formed during the Pliocene to Early Pleistocene [2]. The current oil reserves of around 19.0 million tones have been proved to be hosted in the Lower-Neogene, Middle-Paleogene and Cretaceous sandstone reservoirs [2] (Figure 2). This suggests the promising potential for hydrocarbon exploration and exploitation in this region [3]. The discovery of the Kekeya Oilfield is hence considered to be the second important milestone for petroleum exploration in the Tarim Basin [4].
Two sets of hydrocarbon source rocks have been disclosed in the Kekeya area. They are composed of the Permian lacustrine mudstones within the Pusige Formation and the Middle-Lower Jurassic sequences [3]. Specifically, the Permian lacustrine mudstones in the Pusige Formation contain mainly Type II kerogen with the TOC value of 0.33–4.19% and have entered into the high mature stage with %Ro = 0.84–1.40 [6,7]. The extractable biomarkers within source rocks are enriched in the unknown C30 terpenoids, C29 and C30 rearranged hopanes and C29 regular steranes [8,9]. The δ13C values of individual n-alkanes range from −34.0% to −28.0%, indicating the major role of aquatic organic matter inputs within the Permian Pusige shale sequences. [9,10].
The Jurassic source rocks were deposited in the fluivial/lacustrine bog setting and contain mainly Type II kerogen within the shales and Type III kerogen within the coaly shales and coals [3,6]. The Middle-Lower Jurassic source rocks are depleted in C30 rearranged hopanes and C27-trisnorneohopane (Ts) and also enriched in C29 regular steranes with the C29 ααα 20S/(20S + 20R) value of <0.3, indicating the immature and/or low-matured organic matters within the Jurassic source rocks [6,11,12]. The δ13C values of Jurassic kerogens range from −27.2% to −23.3% [3,13].
The occurrence of multiple source rocks results in a big challenge of hydrocarbon sources in this region. In general, four opinions have been proposed:
(I).
Jurassic source rocks. At the early stage of petroleum exploration, the Carboniferous, Permian and Jurassic source rocks were discovered in the northern area of the southwestern margin of the Tarim Basin, the Carboniferous and Permian source rocks were deposited in the marine setting and the Jurassic source rocks were deposited in the terrestrial environments [3,14]. Previous investigators thought that these three sources were also developed in the Kekeya area. The occurrence of spore pollen, V/Ni < 1 and the enrichment of C29 regular steranes for crude oils from the Kekeya Oilfield led some investigators to believe that crude oils were generated mainly from the Jurassic lacustrine source rocks [15], since the spore pollen can only be detected in the Jurassic Yangye Formation [3,15];
(II).
Permian shales. Some investigators have suggested that crude oils from the Kekeya Oilfield were originated mainly from the Permian source rocks, the relevant evidence refers to the enrichments of Ts, C30 rearranged hopanes, and C27 or C29 regular steranes and depletion of hopanes in oils. The δ13C value of individual n-alkanes range from −32% to −28% [6,8,9,10];
(III).
Permian and Jurassic source rocks. Diterpenoids are considered to be the typical biomarkers to indicate the inputs of higher plants [16]. With the development of petroleum exploration in this region, these compounds were detected in the newly produced crude oils, and these oils are isotopically 2–3% heavier than the Permian-sourced oils. The higher plants act as the major role in the Jurassic source rocks [5]. This indicates that except for the Permian lacustrine shales, the Jurassic source rocks may also have a major contribution [5];
(IV).
Carboniferous and Permian source rocks. Crude oils from the Kekeya Oilfield are relatively enriched in dibenzothiophene, tricyclic and tetracyclic diterpanes and 3β-methyl steranes [17,18]. This indicates that these oils may be originated from pre-Mesozoic formations. The δ13C values of saturated and aromatic fractions suggest the mixed contributions of terrigenous and aquatic alge [19]. These two lines of evidence highlight the major contribution of Carboniferous and Permian source rocks in study area [19]. In fact, only the Permian lacustrine shales and Jurassic source rocks were discovered in this region later [8,11].
The abovementioned research arrives at the relevant conclusions regarding oil sources based mainly on the geochemical data of small pieces of oil samples in the Kekeya structural belt, hence introducing the unpredictable uncertainty into the whole map of oil sources in this region. More recently, a new discovery well, Well FS8, was drilled in the Fusha structural belt (Figure 1b). Crude oils were produced from the Lower-Jurassic Shalitashi sandstone reservoirs with the daily production of 20.6 m3 (Figure 2) [20]. This highlights the promising potential of hydrocarbon resources in the Fusha structural belt. However, several questions induced by this new discovery: where is this oil derived mainly from? Is it generated from the local source rocks or a migrated oil from the Kekeya structural belt? Is this oil similar to oils from the neighbor Well FS4 in the Fusha structural belt? All these questions require us to make a comprehensive reevaluation of hydrocarbon sources in this region. Accordingly, 32 oil samples were collected from the Fusha and Kekeya structural belts; this study focuses on the molecular and stable carbon isotopic compositions of these oil samples to clarify the whole map of oil groups and their possible hydrocarbon sources. The result provides a better understanding of hydrocarbon sources and accumulation mechanisms and hence petroleum exploration in this region.
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Figure 2. Comprehensive stratigraphy histogram of the Southwest Depression, Tarim Basin (modified after [21]).
Figure 2. Comprehensive stratigraphy histogram of the Southwest Depression, Tarim Basin (modified after [21]).
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2. Samples and Methods

2.1. Oil Samples

A total of 32 oil samples were collected from the Kekeya area (Figure 1 and Table 1), including 20 samples from the Lower Neogene Xiehefu Formation (N1x), 5 samples from the Middle Paleogene Kalataer Formation (E2k), 3 samples from the Middle Cretaceous Kukebai Formation (K2k), 2 samples from the Middle Cretaceous Kezilesu Formation (K1kz) and 2 oil samples from the Lower-Jurassic Shalitashi reservoir (J1s) (Figure 1). Most oils are the condensate, while 2 J1s samples are the normal oil.

2.2. Gas Chromatography (GC)

The whole oil GC traces were obtained by using the Agilent 7890B gas chromatograph equipped with a flame ionization detector (FID). A fused silica capillary column (50 m × 0.32 mm i.d.) coated with CP-SIL5CB (film thickness 0.40 mm) was used. The sample was injected using a split ratio of 50:1 and nitrogen was used as carrier gas, with a constant flow rate of 1.0 mL/min. The GC oven was initially set at 35 °C for 15 min, subsequently programmed to 310 °C at 4 °C/min and then maintained at this temperature for 30 min [22]. Although the whole oil GC traces can be used to calculate the parameters including ∑n-C21−/∑n-C22+, carbon preference index (CPI) and odd-even predominance (OEP) of n-alkanes, Pr/Ph, Pr/n-C17 and Ph/n-C18 (Table 2), the quantification of specific compounds needs to run these samples on the gas chromatography-mass spectrometry (GC-MS).

2.3. Gas Chromatography-Mass Spectrometry (GC-MS)

Asphaltenes were removed by precipitation with n-hexane followed by filtration. The de-asphaltened oils were then separated into saturated, aromatic and resins fractions using column chromatography with aluminum oxide as stationary phase, with n-hexane, benzene, and a mixture of dichloromethane and methanol (9:1, v/v) as eluents, respectively. The measurements of GC-MS were conducted on the saturated and aromatic fractions. Prior to GC-MS analysis, two internal standards squalane and adamantane-d16 were added for the quantification of terpenes, steranes, diamondoids and aromatic compounds, respectively.
The GC-MS was equipped with a Hewlett–Packard 6890 gas chromatograph coupled to a Micromass Platform II spectrometer. The HP-5MS column (30 m × 0.25 mm i.d × 0.25 μm) was used for the separation. The GC oven temperature was initially held at 60 °C for 2 min, ramped from 60 °C to 315 °C at 3 °C/min and held for 15 min. The carrier gas was helium, and the flow rate was 1.0 mL/min. The temperature was 250 °C for transfer line and 200 °C for the ion source. The operation conditions of ion source were the electron ionization (EI) mode at 70 eV. The identification of targeted compounds was performed by the Full scan and SIM GC–MS analysis. The mass range was 50–550 Da with the scan time of 1 s for full scan GC–MS analysis [23]. The selected ions of SIM GC–MS analysis were m/z 85 for n-alkanes, m/z 135, 136, 149, 163 and 177 for adamantanes, m/z 187, 188, 201 and 215 for diamantanes, m/z 191 for terpanoids, m/z 217 for steranes, m/z 178 and 192 for phenantheranes, m/z 184 and 198 dibenzothiophenes (DBTs), m/z 231 for triaromatic steroids and m/z 85 for squalane and m/z 152 for adamantane-d15 [23]. Although the quantitative data can be obtained by using GC–MS analysis, gas chromatography isotope ratio mass spectrometry (GC-IRMS) is required to get the stable carbon isotopic compositions of individual n-alkanes.

2.4. Gas Chromatography Isotope Ratio Mass Spectrometry (GC-IRMS)

Stable carbon isotopic compositions of individual compounds were analyzed by a Hewlett–Packard 6890 gas chromatograph connected to a Micromass Isoprime system. HP6890 GC was fitted with a 50 m × 0.32 mm i.d. column coated with a 0.40 μm film of CP-SIL 5CB, leading directly into the combustion interface. The oven temperature was as follows: 30 °C for 15 min, raised from 30 °C to 310 °C at a rate of 3 °C/min and held at 310 °C for 30 min. Helium was used as the carrier gas with a flow rate of 1.5 mL/min. The sample was injected using a split mode with a split ratio of 15:1. The combustion furnace was operated at a temperature of 800 °C and was equipped with CuO and Pt wire as oxidant and catalyst. In order to perform calibration, a CO2 reference gas calibrated against Charcoal Black (a national calibration standard, with a value of −22.43% based on the VPDB standard) was automatically introduced into the IRMS in a series of pulses before and after the array of peaks of interest [22]. The precision of the measurement was typically better than 0.3% relative to the VPDB standard.

3. Results

3.1. Terpenoids, Steranes and Oil Groups

Figure 3 and Table 1 present the chemical compositions of terpenoids and steranes for oil samples from the Kekeya area. Significant differences can be detected on steranes, in particular terpenoids. Based on the distribution patterns of terpenoids and steranes, oil samples from the Kekeya area are classified into four groups:
(1)
Group I: oil samples from KS101. These samples are depleted in tricyclic terpanes, Ts, C30H rearranged hopane and enriched in C29 Norhopane, C30-C35 hopanes and C29 steranes (Figure 3a), resulting in low ratios of C19-C23TT/C30H, Ts/(Ts + Tm) and DiaC30H/C30H (Table 1). C20 TT is the peak of C19-C23TT (Figure 3a). Regular steranes decrease with the order of C29 ααα20R > C27 ααα20R > C28 ααα20R (Figure 3a);
(2)
Group II: oil samples from FS8. Compared with Group I oils, these oils contain more C29 norhopane, C30 rearranged hopane and C29 steranes, resulting in higher ratios of DiaC30H/C29Ts and C29 ααα20R/C27 ααα20R, C23 TT is the peak of C19-C23TT (Figure 3b and Table 1).
(3)
Group: oil samples from FS4, KZ104, KZ106, KZ107, KZ108H, K6, KS7009, K2 (N1x5) and KS103 (E2k). Compared with Group I/II oils, Group III oils contain much less C19-C23TT, C30-C35 hopanes and C27-C29 steranes, but present much higher ratios of C19-C23TT/C30H, Ts/(Ts + Tm), DiaC30H/C30H (Figure 3c and Table 1).
(4)
Group IV: oil samples from K1, K2 (N1x4, N1x7, N1x8), K9, K10, K30, K35, K51, K8001, KX3 and KS103 (N1x8). These oils are even more depleted in C19-C23TT, C30-C35 hopanes and C27-C29 steranes than Group III oils (Figure 3d). The ratios of C19-C23TT/C30H, DiaC30H/C30H and C29 ααα20R/C27 ααα20R are greater than those for Group III oils (Table 1).
Except for terpenoids and steranes, the molecular compositions of these collected oil samples are comprehensively depicted quantitatively including n-alkanes, isoprenoids and diamondoids in the saturated fraction and naphthalenes, phenanthrenes, dibenzothiophenes and triaromatic steranes in the aromatic fraction. The results of compound-specific carbon isotope analysis of n-alkanes were also provided to make a more sophisticated geochemical map of these oil samples at the molecule levels. The details are stated as the following.
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Figure 3. Typical m/z 191 and m/z 217 mass chromatograms showing the distribution of the terpanes and steranes in four oil groups from the Kekeya area.
Figure 3. Typical m/z 191 and m/z 217 mass chromatograms showing the distribution of the terpanes and steranes in four oil groups from the Kekeya area.
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3.2. n-Alkanes and Isoprenoids

Figure 4 presents the typical GC traces of whole oils. Group I oils have the main peak of n-C14 and high concentrations of n-alkanes. Group II oils have the bimodal distribution of n-alkanes with the main peaks of n-C11 and n-C21. Group III/IV oils have the main peak ranging from n-C8 to n-C15 and also high concentrations of n-alkanes. Moreover, the Σn-C21−/Σn-C22+ ratio for Group III/IV oils is obviously greater than that for Group I/II oils. No significant difference can be observed on the other parameters (Table 2).

3.3. Diamondoids

Figure 5 and Table 3 present the molecular compositions of diamondoids in crude oils collected from the Kekeya area. 1-cage adamantanes and 2-cage diamantanes comprise the diamondoids for crude oil samples (Figure 5). Group I oils have the highest concentrations of diamondoids (35,763–45,244 μg/g) and the highest ratios of MAI and MDI (Table 3). In contrast, Group II oils have the lowest concentrations of diamondoids (147–148 μg/g) and contain only adamantanes with no diamantanes detectable (Table 3). The concentrations of diamondoids are 941–2001 μg/g for Group III oils and 755–3884 μg/g for Group IV oils, which are much lower than group I oils, but much higher than Group II oils (Figure 5 and Table 3). The ratio of 1-cage adamantanes to 2-cage diamantanes is 0.54–1.25 (Averaged 0.90) for Group I oils, 3.04–27.55 (Averaged 12.8) for Group III oils and 8.80–35.07 (Averaged 18.8) for Group IV oils, indicating the higher contribution of adamantanes to diamondoids for Group III/IV oils than Group I oils (Figure 5).

3.4. Aromatic Compounds

Naphthalenes, phenanthrenes, dibenzothiophenes and triaromatic steranes are the major components of aromatic compounds in crude oils from the Kekeya area (Figure 6). Several differences in molecular compositions can be observed among various group oils (Figure 6 and Figure 7). Group I oils are extremely enriched in aromatic compounds (62,966–228,944 μg/g), in particular phenanthrenes (22,478–47,204 μg/g) and dibenzothiophenes (5500–12,032 μg/g) relative to the others. Compared with Group I oils, Group II oils contains more triaromatic steranes (133–144 μg/g) with the lowest TA[C20/(C20 + C28)-20R] value of 0.32–0.33, and much less phenanthrenes (734–810 μg/g) and dibenzothiophenes (20–28 μg/g). Group III/IV oils are depleted in triaromatic steranes (<12 μg/g) but with the greater TA[C20/(C20 + C28)-20R] ratio of 0.62–0.82 and present large variations in the absolute concentrations of the other aromatic compounds. For example, phenanthrenes are 209–1893 μg/g for Group III oils and 206–1606 Group IV oils, dibenzothiophenes are 5–297 μg/g for Group III oils and 1–369 μg/g for Group IV oils (Table 4).

3.5. Carbon Isotopic Composition of Individual n-Alkanes for Crude Oils

Table 5 presents the δ13C values of whole oils and individual n-alkanes for crude oils from the Kekeya area. Group I oils are relatively 13C-enriched in the whole oil by presenting around 4% heavier than Group II oils, and 2% heavier than Group III/IV oils (Table 5). This is consistent with the distribution patterns of δ13C values of individual n-alkanes.
The δ13C values of individual n-alkanes for all samples present a decreasing pattern with carbon number increasing. They extend from −30.2% to −26.1% for Group I oils, −32.1% to −29.8% for Group II oils and −32.4% to −27.6% for Group III/IV oils (Figure 8 and Table 5). The δ13C values of individual n-alkanes for Group I oils are 2–4% heavier than that for Group II oils, and around 2% heavier than that for Group III/IV oils.

4. Discussion

4.1. Depositional Environments and Organic Matter Inputs of Oil Sources

Molecular biomarkers in the saturated and aromatic fractions of collected oil samples can be used to constraint the depositional environments and organic matter inputs of source rocks [24]. The ratios of Pr/n-C17 and Ph/n-C18 are 0.06–0.16 and 0.04–0.10, respectively (Table 2). On the crossplots of Pr/n-C17 versus Ph/n-C18 most oils are located at the area of mixed Type II/III organic matter (Figure 9a). This indicates that petroleum source rocks were deposited under the suboxidized conditions with the mixed inputs of terrigenous higher plants and aquatic algae [25,26,27].
The ratios of Pr/Ph and DBT/PHEN for these oil samples are 1.02–1.84 and 0.01–0.26, respectively (Table 2 and Table 4). On the crossplots of Pr/Ph versus DBT/PHEN, these oils are located at the area of marine shale and other lacustrine shales (Figure 9b). This indicates that petroleum source rocks were deposited under the lacustrine settings [28], which is consistent with the previous investigations [3,29]. Specifically, during the middle-to-late Early Permian period, the southwest depression experienced the crustal uplift from east to west. The Permian Pusige Formation was deposited under the lacustrine setting in the Kekeya area. During the Jurassic period, the faulted subsidence occurred with the water depth increasing gradually from the foreland area to the depression, the faulted lacustrine sedimentary system was developed in Kekeya area [30,31]. The predominance of C21TT among tricyclic terpanes further indicates that lacustrine algae act as the major contributor to organic matters within the source rocks [32,33].
Terrestrial higher plants may have a greater contribution to the source rocks for Group II oils. C24 Tetracyclic terpenane is considered to be originated from higher plants and C26 Tricyclic terpenanes are assumed to be generated from aquatic organisms [34,35]. The ratio of C24Ter/C26TT can be used to evaluate the relative inputs of terrestrial higher plants and aquatic algae [34]. It is relatively higher for Group II oils (Table 1). C27 regular steranes are generated from aquatic organisms and algae [36,37], C28 regular steranes are derived from diatoms, coccolithophores, and dinoflagellates, and C29 regular steranes are originated from algae and terrestrial higher plants [24,38]. The ratios of C29-/C27-regular steranes and C29 ααα20R/C27 ααα20R and the concentrations of C29-regular steranes are both higher for Group II oils than those for the other oils (Figure 9c,d). This indicates that terrestrial higher plants have a greater contribution to the source rocks for Group II oils [39], which may be caused by the more oxidized depositional environments as indicated by the greater Pr/Ph ratio (Table 2).

4.2. Thermal Maturity of Crude Oils and Hydrocarbon Mixing

Hopanes and steranes indicate matured oils in the study area. The cross-plot of C29 ββ/(αα + ββ) versus C29 ααα-20S/(20S + 20R) suggests matured oils, while these two ratios for Group I/II oils are relatively lower, indicating lower thermal maturity levels relative to Group III/IV oils (Figure 10a). The C29 20S/(20S + 20R) ratio mostly ranges from 0.50 to 0.55 for Group III/IV oils (Table 1), approaching the equilibrium value (0.52–0.55) [40]. Therefore, care should be taken to this isomerization ratio. The C29 ββ/(ββ + αα) ratio is independent of source organic matter inputs and 0.50–0.58 for Group III/IV oils, which does not approach the equilibrium value (0.67–0.72) [25,41]. The positive correlation between C29 ββ/(ββ + αα) and Ts/(Ts + Tm) suggests that Ts/(Ts + Tm) is significantly affected by thermal stress rather than organic facies (Figure 10b). The Ts/(Ts + Tm) ratio also indicates more matured oils for Group III/IV (Equivalent %Ro = 0.9–1.3) than Group I/II (Equivalent %Ro = 0.6–0.8) [40]. The positive relations between Ts/(Ts + Tm) and diaC30H/C30H and diaC30H/C29 Ts and TA [C20/ (C20+ C28)-20R] highlight the dominated role of thermal stress on molecular compositions and associated parameters (Figure 10c–e) [42,43], resulting in the different distribution patterns of terpanes and steranes (Figure 3).
Adamantanes and diamantanes indicate more matured oils. Due to less thermal stability relative to diamondoids, terpanes, steranes and triaromatic steranes decrease subsequently from Group I to IV, and diamondoids show the opposite trend (Figure 11a–f). The increasing concentrations of diamondoids indicate the increased thermal maturity levels from Group II to IV oils. This is further confirmed by the parameters of MAI and MDI for diamondoids [44]. The crossplot of MDI versus MAI indicates that the equivalent %Ro is generally 1.1–1.3 for Group II oils, 1.3–1.6 for Group III/IV oils and 1.6–1.9 for Group I oils (Figure 10e). The C19-C23TT/C30H ratio presents the positive correlations with adamantanes and diamantanes (except for Group I oils) (Figure 11g,h), indicating thermal stress has the critical role in regulating the evolution path of this parameter.
Hydrocarbon mixing is indicated by the obvious discrepancy in evaluating thermal maturity of crude oils by using molecular biomarkers (Figure 10a) and diamondoids (Figure 10e), respectively. As stated below, for Group I oils, the mixing of crude oils generated from different sources with distinct thermal maturity levels is supposed to be responsible partly for it. Moreover, Group I oils are extremely enriched in diamondoids and aromatic compounds (Figure 5 and Figure 7). This may be caused by the high thermal maturity of their sources (Figure 10e), which facilitates the generation of these compounds resistant to thermal stress. On the other hand, multiple extensive gas charging and cap gas leakage from the reservoirs are also considered to play a part role as indicated by the loss of n-C10–n-C14 (Figure 12) [45], which agrees with previous studies [21,46]. For Group III/IV oils, this is assumed to be induced by the mixing of oils originated from the same source at different thermal maturity levels.

4.3. Possible Oil Sources

In the Kekeya area, the Permian Pusige Formation and the Middle-Lower Jurassic Formations are considered to be the effective source rocks [5,8]. The two sets of source rocks show obvious differences in biomarkers, and the biomarkers within the Permian Pusige Formation are enriched in the unknown C30 terpenoids, C29 and C30 rearranged hopanes and C29 regular steranes [8,9]. While the Middle-Lower Jurassic source rocks contain less C30 rearranged hopanes and Ts, but more C29 regular steranes [9,11]. More importantly, the Permian source rocks and associated oils are isotopically lighter, the relevant Jurassic with the δ13C values of individual n-alkanes range from −34.0% to −28.0% [9,10]. The δ13C values of Jurassic kerogens extend from −27.2% to −23.3% [13], resulting in −30% to −26% for the δ13C values of oils, since the thermal maturation effects usually induce <3% isotopic fractionation between parent kerogen and associated crude oil [47]. The relative enrichments of unknown C30 terpenoids, C29 and C30 rearranged hopanes (Figure 4c,d) and isotopically lighter n-alkanes (−32.4% to −27.6%) (Table 5) support that Group III/IV oils are derived mainly from the Permian Pusige Formation. Our current research demonstrates that the deep burial depth (8000–10,000 m) allows the Permian source rocks to approaching the high mature stage in the Kekeya structural belt (%Ro = 1.3–2.0), shallow burial depth (4000–6000 m) leads to the formation of matured Permian source rocks in the Fusha structural belt (%Ro = 0.6–1.2). Accordingly, the effective Permian source rock for the high-matured Group III/IV oils should be located at the Kekeya structural belt.
Compared with Group III/IV oils, the relative depletion of C30 rearranged hopanes and Ts and enrichment of C29 regular steranes artificially indicate that the Jurassic source rocks may have the major contribution for Group II oils (Figure 3b). However, the isotopically light n-alkanes (−32.1% to −29.8%) suggests the Permian source rocks should play the major role (Figure 8). This discrepancy may be attributed to the lower thermal maturity levels of Group II oils. As stated above, Group II oils present lower thermal maturity than Group III/IV oils. This is indicated by the lower thermal maturity ratios including Σn-C21−/Σn-C22+, C29 20S/(20S + 20R), Ts/(Ts + Tm), C19-C23TT/C30H, diaC30H/C30H, MDI and TA[C20/(C20 + C28)-20R] (Table 1, Table 2, Table 3 and Table 4), resulting in lower absolute concentrations of n-alkanes and diamondoids (Table 2 and Table 3) and higher absolute concentrations of terpanes, steranes and triaromatic steranes within Group II oils (Table 1 and Table 4) as well as 1–2% 13C-depleted n-alkanes (Figure 8). Group II oils were collected from the newly drilled discovery Well FS8 and are assumed to be derived from the local Permian source rocks that have the nearly same thermal maturity levels, since crude oil and the local Permian source rocks share have the nearly same thermal maturity levels. More importantly, the Permian Pusige Formation of Well FS4 has the similar burial depth and hence thermal maturity levels as Well FS8. While the produced oils from the Cretaceous reservoir present greater thermal maturity levels as those oils from the Kekeya structural belt (Figure 10). This indicates oil secondary migration from the Kekeya to Fusha structural belt around Well FS4 (Figure 3b–d).
In comparison with Group III/IV oils, Group I oils show a relative depletion of tricyclic terpanes (Ts), C30 rearranged hopane, and an enrichment in C29 Norhopane, C30-C35 hopanes, and C29 steranes (refer to Table 1). The δ13C values of individual n-alkanes range from −30.2% to −26.1%, resulting in isotopically 2–3% heavier than that for Group III/IV oils. This is generally consistent with the molecular and isotopic compositions of oils generated from the Jurassic source rocks, indicating the contribution of Jurassic source rocks for Group I oils. It is only constrained around the location of Well KS101, since no vital geochemical diagnostic can be detected to indicate the relevant input of Jurassic-associated oils in the other wells. This may be attributed to the vertically permeable passageways around Well KS101, which is indicated by the strong gas washing and leakage (Figure 12) [21,46]. Moreover, its greater burial depth after subsidence from 65–20 Ma results in enough oil can be generated from Jurassic source rocks and then migrate upward to the Cretaceous reservoirs [4,19,48].
However, the Permian source rocks also play a part role. These two oils contain extremely high concentrations of diamondoids and aromatic compounds (Figure 5 and Figure 7). The diamondoids indicate the thermal maturity levels of %Ro = 1.6–1.9 for Group I oils (Figure 10e), which is much greater than that of terpanes and steranes (%Ro = 0.6–0.8) (Figure 10a–d). This discrepancy indicates the mixing of Permian-sourced high-matured oils and Jurassic-sourced matured oils, because the local Jurassic source rocks are located mainly within the matured source kitchen and have not approached the high maturity level in study area [3,12,14]. Less input of isotopically heavier Jurassic-sourced oils lead to lighter δ13C values of individual n-alkanes for oil sample from the upper part of Cretaceous reservoirs of Well KS101 relative to the lower one.
In general, Group I oils are a mixture of the Jurassic-associated matured oil and Permian-sourced high-matured oil. Group II oils were generated from the matured Permian Pusige shale sequences in the Fusa structural belt without the contribution of secondary migrated oil from the Kekeya structural belt. Group III/IV oils were derived mainly from the high-matured Permian Pusige source rocks in the Kekeya structural belt. The distribution patterns of molecular and stable carbon isotopic compositions of collected oils (except for Group I oils) are regulated mainly by thermal stress in this region. The Permian Pusige Formation is therefore supposed to be the major effective source rock in study area. The Middle-Lower Jurassic formations only act as a part role in the area of Well KS101. As stated above, this conclusion partly agrees with the second-to-fourth thoughts and disagrees with the first thought regarding oil sources in study area. Accordingly, this study provides a better understanding of hydrocarbon sources and associated processes of relevant petroleum systems, which should be useful in making a reasonable strategy for petroleum exploration in this region.

5. Conclusions

This study presents the comprehensive quantitative data of molecular and stable carbon isotopic compositions of crude oils from the Kekeya area of Southwest Depression, Tarim Basin, to clarify oil groups and effective oil sources. The following conclusions can be made:
(1)
Lacustrine shale sequences within the Upper-Middle Permian Pusige Formation (P3–2p) are the major effective oil sources. In the Kekeya structural belt, crude oils were generated from deeply buried P3–2p at the late-to-high maturity stage. In the Fusha structural belt, oils produced from the Lower-Jurassic reservoirs (J1s) were generated from the local P3–2p at the middle to late mature stage. The P3–2p-associated oils in the Kekeya structural belt can migrate laterally to the Fusha structural belt, but not to the location of Well FS8;
(2)
The Middle-Lower Jurassic lacustrine shales (J1–2) as the second effective sources are only confined to the area of Well KS101 in the Kekeya structural belt. The J1–2 generated oils can migrate upward and into the Cretaceous sandstone reservoirs of Well KS101, and then mingle with the early charged oils derived from the Permian source rocks;
(3)
The comprehensive quantitative data of crude oils can provide a better understanding of hydrocarbon groups, sources and accumulation process in the Kekeya area of the Southwest Depression, Tarim Basin. Our future work will combine the results of 1D and 2D basin modeling with geochemical data together to make a more sophisticated constrains. This should be useful for petroleum exploration in this region.

Author Contributions

X.G.: Investigation, Formal analysis, Data curation, Writing—original draft. Q.X.: Methodology, Conceptualization, Project administration, Data Curation, Writing—Review & Editing. Z.G.: Investigation, Formal analysis. S.C.: Investigation, Methodology. H.Z.: Validation, Funding acquisition. X.W.: Supervision, Funding acquisition, Resources. Z.X.: Supervision, Validation, Funding acquisition. Z.W.: Supervision, Validation, Funding acquisition, X.X.: Project administration, Supervision. Q.M.: Supervision, Project administration. All authors have read and agreed to the published version of the manuscript.

Funding

We appreciate the grants from National Nature Science Foundation of China (No. 42073066, 41961144023, 41673041), the State Key Laboratory of Organic Geochemistry, GIGCAS (No. SKLOG202013), and the Key Laboratory of Exploration Technologies for Oil and Gas Resources (Yangtze University), Ministry of Education (No. PI2021-05).

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

Authors Haizhu Zhang, Xiang Wang, Zhenping Xu were employed by the company PetroChina Tarim Oilfield Branch. 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. All authors have read and agreed to the published version of the manuscript.

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Figure 1. (a) The location of study area and (b) the tectonic elements and well distribution of the Kekeya area (modified after [5]).
Figure 1. (a) The location of study area and (b) the tectonic elements and well distribution of the Kekeya area (modified after [5]).
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Figure 4. Typical GC traces of crude oils within four groups in the Kekeya area.
Figure 4. Typical GC traces of crude oils within four groups in the Kekeya area.
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Figure 5. The (a) absolute and (b) relative concentrations of diamondoids in oils from the Kekeya area, I–IV indicate the oil groups.
Figure 5. The (a) absolute and (b) relative concentrations of diamondoids in oils from the Kekeya area, I–IV indicate the oil groups.
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Figure 6. Typical total ion chromatography of aromatic fraction for oils from the Kekeya area.
Figure 6. Typical total ion chromatography of aromatic fraction for oils from the Kekeya area.
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Figure 7. The (a) absolute and (b) relative concentrations of aromatics for oils in the Kekeya area, I–IV indicate the oil groups.
Figure 7. The (a) absolute and (b) relative concentrations of aromatics for oils in the Kekeya area, I–IV indicate the oil groups.
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Figure 8. Stable carbon isotope profiles for oils collected from the Kekeya area.
Figure 8. Stable carbon isotope profiles for oils collected from the Kekeya area.
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Figure 9. The crossplots of biomarker parameters referring to depositional environments and organic matter inputs for oil sources in the Kekeya area. (a) Pr/n-C17 versus Ph/n-C18; (b) Pr/Ph versus DBT/PHEN; (c) C29/C27-Regular steranes versus C24Te/C26TT; (d) C29 Regular steranes (μg/g) versus C24Te/C26TT.
Figure 9. The crossplots of biomarker parameters referring to depositional environments and organic matter inputs for oil sources in the Kekeya area. (a) Pr/n-C17 versus Ph/n-C18; (b) Pr/Ph versus DBT/PHEN; (c) C29/C27-Regular steranes versus C24Te/C26TT; (d) C29 Regular steranes (μg/g) versus C24Te/C26TT.
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Figure 10. The crossplots of thermal maturity parameters for crude oils from the Kekeya area. (a) C29-ββ/(αα + ββ) versus C29-ααα20S/(20S + 20R); (b) Ts/(Ts + Tm) versus C29 ββ/(αα + ββ); (c) Ts/(Ts + Tm) versus diaC30H/C29 Ts; (d) Ts/(Ts + Tm) versus TA [C20/(C20 + C28)-20R)]; (e) MAI versus MDI.
Figure 10. The crossplots of thermal maturity parameters for crude oils from the Kekeya area. (a) C29-ββ/(αα + ββ) versus C29-ααα20S/(20S + 20R); (b) Ts/(Ts + Tm) versus C29 ββ/(αα + ββ); (c) Ts/(Ts + Tm) versus diaC30H/C29 Ts; (d) Ts/(Ts + Tm) versus TA [C20/(C20 + C28)-20R)]; (e) MAI versus MDI.
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Figure 11. The crossplots of molecular parameters and absolute concentrations for crude oils from the Kekeya area. (a) C19-C23TT (μg/g) versus C30-C35H (μg/g); (b) C19-C23TT/C30H versus C19-C23TT (μg/g); (c) C19-C23TT/C30H versus C30-C35H (μg/g); (d) C19-C23TT/C30H versus DiaC30H; (e) C19-C23TT/C30H versus C29 steranes (μg/g); (f) C19-C23TT/C30H versus Triaromatic steranes (μg/g); (g) C19-C23TT/C30H versus Admantanes (μg/g); (h) C19-C23TT/C30H versus Diamondoids (μg/g).
Figure 11. The crossplots of molecular parameters and absolute concentrations for crude oils from the Kekeya area. (a) C19-C23TT (μg/g) versus C30-C35H (μg/g); (b) C19-C23TT/C30H versus C19-C23TT (μg/g); (c) C19-C23TT/C30H versus C30-C35H (μg/g); (d) C19-C23TT/C30H versus DiaC30H; (e) C19-C23TT/C30H versus C29 steranes (μg/g); (f) C19-C23TT/C30H versus Triaromatic steranes (μg/g); (g) C19-C23TT/C30H versus Admantanes (μg/g); (h) C19-C23TT/C30H versus Diamondoids (μg/g).
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Figure 12. The crossplots of mole concentration versus carbon number of n-alkanes for collected oil samples, indicating the occurrence of gas washing in the Kekeya area.
Figure 12. The crossplots of mole concentration versus carbon number of n-alkanes for collected oil samples, indicating the occurrence of gas washing in the Kekeya area.
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Table 1. Molecular parameters and concentrations of terpanes and steranes for oil samples from the Kekeya area.
Table 1. Molecular parameters and concentrations of terpanes and steranes for oil samples from the Kekeya area.
Oil GroupWellDepth
(m)		FormationC19-C23TT/
C30H		C24Ter/
C26TT		Ts/
(Ts + Tm)		DiaC30H/
C30H		Ga/
C30H		C29-ααα20S/
(20S + 20R)		C29-αββ/
(αββ + ααα)		C19-23TT (μg/g)DiaC30H (μg/g)C30-35H (μg/g)Regular Sterane (μg/g)
C27C28C29
Group IKS1016664K10.6 1.5 0.4 0.1 0.2 0.4 0.4 2684815484084289
6821K10.7 1.2 0.4 0.1 0.2 0.4 0.4 50043264769132452
Group IIFS83882J1s0.3 3.0 0.5 0.2 0.0 0.5 0.5 302180262026226614
3899J1s0.3 3.0 0.5 0.2 0.0 0.5 0.5 285177250028222599
Group IIIFS42394K21.1 2.0 0.7 0.6 0.1 0.6 0.6 2521476491378214
FS42396K21.2 2.0 0.7 0.6 0.1 0.5 0.5 178964111150140
FS42789K21.4 1.4 0.8 0.7 0.1 0.5 0.6 1397528393399
KS1036336E2k4.2 1.1 0.8 1.8 0.1 0.5 0.6 196911361158147
KZ108H6755E2k23.2 1.2 0.8 1.1 0.1 0.5 0.6 216771821050129
K23323N13.8 1.3 0.8 1.6 0.3 0.5 0.5 167761421769172
K23776N1x7 23.6 0.7 0.6 0.3 0.2 0.5 0.5 201012
K10/N11.0 0.5 0.7 0.5 0.3 0.5 0.5 1383911433
KS70093877N1x7 20.7 0.8 0.7 0.4 0.4 0.5 0.5 123724402257154
KS70093944N1x7 23.4 0.9 0.8 1.7 0.3 0.5 0.5 153821321352131
Group IVK1/N112.9 1.4 0.9 3.8 0.4 0.5 0.5 99322682355
K23172N1x4 112.5 0.8 0.8 1.3 0.0 0.4 0.5 1522012
K23221N1x4 28.8 1.1 0.7 1.9 0.1 0.6 0.5 1021025
K23273N17.5 1.3 0.8 3.3 0.4 0.5 0.6 119574262252
K23804N1x86.6 1.0 0.6 0.5 0.2 0.4 0.4 5841610926
K63322N1x47.1 0.9 0.7 2.8 0.4 0.5 0.6 5524211156145
K93030N26.1 1.5 0.8 1.5 0.2 0.5 0.5 88223892154
K93101N111.1 1.0 0.7 2.9 0.1 0.5 0.6 510012
K93871N16.1 0.9 0.7 5.0 0.2 0.5 0.5 711726101950
K303805N1x511.9 0.8 0.9 2.9 0.3 0.5 0.6 1645136
K35/N1x5 230.5 1.0 0.6 2.9 0.3 0.3 0.6 700001
K513703N1x227.5 0.9 0.7 3.9 0.7 0.5 0.6 1322013
K80013929N1x810.3 0.7 0.7 1.9 0.3 0.5 0.6 1123023
KX33793N1x811.0 1.0 0.8 1.2 0.3 0.5 0.5 69921339
KS1033867N1x815.9 1.1 0.9 3.4 0.4 0.5 0.5 104242181848
KZ1046348E8.1 0.9 0.9 3.5 0.4 0.5 0.6 1034740113180
KZ106/E2k8.7 0.9 0.9 3.7 0.3 0.5 0.6 109484992876
KZ1076269E9.1 1.1 0.8 2.7 0.2 0.5 0.6 92304081949
Note: C19-C23TT: the total concentrations of C19-C23 tricyclic terpanes; C30H: C3017α(H), 21α(H)-hopane; C24Ter: C24 tetracyclic terpane; C26TT: C26 tricyclic terpanes; Ts:C2718α(H), 21β(H)-22, 29, 30-trisnorneohopane; Tm:C2717α(H), 21β(H)-22,29,30-trisnorhopane; DiaC30H: C3017α(H)-diahopane; Ga: Gammacerane; C30-35H: the total concentrations of C30-C35 hopanes.
Table 2. Molecular parameters and concentrations of n-alkanes and isoprenoids for oil samples from the Kekeya area.
Table 2. Molecular parameters and concentrations of n-alkanes and isoprenoids for oil samples from the Kekeya area.
Oil GroupWellDepth (m)FormationMain Peak ∑n-C21−/∑n-C22+CPIOEPPr/PhPr/n-C17Ph/n-C18n-Alkanes (μg/g)
Group IKS1016664K1n-C144.021.080.891.250.060.05358,636
6821K1n-C143.101.140.901.080.080.09314,590
Group IIFS83882J1sn-C213.051.040.951.530.150.10254,729
3899J1sn-C213.011.110.941.550.160.10264,203
Group IIIFS42394K2n-C133.341.190.911.650.080.06303,792
FS42396K2n-C134.831.190.921.780.080.05417,742
FS42789K2n-C132.961.110.931.540.060.04612,909
KS1036336E2kn-C123.481.090.921.160.080.07307,870
KZ108H6755E2k2n-C83.051.090.931.180.070.06335,809
K23323N1n-C112.851.130.951.550.160.10218,293
K23776N1x7 2n-C1023.491.130.931.230.070.06495,750
K10/N1n-C1220.471.080.941.380.070.06475,483
KS70093877N1x7 2n-C132.781.090.921.060.060.06325,145
KS70093944N1x7 2n-C103.501.100.931.020.060.06436,337
Group IVK1/N1n-C153.801.100.91.270.070.06482,465
K23172N1x4 1n-C1019.571.140.641.230.070.07466,475
K23221N1x4 2n-C910.451.100.921.20.070.06425,773
K23273N1n-C142.811.090.961.10.070.06295,270
K23804N1x8n-C1026.111.160.921.380.070.06474,613
K63322N1x4n-C152.671.110.921.120.060.06418,273
K93030N2n-C126.461.120.931.180.060.06258,799
K93101N1n-C134.211.150.931.670.090.06239,626
K93871N1n-C126.571.100.941.190.060.06389,962
K303805N1x5n-C117.301.110.941.120.060.05483,963
K35/N1x5 2n-C1132.401.040.951.220.100.07358,932
K513703N1x2n-C127.601.120.921.210.070.06438,024
K80013929N1x8n-C108.611.100.951.160.060.06506,190
KX33793N1x8n-C1020.181.100.621.840.100.07482,712
KS1033867N1x8n-C116.681.100.921.100.070.07499,034
KZ1046348En-C95.391.110.931.170.060.05354,246
KZ106/E2kn-C104.891.110.921.130.060.05284,251
KZ1076269En-C95.701.100.901.210.060.05342,163
Note: CPI = {(C25 + C27 + C29 + C31 + C33) [1/(C24 + C26 + C28 + C30 + C32) +1/(C26 + C28 + C30 + C32 + C34)]}/2; OEP = {(Ci + 6 × Ci + 2+Ci + 4)/[4 × Ci + 1+Ci + 3)]}m, i + 2: Main peak carbon, m: (−1) i + 1.
Table 3. Molecular parameters and concentrations of diamondoids for crude oil samples collected from the Kekeya area.
Table 3. Molecular parameters and concentrations of diamondoids for crude oil samples collected from the Kekeya area.
Oil GroupWellDepth (m)FormationMAI MDI%RoADMs/DAMsC0-ADM (μg/g) C1-ADM (μg/g)C2-ADM (μg/g)C3-ADM (μg/g)C4-ADM (μg/g)C0-4-ADMs (μg/g)C0-DAM (μg/g)C1-DAM (μg/g)C2-DAM (μg/g)C3-DAM (μg/g)C0-3-DAMs (μg/g)Diamondoids (μg/g)
Group IKS1016664K10.84 0.60 1.891.25214281385988567492225113316483616765184220,13245,244
6821K10.77 0.59 1.870.543770334114732366712549349594838020221623,21435,763
Group IIFS83882J1s0.64 0.00 n.a n.a622752916147n.an.an.an.an.a147
3899J1s0.67 0.00 n.a n.a623752816148n.an.an.an.an.a148
Group IIIFS42394K20.70 0.33 1.23 15.43 58328785507202187918524391222001
FS42396K20.68 0.36 1.32 13.02 40267711459189166518544691281793
FS42789K20.68 0.35 1.30 8.46 312045523761681331236457141571488
KS1036336E2k0.60 0.32 1.22 3.04 201154142731639847914286183241308
KZ108H6755E2k20.73 0.39 1.39 9.19 3718342925798100325453091091113
K23323N10.66 0.45 1.53 7.75 13112314323313107466148281441218
K23776N1x7 20.72 0.40 1.42 27.55 5729874144417717181225205621780
KS70093877N1x7 20.69 0.50 1.65 7.26 211293662551038742251409120994
KS70093944N1x7 20.70 0.36 1.30 12.36 2314536724097871142824570941
Group IVK1/N10.78 0.42 1.47 14.04 584079186182632264336552121612425
K23172N1x4 10.71 0.39 1.38 29.93 432306373981491456821174491505
K23221N1x4 20.71 0.37 1.33 23.96 351634152489595681513440996
K23273N10.70 0.32 1.21 12.14 1810829319683698102219758755
K23804N1x80.73 0.35 1.30 27.81 6132780250319818921329215681960
K63322N1x40.68 0.36 1.32 11.05 201433962611109301634277841014
K93030N20.71 0.33 1.25 17.33 49280777516201182122443371051926
K93101N10.71 0.39 1.38 8.80 917716501168527351489141114273713884
K93871N10.70 0.34 1.27 15.69 291604492929910281426205661094
K10/N10.71 0.32 1.22 24.19 331574082519694571513339984
K303805N1x50.71 0.33 1.24 17.12 3520453433111712211428245711293
K35/N1x5 20.68 0.36 1.31 35.07 4426476148719817541119155501804
K513703N1x20.69 0.36 1.31 17.29 3323167445617315661937287911657
K80013929N1x80.72 0.38 1.37 16.86 53307794505186184521433881091954
KX33793N1x80.76 0.39 1.38 32.29 8241794952520121721724225672240
KS1033867N1x80.72 0.38 1.36 20.90 61363910548206208723393261002187
KZ1046348E0.72 0.40 1.41 12.21 46258627378152146034463461201580
KZ106/E2k0.68 0.36 1.32 12.23 543528395352171997376451121632161
KZ1076269E0.73 0.38 1.36 14.29 46266634357138144128393051011542
Note: MAI = 1-methyladamantane/(1-methyladamantane + 2-methyladamantane); MDI = 4-methyldiamantane/(1-methyldiamantane + 3-methyldiamantane + 4-methyldiamantane); %Ro = 2.4322 × MDI + 0.4389; ADMs/DAMs: the total concentration ratio of adamantanes to diamantanes; ADM: adamantane; DAM: diamantane.
Table 4. Molecular parameters and concentrations of aromatics for crude oils from the Kekeya area.
Table 4. Molecular parameters and concentrations of aromatics for crude oils from the Kekeya area.
Oil GroupWellDepth (m)FormationNaphthalenes (μg/g)Phenanthrenes (μg/g)Dibenzothiophenes (μg/g)Triaromatic Steroids (μg/g)Aromatics (μg/g)DNRMPI1MPI2MPRMDRDBT/PTA[C20/(C20 + C28)-20R]
Group IKS1016664K194122,47855002528,9441.87 0.14 0.16 0.76 4.09 0.23 0.55
6821K1369647,20412,0323462,9664.59 0.13 0.14 0.77 4.86 0.25 0.69
Group IIFS83882J1s12537342013321401.89 0.57 0.54 0.80 5.65 0.01 0.33
3899J1s2938102814412751.36 0.59 0.57 0.84 5.05 0.02 0.32
Group IIIFS42394K28751372681223273.32 0.39 0.39 0.73 7.02 0.04 0.62
FS42396K27451327711021531.55 0.43 0.46 0.78 8.15 0.04 0.66
FS42789K29771893100929791.98 0.38 0.41 0.73 7.70 0.05 0.65
KS1036336E2k195121582014920.75 0.47 0.49 0.58 3.42 0.09 /
KZ108H6755E2k28209502221.75 0.82 1.05 0.80 2.54 0.04 /
K23323N12575824163335659.23 0.39 0.44 0.68 1.22 0.19 0.71
K23776N1x7 2226559511028713.60 0.38 0.41 0.76 4.19 0.03 /
K10/N1227958511028753.80 0.41 0.44 0.76 4.04 0.03 /
KS70093877N1x7 212091252297827663.65 0.41 0.44 0.67 1.25 0.26 0.81
KS70093944N1x7 2181205188414151.20 0.44 0.47 0.71 1.27 0.17 0.76
Group IVK1/N13396212819896.80 0.55 0.62 0.72 2.03 0.03 0.64
K23172N1x4 1244147112029243.68 0.37 0.41 0.70 0.62 0.03 /
K23221N1x4 2205910452390334310.17 0.40 0.43 0.65 1.17 0.25 /
K23273N150839111210021.37 0.42 0.47 0.68 1.26 0.09 0.73
K23804N1x828206102352.48 0.51 0.59 0.83 /0.01 /
K63322N1x41561192210015580.76 0.43 0.47 0.67 1.21 0.17 /
K93030N25573610729002.14 0.41 0.44 0.74 1.47 0.11 0.74
K93101N125751606329045103.76 0.46 0.49 0.79 1.63 0.25 /
K93871N152963126111421.30 0.47 0.50 0.79 1.62 0.11 0.79
K303805N1x519161016192031245.59 0.45 0.46 0.72 1.43 0.22 /
K35/N1x5 22575643145033634.18 0.43 0.46 0.80 1.34 0.25 /
K513703N1x21951522103025763.53 0.41 0.43 0.74 1.43 0.23 /
K80013929N1x876911802360218516.34 0.46 0.48 0.79 1.81 0.24 /
KX33793N1x8154470173023186.57 0.42 0.44 0.80 1.47 0.13 /
KS1033867N1x85392712831111/0.46 0.48 0.79 1.65 0.12 0.82
KZ1046348E12729330423/0.92 1.10 0.87 2.13 0.02 /
KZ106/E2k9191160220022993.01 0.48 0.50 0.77 1.69 0.23 /
KZ1076269E7556190582/0.65 0.71 0.82 2.54 0.02 /
Note: DNR = (2,6-dimethylnaphthalene + dimethylnaphthalene)/1,5-dimethylnaphthalene; MPI1 = 1.5 × (2-methylphenanthrene + 3-methylphenanthrene)/(phenanthrene + 1-methylphenanthrene + 9-methylphenanthrene); MPI2 = 3 × (2-methylphenanthrene)/(phenanthrene + 1-methylphenanthrene + 9-methylphenanthrene); MPR = (3-methylphenanthrene + 2-methylphenanthrene)/(9-methylphenanthrene + 1-methylphenanthrene); MDR = 4-methyldibenzothiophene /1-methyldibenzothiophene; TA: Triaromatic; /: The content is too low to be detected.
Table 5. The δ13C values of whole oil and individual n-alkanes for oil samples collected from the Kekeya area.
Table 5. The δ13C values of whole oil and individual n-alkanes for oil samples collected from the Kekeya area.
Oil
Group		WellDepth
(m)		Formationδ13Coil
(%)		n-C12n-C13n-C14n-C15n-C16n-C17n-C18n-C19n-C20n-C21n-C22n-C23n-C24n-C25n-C26n-C27n-C28n-C29n-C30n-C31n-C32
Group IKS1016664K1−27.7−27.8−28−28.6−28.9−29−29.2−29.5−29.5−29−29.5−29.8−30.2−29.5−30.2−29.8−29.3−29.5−29−29.1−29.6/
6821K1−26.9−26.1−26.5−27.3−27.3−27.8−28−28.3−27.4−28.2−27.9−28.1−28.3−27.9−28.9−28.4−28.9−27.9−28.1−28−28.2−28.6
Group IIFS83882J1s−28.9−30.1−30.5−30−30.6−30.6−30.4−30−32−31.5−31.9−31.6−31.5−31.4−31.6−31.5−31.8−31.1−31.1−31−30.8−30.7
3899J1s−28.6−30.4−30.2−30.1−30.1−30.1−30.5−29.8−31.3−31.6−32.1−31.3−31.9−31.6−31.5−31.7−31.4−30.8−31.3−31.1−30.8−30.2
Group IIIFS42394K2−30.8−28.4−28.6−28.9−29.3−28.9−29−28.7−30.1−29.8−30−30.1−29.8−30.1−30.5−30.2−30.7−30.8−30.8−30.8−30.4−30.4
FS42396K2−28.1−29.2−29.3−29.1−29.7−29.7−29.5−30−30−29.6−29.8−30.1−30.6−30.6−30.3−30.9−31.2−30.7−30.5−30.3−31.1−30.1
FS42789K2−28.9−29.2−29.9−29.4−29.9−29.6−29.2−29−30−30.1−29.3−30−30.5−30.3−30.6−30.6−30.9−30.1−29.3−30.7−29.2/
KS1036336E2k−29.8−29.7−29.7−29.8−30.3−29.8−29.9−29.3−30−30.7−31.3−30.9−31.2−30.8−30.9−31.4−31.4−31.4−31.7−30.7−30.9−30.5
KZ108H6755E2k2−29.1−29.2−29−29.5−29.7−29.6−30.2−29.6−29.9−30.2−30.7−30.5−30.9−30.2−30.7−31.2−31.4−31−31−30.5//
K23323N1−29.8−29.1−28.7−29−29.5−29.5−29.7−29.7−30.5−30.1−30.3−30.8−31.2−30.7−31.2−31.4//////
K23776N1x7 2−29.5−29.2−29−29.5−30−29.4−29.7−29.2−30.7−30.5−30.9−30.8−30.8−30.4−31.3−31.2−31.4−31.4−31−31.2−30.9−30.3
K10/N1−27.7−27.8−28−28.5−29.1−28.9−29.1−29.2−30.1−29.8−30.7−30.8−30.9−30.4−30.6−31.5−31.3−32.4////
KS70093877N1x7 2−29.6−29.1−29.2−29−29.6−29.4−29.3−29.7−30.5−30.3−30.7−30.4−30.7−30.9−31.1−30.8−31−31.1−31.2−30.3−30.4−30.7
KS70093944N1x72−28.9−29.1−29.1−29.4−29.8−29.2−29.4−29.2−29.8−30.3−30.8−30.4−30.7−30.4−31.2−31.2−31.5−30.9−31.1−30.4−30.2−29.9
Group IVK1/N1−29.2−28.7−28.7−29.1−29.7−29.3−29.5−29.6−30.2−30.1−30.4−30.4−31.2−30.9−30.9−31.8−32.1−31.5−31.4−31.8//
K23172N1x4 1−29.6−27.6−28.3−28.2−28.9−29.3−29.7−29.1−30.3−29.8−30.7−30.6−30.5−30.4−30.6−31.3−31.3−31.5−31.4−31.1//
K23221N1x4 2−29.2−29.1−29.2−29.6−29.8−29.9−29.6−29.4−30.4−29.9−30.5−30.9−30.3−30.3−31.4−31−31−31.1−30.3///
K23273N1−29.1−28.8−29.1−29.5−29.6−29.4−29.3−29.9−30.6−30.4−30.3−30.2−30.9−30.3−31.3−31−30.8−30.9−31.3−30.7−30.6−30.4
K23804N1x8−29.2−28.6−28.7−29.1−29.6−29.4−29.2−29−29.9−30−30.6−30.5−30.1−30.6−30.8−30.8−31.5−31.4−31.3−31.3−30.3−29.7
K63322N1x4−29.6−29.1−28.9−29.2−29.8−29.4−29.8−29.6−30.3−30.1−30.7−31.2−31.1−30.9−31.4−31.6−32.1−31.4−31.7−31.6−30.9−30.9
K93030N2−27.9−28.4−28.5−28.5−29.3−29−29.7−28.9−30.6−30.1−30.6−30.4−31−30.5−31.5−31.6−31.7−32−32−31//
K93101N1−29.5−28.8−28.8−29.1−29.7−29.4−29.4−29.1−30.7−30−30.8−30.5−30.7−30.9−31.2−30.9−31.7−31.2−31.9−30.9−30.6−30.1
K93871N1−29.8−28.6−28.8−29.8−29.7−29.3−29.9−29.5−30.8−30.6−31−30.9−31.1−30.6−31.7−31.5−32.4−32.1−32.3−31.2−31.1−31.1
K303805N1x5−29.6−27.6−28.3−28.3−29.3−29.2−29.2−28.9−30.1−29.5−30.4−30.4−30.7−30.3−31.2−31.4−31.7−31.8−31.6−31−31.2−29.2
K35/N1x5 2−29.4−28.9−29.2−29.1−29.7−29.6−30−29.5−30.5−30.7−30.4−30.8−31.4−30.7////////
K513703N1x2−29.4−28.7−28.8−29−29.9−29.6−29.4−29.3−30.3−30.2−30.9−30.7−31.4−31−31.3−31.4−31.5−31.7−31.2///
K80013929N1x8−29.6−27.9−28.2−28.4−29.2−29.1−29.3−28.9−29.3−29.8−30.3−30.3−30.8−30.5−31.1−31.6−32−32.1−32.3−31.2−31.6/
KX33793N1x8−29.2−28.4−29−29.5−29.9−29.4−29.8−30−30.6−29.7−30.1−30.5−30.9−30.7−31.4−32.1−31.3−31.9////
KS1033867N1x8−27.6−28.2−28.3−28.4−29.3−29.2−29.1−29.3−30.3−29.6−29.6−30.4−30.9−30.8−30.8−31.5−31.6−31.6−32.3−31.1//
KZ1046348E−29.4−27.8−28−28.6−28.9−29−29.2−29.5−29.5−29−29.5−29.8−30.2−29.5−30.2−29.8−29.3−29.5−29−29.1−29.6/
KZ106/E2k−29.4−26.1−26.5−27.3−27.3−27.8−28−28.3−27.4−28.2−27.9−28.1−28.3−27.9−28.9−28.4−28.9−27.9−28.1−28−28.2−28.6
KZ1076269E−29.3−30.1−30.5−30−30.6−30.6−30.4−30−32−31.5−31.9−31.6−31.5−31.4−31.6−31.5−31.8−31.1−31.1−31−30.8−30.7
Note: /: The content is too low to be detected.
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Gao, X.; Xiao, Q.; Ge, Z.; Cai, S.; Zhang, H.; Wang, X.; Xu, Z.; Wang, Z.; Xie, X.; Meng, Q. Molecular and Carbon Isotopic Compositions of Crude Oils from the Kekeya Area of the Southwest Depression, Tarim Basin: Implications for Oil Groups and Effective Sources. Energies 2024, 17, 760. https://0-doi-org.brum.beds.ac.uk/10.3390/en17030760

AMA Style

Gao X, Xiao Q, Ge Z, Cai S, Zhang H, Wang X, Xu Z, Wang Z, Xie X, Meng Q. Molecular and Carbon Isotopic Compositions of Crude Oils from the Kekeya Area of the Southwest Depression, Tarim Basin: Implications for Oil Groups and Effective Sources. Energies. 2024; 17(3):760. https://0-doi-org.brum.beds.ac.uk/10.3390/en17030760

Chicago/Turabian Style

Gao, Xiaojie, Qilin Xiao, Zhushi Ge, Suyang Cai, Haizhu Zhang, Xiang Wang, Zhenping Xu, Zhanghu Wang, Xiaomin Xie, and Qiang Meng. 2024. "Molecular and Carbon Isotopic Compositions of Crude Oils from the Kekeya Area of the Southwest Depression, Tarim Basin: Implications for Oil Groups and Effective Sources" Energies 17, no. 3: 760. https://0-doi-org.brum.beds.ac.uk/10.3390/en17030760

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