Organic carbon isotopic analysis is a significant approach for oil-source correlation, yet organic carbon isotopic behavior during oil expulsion from saline lacustrine source rocks is not well constrained, and this hinders its wide application for fingerprinting oils generated by saline lacustrine source rock. To resolve this puzzle, semiclosed hydrous pyrolysis was conducted on typical saline lacustrine source rocks from the Qianjiang Formation (type I kerogen) and Xingouzui Formation (type II kerogen) sampled in the Jianghan Basin, China, under high-temperature high-pressure conditions (T = 275℃–400℃; P = 65–125 MPa). Experimental results show that there is minor carbon isotopic fractionation (<3‰) between pyrolyzed and nonpyrolyzed retained oil fractions during the main oil generation/expulsion stage of both type I and II source rocks. Carbon isotopic fractionations between expelled and retained oil fractions are also minor (<2‰) during this stage. The δ13C values of retained and expelled oil fractions generated by the type I saline lacustrine source rock correlate positively with the degree of oil expulsion, whereas the influence of oil expulsion on the δ13C values of oil fractions generated by the type II source rock was not consistent. In addition, carbon isotopic analysis also unravels the mixing of oil-associated gases with different maturity levels and/or generated via different processes. Outcomes of this study demonstrate that oil expulsion from type I and II saline lacustrine source rocks cannot be able to result in large-degree carbon isotopic fractionation, indicating that carbon isotopic analysis is a feasible approach for conducting oil-source correlation works in saline lacustrine petroleum systems.

Oil is generated through the thermal degradation of kerogen in hydrocarbon source rock and expelled after migrating within the source rock [1-3]. Oil migration within the source rock can be mainly through diffusion in organic matter networks, whole-phase flow, migration of a liquid-saturated gas phase, and so on [4-6]. Only portions of generated oils are expelled [7], and oil retained within source rocks is an important component of the rock. Besides the economic significance of retained oils (i.e., shale oil [8]), retained oil also has the ability to enhance the hydrocarbon-generating potential of source rocks because it has greater gas generation potential than overmature kerogen, especially for C2–C5 gaseous hydrocarbons [9-11].

Fractionation of organic compound classes occurs during oil expulsion. In general, organic fractions with higher molecular weight and higher degrees of polarity are more likely to be retained in source rocks rather than being expelled out of rocks [12-14], and molecular fractionations within individual compound classes may not be substantial [15, 16]. Compared with polar compounds and aromatic hydrocarbons, paraffins are more readily to be expelled [14, 16, 17]. Therefore, the degree of oil expulsion has a significant impact on chemical compositions of expelled oils [18]. Previous studies also showed that the expulsion behavior of organic compounds is mainly influenced by the selective adsorption of organic matter in source rocks [14, 19, 20].

The stable carbon isotopic ratio (δ13C) of organic fractions in oils is useful for oil-source correlations as evidenced by many field-based [21, 22] and laboratory studies [23-25]. Carbon isotopic fractionation between expelled oil and kerogen is minor [23, 24] and is up to 4‰ at the oil generation peak [25]. Previous studies have also investigated the retention/expulsion behavior of organic carbon isotopes for type II and III source rocks. The influence of oil expulsion on organic carbon isotopic compositions of n-alkanes yielded by type II source rocks is insignificant [10, 26]; however, oil expulsion indeed results in lighter δ13C values for n-alkanes in expelled oils compared with those in retained oils (~4‰) generated by type III source rocks [26]. The authors interpreted that such carbon isotopic fractionation may be caused by the common occurrence of nanometer-scale pores within organic matter in type III source rocks (e.g., vitrinite), which enhances the selective adsorption of heavier carbon isotopes [26]. The saline lacustrine source rock is an important type of oil-gas source rock for the large-scale hydrocarbon accumulation worldwide, such as the Green River petroleum system in America [27] and the Jianghan oil field in central China (Figure 1) [28]. Both type I and type II source rocks can be formed in saline lacustrine settings [28]. However, it is not well constrained whether the organic matter type has a significant impact on organic carbon isotopic behavior during oil expulsion from saline lacustrine source rocks. This is fundamental for understanding whether oil expulsion can greatly affect oil-source correlations using stable organic carbon isotopic analysis in saline lacustrine petroleum systems. Therefore, the primary objective of this study was to evaluate carbon isotopic partitioning of organic fractions with variable polarities (saturates, aromatics, resins, and asphaltenes) in oils generated and expelled from the type I and II saline lacustrine source rocks using hydrous pyrolysis experiments.

Hydrous pyrolysis is a widely utilized laboratory method for investigating physical/chemical variations during hydrocarbon generation/expulsion, in which immature organic-rich sediments are pyrolyzed into immiscible oils which resembles natural oils [23, 29]. This artificial technique has been successfully applied to characterize the carbon isotopic behavior of organic fractions/compounds during oil generation/expulsion in sedimentary basins worldwide [30-34]. In this study, immature type I saline lacustrine source rock samples were taken from the Eocene Qianjiang Formation in the Jianghan Basin, central China (Figures 1 and 2). These source rock samples were analyzed by hydrous pyrolysis experiments in a wide temperature range (275°C–400°C), which corresponds to the oil generative window of source rocks [35, 36]. Carbon isotopic compositions of retained oil fractions, expelled oil fractions, and hydrocarbon gases yielded during hydrous pyrolysis of source rock samples were determined in order to investigate the organic carbon isotopic behavior during oil expulsion. Hydrous pyrolysis and organic carbon isotopic analysis were also performed for a type II saline lacustrine source rock samples (Eocene Xingouzui Formation) from the same basin for comparison purposes (Figures 1 and 2).

2.1. Source Rock Samples

Source rocks used in this study were sampled from the Eocene Qianjiang Formation and Xingouzui Formation intersected by drill holes Wang 56 and Xin 521, respectively, in the Qianjiang Depression of the Jianghan Basin (Figure 1). The Jianghan Basin is a characteristic terrestrial petroliferous basin in central China, with abundant oil and gas generated from saline lacustrine hydrocarbon source rocks [28]. The Jianghan Basin is located in the central part of the Yangtze Platform and formed during the late stage of the Yanshanian movement [37]. In this study, source rocks samples were taken from the Qianjiang and Xingouzui formations deposited in the Qijiang Depression of the Jianghan Basin. Several episodes of tectonic evolution were identified in the Qianjiang Depression, including the fault-controlled subsidence, depression, and uplifting [38]. The Eocene Qianjiang Formation is mainly composed of mudstone, evaporates, carbonates, silty sandstone, and gypsum [39]. The Eocene Xingouzui Formation is mainly composed of mudstones, gypsum, dolomite, and silty sandstone [37]. Both Qianjiang and Xingouzui formations were deposited in saline lacustrine settings [40]. Source rocks within the Qianjiang Formation generally have total organic carbon (TOC) values distributed in 0.71%–9.02%, and source rocks within the Xingouzui Formation generally have TOC values in 0.91%–4.80% [37-39]. High contents of organic matter in these two strata units suggest that they have favorable hydrocarbon generative capacities [37-39]. Besides, source rocks from both Qianjiang and Xingouzui formations are in a wide thermal maturity range (low mature to over mature) [38], and those with low maturity levels are ideal samples for pyrolysis studies. Two high TOC and low mature source rock samples from the Qianjiang and Xingouzui formations were selected in this study for hydrous pyrolysis experimental studies.

2.2. Methodology

2.2.1. Hydrous Pyrolysis

Hydrous pyrolysis experiments were conducted in a wide temperature range (275℃, 400℃) in a semiclosed system as described in the References 1-3, 41. There are several main subsets in the hardware experimental system [2]: (1) reaction (autoclave pyrolyzing samples); (2) heating (temperature controls); (3) hydraulic control (lithostatic pressure controls); (4) fluid replenishment (fluid pressure controls); and (5) collecting subsystems (oil/gas collection). Oil/gas expulsion was controlled by a two-position, three-way solenoid valve, which occurs when the fluid pressure increment exceeds the preset pressure threshold (5 MPa). The solenoid valves open automatically to release oil/gas when the increment in fluid pressure reaches 5 MPa while close when the decrease in fluid pressure reaches 5 MPa [1-3]. Pyrolysis sample was drilled as a cylinder from the core rock sample with a diameter of ~2.5 cm and height of ~4 cm (~40 g in mass). After placing the cylinder pyrolysis sample into the autoclave, distilled water was added to fill up the remaining space of the autoclave. Before running samples, a leak test was conducted. The experiment was increased to the preset temperature at a rate of 1℃/min and held for 24 hours. After the pyrolysis was finished, the temperature of the autoclave would be decreased in order to collect hydrocarbon products. Gas was collected when temperature decreased to 150℃, and associated gas condensate was collected with a cold trap. There are two types of liquid oil products during hydrous pyrolysis experiments at room temperatures, including expelled immiscible oils and retained oils. The expelled immiscible oil consists of liquid hydrocarbons leached from pipelines and autoclave by dichloromethane and condensates collected by the cold trap. The retained oil is collected through organic extraction of pyrolyzed rock samples by chloroform.

2.2.2. Organic Geochemistry Analysis of Source Rock Samples

Organic geochemical features of source rock samples that underwent hydrous pyrolysis were determined using a Rock-EVAL6 instrument [5, 8]. Three peaks were measured during the experiment. The peak S1 denotes amounts of free hydrocarbons, which were generated by heating source rock samples at 300℃ for 3 minutes at the initial stage of pyrolysis. The peak S2 denotes pyrolyzable hydrocarbons, which were generated by heating source rocks from 300℃ to 600℃ (25℃/min) for 1 minute. The peak S3 denotes CO2. It is noted that CO2 was generated from 300℃ to 400℃ but can only be captured by the thermal conductivity detector during the cooling stage. When the temperature reaches 600℃, the system would remain at this temperature point for 8 minutes in order to measure the amount of inert residual organic carbon. TOC is the sum of the total pyrolyzable carbon and residual carbon. Tmax represents the temperature when maximum generation of hydrocarbons was released from cracking of kerogen. Hydrogen index (HI) equals S2/TOC × 100.

2.2.3. Liquid Chromatography and Carbon Isotopic Analyses of Oil Fractions

Asphaltene in retained and expelled oils was collected through precipitating with petroleum ether (about 0.05 g sample/30 mL solvent) [15-18]. The asphaltene-removed oils were further separated into saturated, aromatic, and resin fractions using alumina columns with a sequential solvent eluting procedure (n-hexane, toluene, and chloroform). The stable carbon isotopic ratios (δ13C) of organic fractions were determined using a Finnigan MAT Delta Plus XLDELTA plus XL C003 mass spectrometer. The δ13C values are reported in “δ” notation as per mil (‰) deviations from the δ13C value of the Vienna Peedee Belemnite.

2.2.4. Molecular and Carbon Isotopic Analyses of Gas

Gas composition was analyzed using a gas chromatograph (GC, CP 3800, SpectraLab Scientific) fitted with a PoraPLOT Q capillary column (30 m, 0.25 mm i.d., 0.25 µm) [20-23]. The oven was initially set at 70℃ for 6 minutes and programmed with a heating rate of 15℃/min until 180℃ holding for 4 minutes. Hydrocarbon gas species was quantified using an external standard method [1-3]. Organic carbon isotopic ratios of individual hydrocarbon gas species were analyzed using a Finnigan MAT 253 mass spectrometer. Isotopic results were generally reproducible less than ±0.3‰.

3.1. Hydrocarbon Yields and Organic Geochemical Features of Pyrolyzed Source Rocks

Yields of hydrocarbons generated by hydrous pyrolysis experiments are listed in Table 1. In general, hydrocarbon yields of hydrous pyrolysis experiments for the sample Wang 56 are higher than those of the experiments for the sample Xin 521, and this is caused by the higher organic matter content in the sample Wang 56 (Table 1). Compared with hydrous pyrolysis conducted under higher temperature conditions, yields for expelled oil are relatively lower in hydrous pyrolysis experiments at 275℃–300℃, with values of 0.349–0.439 mg/g·TOC and 1.82–2.39 mg/g·TOC for the samples Wang 56 and Xin 521, respectively (Table 1). After 325℃, there is a drastic increase in the yields of expelled oil and gas (Table 1). Such increasing trends are accompanied with the increase of retained oil yields which reach the peak at 350℃ (Table 1). The yields of retained oils decrease from 350℃ to 400℃, whereas the yields of expelled oils and gas still exhibit increasing trends at this temperature interval for experiments using the sample Xin 521 (Table 1). For experiments using the sample Wang 56, the yields of expelled oils fluctuate during this temperature interval.

Samples from both Qianjiang and Xingouzui Formations have elevated organic matter abundance, with TOC (%) values of 8.27% and 2.6% for the samples Wang 56 and Xin 521, respectively (Table 1). The Tmax values of samples Wang 56 and Xin 521 are 434°C and 432°C, respectively (Table 2), suggesting that they are low mature with respect to thermal maturation [2]. The Tmax and HI values indicate that the samples Wang 56 and Xin 521 belong to type I and II source rocks, respectively (Figure 3). Pyrolyzed samples have lower TOC and HI values (Table 3) compared with unpyrolyzed samples (Table 2), and TOC and HI values display negative correlations with pyrolysis temperatures (Figures 4(a) and 4(b)).

3.2. Carbon Isotopic Ratios of Individual Organic Fractions in Retained and Expelled Oils

Proportions and carbon isotopic ratios of individual organic fractions are listed in Tables 3 and 4, respectively. In hydrous pyrolysis experiments for the sample Wang 56, resin occupies the largest percentage in both retained oils and expelled oils at the initial stage (<325℃). Compared with expelled oils, retained oils contain higher proportions of resin and aromatic fractions but lower proportions of asphaltene (Figures 5(a) and (b)). Expelled oils and retained oils contain comparable proportions of the saturated fraction (Figures 5(a) and (b)). In hydrous pyrolysis experiments for the sample Xin 521, expelled oils have lower proportions of resin but higher proportions of saturated fractions than retained oils (Figures 5(c) and 5(d)). The proportions of aromatic fractions are similar in both expelled and retained oils, yet expelled oils have slightly lower proportions of asphaltene (Figures 5(c) and 5(d)). Generally, individual organic fractions in expelled oils have more 13C-depleted carbon isotopic ratios compared with retained oils yielded by hydrous pyrolysis for both the samples Wang 56 and Xin 521 (Figures 6(a)–6(d)). Resin and asphaltene fractions have higher δ13C values compared with saturated and aromatic fractions in both expelled and retained oils (Figures 6(a)–6(d)). The carbon isotopic fractionation between oils in unpyrolyzed samples and oils yielded during hydrous pyrolysis (both expelled and retained oils) is minor, which is up to 2.5‰ and 2.4‰ for the experiments using samples Wang 56 and Xin 521, respectively (Figures 6(a)–6(d)).

3.3. Molecular and Carbon Isotopic Compositions of Hydrocarbon Gases

Gas compositions and carbon isotopic ratios of hydrocarbon gases, including methane, ethane, and propane, are listed in Table 5. Higher proportions of hydrocarbon gases are yielded in hydrous pyrolysis experiments for the sample Wang 56 compared with experiments for the sample Xin 521 (Figures 7(a) and 7(b)). Methane, ethane, and propane generally exhibit increasing trends with the rise of pyrolysis temperature, and methane is the most abundant hydrocarbon gas species yielded in hydrous pyrolysis for both the samples (Figures 7(a) and 7(b)). Methane has more 13C-depleted carbon isotopic ratios compared with ethane and propane (Figures 7(c) and 7(d)).

4.1. Influence of Maturation on the Retention Behavior of Organic Carbon Isotopes

Thermal maturation is an important factor affecting the retention behavior of organic carbon isotopes. Differences between δ13C values of pyrolyzed and unpyrolyzed retained individual organic fractions (Dδ13C) are minor (<3‰) throughout the whole course of the study (Figures 8(a)–8(d)). The Dδ13C values of aromatic hydrocarbons, resin, and asphaltene fractions exhibit positive correlations with pyrolysis temperatures (Figures 8(b)–8(d)), suggesting that pyrolyzed retained fractions become isotopically heavier relative to unpyrolyzed fractions with the increase of thermal maturity levels. This is consistent with the knowledge that the cleavage of 13C–12C bonds requires higher energy compared with that for the cleavage of 12C–12C bonds [25]. Therefore, more 13C isotopes are present in retained fractions generated at higher maturity levels. However, there is a weak correlation between Dδ13C values of saturated hydrocarbons and pyrolysis temperatures, and this pattern suggests that the influence of maturation on the retention behavior of organic carbon isotopes of saturated hydrocarbons is not systematic.

HI (HI = S2/TOC) is a commonly used parameter for differentiating organic matter types of source rocks and can be used to estimate the quantity of pyrolyzable organic matter in source rocks [42]. Generally, type I source rocks are characterized by higher HI values compared with type II and type III source rocks [42]. Quantities of pyrolyzable organic matter can be affected by thermal maturation and organic facies of source [2]. In experiments using both samples Wang56 and Xin521, HI values display decreasing trends with the increase of pyrolysis temperatures (Figure 4(b)), supporting that thermal maturation is an important factor resulting in the decrease in the proportions of pyrolyzable organic matter in source rocks. In this study, Dδ13C values of saturated hydrocarbons, resin, and asphaltene exhibit negative correlations with HI values (Figures 8(e), (g), and (h)). This indicates that organic fractions generated from source rocks with lower contents of pyrolyzable organic matter are characterized by more 13C-enriched carbon isotopic compositions relative to unpyrolyzed fractions. Thermal maturation can result in the depletion of pyrolyzable organic matter in source rocks as shown in Figure 4(b), and oils generated at higher maturity levels tend to be enriched in heavier carbon isotopes as the cleavage of 12C–13C bonds requires higher energies than those for the cleavage of 12C–12C bonds [25]. Therefore, oils generated from source rocks with lower contents of pyrolyzable organic matter can have 13C-enriched carbon isotopic compositions. In contrast to resin and asphaltene fractions (Figures 8(c) and 8(d)), Dδ13C values of saturated hydrocarbons do not display rigorous correlations with pyrolysis temperatures (Figure 8(a)), suggesting that thermal maturation may not be the dominant factor affecting the carbon isotopic systematics of retained saturated hydrocarbons. Instead, the negative correlations between HI and Dδ13C values of saturated hydrocarbons suggest that the relative proportions of hydrogen-rich, pyrolyzable organic matter may be an important factor controlling the carbon isotopic fractionation of saturated hydrocarbon. Data in this study and past studies have found that hydrogen-depleted fractions (e.g., asphaltene) can have more 13C-enriched carbon isotopic compositions compared with hydrogen-rich fractions (e.g., saturated hydrocarbons; Table 4) [42, 43]. This suggests that hydrogen-enriched fractions may have greater affinity to lighter carbon isotopes during the C–C bond cleavage in hydrocarbon generation. In addition to thermal maturation, the proportion of hydrogen-rich organic matter is also affected by organic facies of source rocks [44-47]. For example, in this study, type I source rock sample Wang56 has a higher HI value (756 mg/g·TOC) compared with the type II source rock sample Xin521 (267 mg/g·TOC), suggesting that they have deposited in different environments. Therefore, the negative correlation between HI and Dδ13C values of saturated hydrocarbons suggests that the carbon isotopic fractionation between nonpyrolyzed and retained pyrolyzed saturated hydrocarbons is likely influenced by the quantity of hydrogen-rich pyrolyzable organic matter.

The Dδ13C values of aromatic hydrocarbons do not display rigorous correlations with HI values (Figure 8(f)). This suggests that the quantity of hydrogen-rich pyrolyzable organic matter in source rock is not a key factor influencing the carbon isotopic fractionation between nonpyrolyzed and pyrolyzed retained aromatic hydrocarbons. In addition, Dδ13C values of resin and asphaltene fractions also exhibit negative correlations with HI values (Figures 8(g) and 8(h)). The R2 value for the correlation between Dδ13C values of resin and HI values (0.40) is similar to that for the correlation between Dδ13C values of resin and pyrolysis temperature (0.50; Figures 8(c) and 8(g)). Such features suggest that both the quantity of pyrolyzable organic matter and thermal maturation affect the carbon isotopic fractionation between nonpyrolyzed and pyrolyzed retained resin fractions. However, the R2 value for the correlation between Dδ13C values of asphaltene and HI values (0.51) is much smaller compared with that for the correlation between Dδ13C values of asphaltene and pyrolysis temperature (0.90; Figures 8(d) and 8(h)), suggesting that thermal maturation is likely the dominant factor affecting the carbon isotopic fractionation between nonpyrolyzed and pyrolyzed retained asphaltene fractions.

4.2. Influence of Maturation on Carbon Isotopic Fractionation Between Expelled and Retained Oil Fractions

Differences between δ13C ratios of expelled and retained individual organic fractions (saturates, aromatics, resins, and asphaltenes) generated by both samples Wang56 and Xin 521 are less than 2‰ during the whole range of pyrolysis temperatures (Figures 9(a) and 9(b)). This suggests that hydrocarbon generation/expulsion does not result in significant carbon isotopic fractionation, which is consistent with previous laboratory investigations [23-25], and supports that carbon isotopic analysis is a useful approach for oil-source correlation works [21, 22, 48]. Regardless of the minor carbon isotopic fractionation between expelled and retained oil fractions, there are obvious correlations between thermal maturation and the δ13C difference between expelled and retained oils (δ13Ce-r), especially in the experiments using the sample Wang56.

4.2.1. Type I Source Rock

In the experiment using the sample Wang56, the expelled individual oil fractions have lower δ13C ratios than the retained fractions at the preoil stage (275℃; Figure 9(a)). Oils expelled at this stage are mainly generated via the thermal cracking of preexisting bitumen in the source rock [10]. The C–C bonds in bitumen with lighter carbon isotopes are easier to be cleaved [25], which can result in lighter carbon isotopic compositions of expelled oils relative to retained oils. Thus, carbon isotopic depletion of expelled oil fractions relative to retained fractions observed in the experiments using the sample Wang56 is consistent with previous literature. At temperatures higher than 300℃ (main oil generation stage), thermal cracking of kerogen becomes the dominant process for oil generation [10]. The δ13Ce-r values of saturated hydrocarbons display a decreasing correlation with pyrolysis temperatures, suggesting that expelled saturated hydrocarbons gradually become more isotopically depleted relative to retained saturates during this stage (Figure 9(a)). Thus, the negative correlation between δ13Ce-r values of saturates and pyrolysis temperatures suggests that thermal maturation is the key factor controlling the retention of 13C isotopes of saturates and larger quantities of 13C isotopes are retained within source rocks rather than expelled at higher maturity levels. Both retained and expelled saturated hydrocarbons become isotopically heavier with the increase of thermal maturity level (Figures 6(a) and 6(b)). Thus, the preferential retention of 13C isotopes suggests that the expelled saturated hydrocarbons have weaker affinities to heavier carbon isotopes (13C) compared with the retained organic matter. Similarly, the δ13Ce-r values of aromatic fractions also exhibit a decreasing trend with the increase of pyrolysis temperatures of 300℃–400℃ (Figure 9(a)). This supports the fact that the affinity of retained aromatic hydrocarbons to heavier carbon isotopes becomes greater with the increase of thermal maturity levels. Retained organic matter generally has greater polarities compared with the expelled oils, especially saturated and aromatic hydrocarbons, and polarities of expelled oils tend to become weaker with the increase of thermal maturity levels during the main oil generation stage [1-3]. Therefore, carbon isotopic fractionation patterns of saturated and aromatic hydrocarbons are consistent with the physicochemical changes of expelled oils during the main oil generation stage.

The δ13Ce-r values of resin exhibit a decreasing trend between 325℃ and 375℃ (Figure 9(a)), which is similar to saturated and aromatic hydrocarbons. Whereas the δ13Ce-r values of resin exhibit increasing trends at the incipient oil generation (300℃, 325℃) and gas generation stages (375℃, 400℃). In contrast to saturated and aromatic hydrocarbons, the expelled resin is a highly polar fraction and generally has a greater affinity to heavier carbon isotopes, which has been proven by past field studies [39, 43]. Thus, the enhanced affinity to heavier carbon isotopes may have significantly affected carbon isotopic fractionation between expelled and retained resin fractions during the incipient oil generation and gas generation stages. This can result in positive correlations between δ13Ce-r values of resin and temperatures as observed in this study.

Carbon isotopic fractionation between expelled and retained asphaltene fractions exhibits distinctive patterns compared with saturates, aromatics, and resin fractions. The δ13Ce-r values of asphaltene display a positive correlation with pyrolysis temperatures from 300℃ to 375℃ (Figure 9(a)). This correlation suggests that increased quantities of heavier carbon isotopes are more readily to be expelled rather than retained within the source rock with the increase in pyrolysis temperature. Asphaltene is the most polar fraction within oils and has structures similar to kerogen; therefore, asphaltene has great affinity to heavier carbon isotopes compared with saturated, aromatic, and resin fractions [1-3]. In this study, positive correlations between δ13Ce-r values of asphaltene and temperatures suggest that the affinity of expelled asphaltene fraction to heavier carbon isotopes gradually becomes greater with the increase of thermal maturity level during the main oil generation stage. The δ13Ce-r value of asphaltene drops from 375℃ to 400℃, during which the major gas generation starts. With the gas generation via the thermal decomposition of kerogen and/or retained bitumen within the source rock, polarities of organic matter preserved in source rocks become greater due to the expulsion of hydrocarbon gases (e.g., methane, ethane, and propane). Therefore, the decrease in the δ13Ce-r value of asphaltene may have been attributed to the increased polarity of organic matter in source rocks, which have greater affinity to heavier carbon isotopes compared with the expelled asphaltene fraction.

4.2.2. Type II Source Rock

In the experiments using the sample Xin521, carbon isotopic fractionation patterns during the expulsion of aromatic and resin are similar to those of the experiments using the sample Wang56, as shown by similar correlations between δ13Ce-r values and pyrolysis temperatures in the main oil generation stage (300℃, 400℃; Figures 9(a) and 9(b)). The sample Xin521 belongs to the type II source rock according to pyrolysis data, yet the sample Wang56 belongs to the type I source rock (Figure 3). Therefore, organic matter type does not significantly affect the expulsion behavior of organic carbon isotopes of aromatic and resin fractions during the main oil generation stage.

In contrast, expulsion behaviors of organic carbon isotopes of saturated hydrocarbons and asphaltene generated by the sample Xin521 are different from those of the sample Wang56 during the main oil generation stage (Figure 9(b)). Stable carbon isotopic ratios of both retained and expelled saturated hydrocarbons do not exhibit significant variations with the increase of pyrolysis temperatures but fluctuate in narrow ranges (δ13C = −28.7‰ to −27.8‰ and δ13C = −28.9‰ to −28.2‰, respectively (Figures 6(c) and 6(d) and Table 5). Therefore, oil expulsion does not significantly affect the carbon isotopic systematics of saturated hydrocarbons generated by the sample Xin521. The δ13C values of retained asphaltene fractions exhibit a positive correlation with pyrolysis temperatures (Figure 6(c)). This can be attributed to the maturation-induced C–C bond cleavage because the cleavage of 12C–13C requires higher energy compared with the cleavage of 12C–12C [25]. Thus, oil fractions generated at higher maturity levels tend to have higher δ13C values. However, δ13C values of expelled asphaltene fractions do not exhibit systematic variations with the increase of pyrolysis temperatures (Figure 6(d)). Therefore, oil expulsion may affect the carbon isotopic ratios of asphaltene fractions in the expelled oils, yet its influence may not be systematic.

4.3. Influence of Oil Expulsion on Carbon Isotopic Systematics of Organic Fractions

Past studies have shown that oil expulsion is likely an important factor controlling the carbon isotopic systematics of oils, and such an impact is directly related with organic matter type [10, 26]. For example, authors found that the influence of oil expulsion on carbon isotopic compositions of oils expelled from type II source rocks is insignificant [26]. However, oils expelled from type III source rocks have different δ13C values compared with retained oils as type III source rocks generally contain higher proportions of vitrinite, which is characterized by abundant nanometer-scale pores. Thus, the carbon isotopic fractionation may be attributed to the selective adsorption of heavier carbon isotopes by porous organic matter [26].

In the experiments using the sample Wang56, δ13C values of saturated, aromatic, resin, and asphaltene fractions in expelled oils exhibit positive correlations with the ratios of expelled/retained individual fractions (Figure 10(a)). Similarly, δ13C values of retained fractions also display rigorous positive correlations with the ratios of expelled/retained individual fractions (Figure 10(b)). Such consistent carbon isotopic partitioning patterns suggest that oil expulsion is an important factor affecting the carbon isotopic systematics of oils generated/expelled by type I source rocks. Oil fractions generated/expelled at higher degrees of expulsion tend to be characterized by more 13C-enriched carbon isotopic compositions. In contrast, oil expulsion has minor impacts on the carbon isotopic systematics of expelled oil fractions generated by the sample Xin521 as suggested by their weak correlations (Figure 10(c)). The correlations between δ13C values of retained aromatic hydrocarbons, resin, and asphaltene fractions exhibit positive correlations with the ratios of expelled/retained individual fractions (Figure 10(d)). Therefore, oil expulsion seems not to be able to significantly affect carbon isotopic features of oils expelled from type II source rocks [26]. However, oil expulsion may have affected the carbon isotopic systematics of retained aromatic, resin, and asphaltene fractions generated by this type of source rock, and these fractions generated from source rocks with a higher degree of oil expulsion tend to be more enriched in heavier carbon isotopes.

The positive correlations between δ13C values of either retained or expelled oil fractions and the ratios of expelled/retained fractions can be explained as a result of thermally induced C–C bond cleavage. The cleavage of 13C–12C bonds requires higher energy compared with the cleavage of 12C–12C bonds [25]. Thus, oils generated/expelled at higher maturity levels are characterized by more 13C-enriched carbon isotopic compositions, which is also observed in this study (Figures 6(a) and 6(b)). Higher degree of oil expulsion generally occurs at higher maturity levels; thus, oil fractions generated/expelled from source rocks with a higher degree of expulsion efficiency tend to have more 13C-enriched carbon isotopic compositions. As for the organic fractions with δ13C values not correlating the ratios of expelled/retained fractions, it is likely that carbon isotopic variations of these fractions during oil generation/expulsion are not systematic. Outcomes of this study support that there is negligible carbon isotopic fractionation during oil expulsion, and this is quite different from those in type III source rocks. Previous pyrolysis experimental studies revealed that oil expulsion from type III source rocks can result in carbon isotopic fractionation up to 4‰, and such a large degree of isotopic fractionation may contribute to biased insights when using their δ13C values for fingerprinting hydrocarbon source and/or characterizing maturity levels [26]. Such a scenario is likely caused by the presence of abundant organic pores in vitrinite, which is a type of dominant organic material in the type III source rocks, and those nanoscale organic pores have the potential to selectively adsorb heavier carbon isotopes during oil expulsion [26]. The minor impact of oil expulsion on carbon isotopic behavior also suggests that the studied samples mainly contain oil-prone organic matter instead of gas-prone organic matter (e.g., vitrinite).

4.4. Gas Mixing During the Main Oil Generation/Expulsion Stage

Mixing of hydrocarbon gases derived from different sources and/or thermal maturity levels is a common process in petroliferous basins [41, 45] and can occur during artificial pyrolysis experiments [2, 47]. The binary plot of δ13Cn versus reciprocal of proportions of Cn gas is a typical diagram to identify the mixing nature of the corresponding gas species [48]. In this study, there are linear correlations between δ13C ratios and reciprocals of gas proportions in the experiments using both samples Wang56 and Xin521 (Figures 11(a) and 11(b)). These correlations suggest the potential occurrence of gas mixing under semiclosed conditions.

In the experiments using the sample Wang56, δ13C values of methane and ethane do not display acceptable regressions with the reciprocals of gas proportions, whereas δ13C values of propane exhibit a negative correlation with 1/C3 (Figure 11(a)). Such patterns suggest that binary mixing of propane occurs during the main stage of oil generation (low propane contents and low δ13C values vs high propane contents and high δ13C values). The proportion of propane rises with the increase of pyrolysis temperatures (Table 5 and Figure 7(a)); thus, the content of propane is directly affected by thermal maturity levels. The carbon isotopic pattern for propane is mainly attributed to thermal maturation. Hydrocarbon gases generated at higher maturity levels are more enriched in heavier carbon isotopes compared with those generated at lower maturity levels as kerogen becomes isotopically heavier with the increase of thermal maturity levels [2, 47]. Unlike propane generated during the experiments, regressions of δ13C1–1/C1 and δ13C2–1/C2 values are not as rigorous as that for δ13C3–1/C3 (Figure 11(a)). This feature suggests that the binary mixing of these two gas species may have been weak during the main stage of oil generation.

In the experiments using the sample Xin521, δ13C values of methane, ethane, and propane display acceptable regressions with the reciprocals of gas proportions (Figure 11(b)). This pattern suggests that the binary mixing of these gas species occurs during the main stage of oil generation. The δ13C values of methane display a negative correlation with 1/C1 values (Figure 11(b)), and this pattern indicates that the two components are characterized by low methane contents/lower δ13C and high methane contents/higher δ13C, respectively. Similar to propane generated by the sample Wang56, such a methane mixing pattern may also be attributed to the mixing of gases generated from different maturity levels. In contrast, δ13C2 and δ13C3 ratios positively correlate 1/C2 and 1/C3 values, respectively (Figure 11(b)). This suggests that the mechanism of binary mixing of these gas species may be different from that for methane. Hydrocarbon gases generated via the thermal decomposition of kerogen can exhibit negative correlations in the plot of δ13Cn–1/Cn as kerogen becomes isotopically heavier with the increase in the amount of gas generated [47-51]. In addition to the degradation of kerogen, light hydrocarbon gases can be generated via the thermal decomposition of preexisting bitumen or oxygenated compounds formed at lower maturity levels. Gases generated via this process can have lower δ13C values compared with gases decomposed from kerogen contemporaneously [52]. Thermal degradation of preexisting organics is affected by oil expulsion/retention process as oils retained within source rocks are significant precursors for gas generation [1-3]. Therefore, the mixing of gases generated from these two processes is a potential mechanism responsible for the positive correlations in the plots of δ13C2–1/C2 and δ13C3–1/C3. Indeed, thermal evolution patterns of carbon isotopic ratios also support that mixing of gases with different genesis occurred during experiments. For example, carbon isotopic ratios of methane show an initial decrease between 300℃ and 350℃, followed by a subsequent increase (Figure 7(c)). In the early stages, gas primarily forms through the C–C cleavage of compounds such as carboxyl and carbonyl, which typically have heavier δ13C values [1-3]. As thermal maturity levels increase, larger quantities of kerogen-derived gas are generated, characterized by lighter δ13C values [2]. This could lead to the initial decrease in δ13C values from 300°C to 350°C [3]. Subsequently, the increase in δ13C values could be primarily attributed to the larger amount of gas derived from the thermal decomposition of kerogen and oil. The δ13C values of these two fractions become isotopically heavier as maturity levels rise [3]. In contrast to experiments using the sample Xin521, carbon isotopic data of ethane and propane generated from the experiments using the sample Wang56 do not display similar correlations in the plots of δ13Cn–1/Cn (Figure 11(a)). This suggests that mixing of gases generated via different process may have not occurred. This may be attributed to the higher hydrocarbon generation potential of kerogen in the sample Wang56 (HI = 756 mg/g·TOC) compared with the sample Xin521 (HI = 267 mg/g·TOC), and kerogen decomposition is likely the dominant mechanism for the gas generation (e.g., ethane and propane) from the sample Wang56.

Carbon isotopic reversal is a common feature for hydrocarbon gases (δ13Cmethane > δ13Cethane and/or δ13Cethane > δ13Cpropane), which has been documented in field-based studies worldwide [46, 47, 49, 51-53]. Previous studies have shown that several processes can result in the carbon isotopic reversal of gases, including mixing of gases of various genesis [46, 53-56], thermochemical sulfate reduction [57], redox reaction [48], and gas adsorption/desorption/diffusion [58, 59]. In experiments using the sample Wang56, carbon isotopic data of methane, ethane, and propane do not display significant isotopic reversals during the whole course of study (Figure 11(c)). In experiments using the sample Xin521, carbon isotopic reversals are also not documented in the data derived from the experiments conducted at 325℃–400℃ (Figure 11(d)). However, partial carbon isotopic reversal is observed in the experiment conducted at 300℃ (Figure 11(d); δ13Cethane > δ13Cpropane). Mixing of ethane and propane occurred in the experiments using the sample Xin521, which thus may be responsible for the occurrence of partial carbon isotopic reversal [46, 54, 60].

4.5. Geological Implications of Hydrous Pyrolysis Data

Carbon isotopic compositions of individual organic fractions of oils/organic extracts are widely utilized indicators for oil-source correlations [43, 61-67]. Regardless of the carbon isotopic fractionation patterns between expelled and retained oils observed in this study, the utility of δ13C values for oil-source correlation works is not compromised because the degrees of fractionation are generally minor (<3‰). The binary plot of δ13C values of saturated hydrocarbons and aromatic hydrocarbons is commonly utilized for differentiating oils derived from terrigenous and marine source rocks [61]. Carbon isotopic data of both retained and expelled oils obtained in this study locate in the region of “terrigenous-derived oils” (Figure 12(a)). This is consistent with the fact that both the source rock samples Wang56 and Xin521 were deposited in saline lacustrine settings [62-64]. Thus, this provides robust support for the application of the δ13Caro–δ13Csat genetic diagram for inferring the depositional environments of saline lacustrine source rocks. Besides, oils generated by hydrous pyrolysis are situated in the vicinity of retained oils within the respective nonpyrolyzed source rock samples in the plot of δ13Caro–δ13Csat, and data from two series of experiments do not overlap with each other (Figure 12). Such patterns support that this plot is feasible for classifying oils generated by different source rocks.

Molecular and stable isotopic compositions of light gaseous hydrocarbons (such as methane, ethane, and propane) are usually utilized for gas-source correlations [55, 57, 66]. Several binary genetic empirical diagrams for determining the gas genesis have been proposed in previous studies. Among them, the cross plot of molecular proportions of C1/(C2 + C3) versus δ13C1 is one of the most significant plots for identifying gases generated by different processes (e.g., microbial activity and thermal maturation of organic matter and abiotic genesis; Bernard et al., 1978). This plot has been recently updated by researchers using a large worldwide data set [67]. Gas samples from both series of experiments are all located in the region of “thermogenic gas,” which is consistent with the fact that hydrocarbon gases are dominantly derived from thermal decomposition of organic precursors in the source rock. Authors suggested that maturation-controlled molecular and isotopic variations of gases should follow a “maturity curve” as shown in (Figure 12(b)) [67]. However, gas isotope data obtained in this study do not strictly follow such path (Figure 12(b)). Therefore, experimental data obtained in this study suggest that although the plot of C1/(C2 + C3) versus δ13C1 can be used to effectively identify genesis of hydrocarbon gases generated from type I and II saline lacustrine source rocks, cautions are still required to determine inter-thermal maturity levels of gases.

Two series of semiclosed hydrous pyrolysis experiments were conducted on type I and II saline lacustrine source rocks taken from the Jianghan Basin, central China, in order to investigate the carbon isotopic behaviors of organic fractions during oil generation/expulsion. Minor carbon isotopic fractionation (<3‰) between pyrolyzed and nonpyrolyzed retained oil fractions occurs during oil generation, which can be affected by thermal maturation and quantities of hydrogen-rich pyrolyzable organic matter in the source rock. An increase in the thermal maturation levels and/or a decrease in the amounts of pyrolyzable organic matter in source rock can result in the preferential retention of 12C over 13C isotopes. Carbon isotopic fractionation between expelled and retained oil fractions is also minor (<2‰) during the main oil generation/expulsion stage. The carbon isotopic fractionation between expelled and retained oil fractions is likely mainly controlled by the relative affinity of 13C/12C isotopes to the individual oil fractions. Oil expulsion has significant impacts on carbon isotopic compositions of retained and expelled oil fractions generated by type I saline lacustrine source rocks, yet its impact is minor on carbon isotopic compositions of oil fractions generated by type II source rocks. Oil generation/expulsion is associated with gas generation, and mixing of gases with different maturity levels and/or generated via different processes likely occurs during the main oil generation/expulsion stage. Partial carbon isotopic reversal (δ13C213C3) was documented in the experiment using the type II lacustrine source rock at the incipient oil generation stage (300℃), which may have been caused by the mixing of gases generated by different precursors (e.g., kerogen and preexisting bitumen). Outcomes of this study demonstrate that carbon isotopic analysis of individual oil fractions can be an effective approach for inferring source rocks of oils and determining the genesis of associated hydrocarbon gases.

Data will be available on request.

The authors declare that there is no conflict of interest.

This work was supported by the Joint Fund of National Natural Science Foundation of China (No. U20B6001). We would like to express special thanks to Section Editor Xiangyang Xie and Associate Editor Zheng Zhou for handling our manuscript and three anonymous reviewers for their constructive comments and suggestions.

Exclusive Licensee GeoScienceWorld. Distributed under a Creative Commons Attribution License (CC BY 4.0).