Hydrocarbon-generating characteristics of Xujiahe coal-bearing source rocks in the continuous sedimentary environment of the Southwest Sichuan

Tao Yang; Kun Xu; Jungang Lu; Bai Liu; Jianbo Liao

doi:10.1515/geo-2022-0754

Article Open Access

Hydrocarbon-generating characteristics of Xujiahe coal-bearing source rocks in the continuous sedimentary environment of the Southwest Sichuan

Tao Yang , Kun Xu , Jungang Lu , Bai Liu and Jianbo Liao

Published/Copyright: February 10, 2025

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From the journal Open Geosciences Volume 17 Issue 1

Abstract

The characteristics of coal-bearing source rocks in the continuous sedimentary environment of the Xujiahe Formation are discussed in the Southwest Sichuan through the thermal simulation and organic geochemical experiments, and the quantitative relationships are determined between carbon isotopes, maturity, and distinct organic matter (OM) types. The results show that the source rocks of the first member and third member of the Xujiahe Formation are mainly formed in the transitional sedimentary environment, while the source rocks of the fifth member of the Xujiahe Formation are primarily developed in the continental sedimentary environment, controlled by the continuous sedimentary environment of the Xujiahe Formation. The vitrinite and inertinite content increases with the liptinite content decreasing from the first member to the fifth member of the Xujiahe Formation. Moreover, the OM of the source rocks is typed II₂ in the first member of the Xujiahe Formation, with the III OM in the third member and fifth member of the Xujiahe Formation. Additionally, the quantitative relationship is identified between methane carbon isotopes and maturities of the humic and mixed gas. The quantitative relationship is presented at δ ¹³C₁ = 12.21 ln Ro – 39.72 (humic gas) and δ ¹³C₁ = 8.07 ln Ro – 38.3 (mixed gas). The ethane carbon isotopes range from −24.93 to −21.07‰ in the first member and the third member of the Xujiahe source rocks, and are between −29.31 and −26.78‰ in the fifth member of the Xujiahe source rocks, which can effectively distinguish the OM type by ethane carbon isotopes. The emergence of new recombined heavy hydrocarbons leads to the carbon isotope inversion of heavy hydrocarbon in the high thermal evolution stage. This study has contributed to the hydrocarbon-generating characteristics of source rocks in the continuous sedimentary environment and the fine gas source correlation.

Keywords: hydrocarbon-generating characteristics; coal-bearing source rocks; sedimentary environments; carbon isotope; Xujiahe Formation; Sichuan Basin

1 Introduction

The hydrocarbon-generating process in the source rocks is the conversion of organic matter (OM) to oil and gas, and the main controlling factor of hydrocarbon-generating characteristics is the OM [1]. The correlation between hydrocarbon-generating characteristics of source rocks and parent material source, sedimentary environment, and thermal maturity has been well studied over recent decades [2–5]. Previous studies have also proved that the OM components are also heterogeneous in the source rocks [1], which varies greatly in different sedimentary environments [6,7]. However, the effect of this heterogeneity on the difference of hydrocarbon-generating materials from source rocks is still uncertain. Especially, the relationship between OM components and hydrocarbon-generating characteristics still lack systematic research in the continuous sedimentary environment, which is bound to further affect the oil–gas source correlation.

The neural network method and cluster analysis are used to explore mineral enrichment [8–11]. However, the thermal simulation experiments of open and closed systems are mainly used directly in the study of hydrocarbon-generating characteristics, evolution, and carbon isotopes of the coal-bearing gas [12–15]. These studies can provide a basis for predicting the amount and characteristics of coal-bearing gas. However, the coal-bearing source rocks with distinct OM components, developed in different sedimentary environments, would inevitably lead to different hydrocarbon-generating characteristics [12,16]. The quantitative study of source rock products is vitally important for fine hydrocarbon source correlation in the continuous sedimentary environment [15,17].

The Xujiahe source rocks were developed in the open and semi-open lake basins, which is continuous in the sedimentary environment [18,19]. The source rocks of the first member of the Xujiahe Formation (T₃x¹) were developed in the transitional sedimentary environment [20,21], and the source rocks of the fifth member of the Xujiahe Formation (T₃x⁵) were formed in the continental sedimentary environment [19,20]. However, some scholars believe that the sedimentary environment of source rocks in the third member of the Xujiahe Formation (T₃x³) was developed in the transitional sedimentary environment and the sea water had not receded from the lake basin, while others believe that the T₃x³ source rocks were formed in the continental sedimentary environment and the transitional sedimentary environment ended in the second member of the Xujiahe Formation [21,22]. In short, the sedimentary environment of Xujiahe source rocks is still controversial, and some deficiencies exist in the quantitative relationship between hydrocarbon-generating characteristics and the sedimentary environment.

This article selects the T₃x¹, T₃x³, and T₃x⁵ source rocks that are developed in the continuous sedimentary environments, discusses the forming environment of Xujiahe coal-bearing source rocks, and identifies the relationship between carbon isotope, maturity, and OM, based on thermal simulation and organic geochemical experiments. This study has a guiding significance for the hydrocarbon-generating characteristics and the fine gas–source correlation in the continuous sedimentary environment.

2 Geological setting

The Sichuan Basin is a sedimentary basin, located on one side of the Yangtze Craton basin, which is a secondary tectonic unit of the Craton basin, surrounded by Longmen Mountains, Micang Mountains, Daba Mountains, Qiyue Mountains, and Dalou Mountains (Figure 1a). The continental strata of the Upper Triassic Xujiahe Formation are distributed as the boundary, covering an area of about 180,000 square kilometers. Moreover, the Sichuan Basin is wholly diamond-shaped and is divided into the west depression belt, the north depression belt, the east high-steep structural belt, the southeast low-steep structural belt, the south gentle structural belt, and the central bulge according to the structural morphology [23–25]. The study area is located in the Southwest Sichuan, belonging to the west depression belt (Figure 1a).

Figure 1

Outcrop location and lithology column: (a) location of the study area and (b) schematic diagram of Xujiahe lithology.

The development of Xujiahe coal-bearing source rocks is controlled by the lacustrine basin, Paleo-Tethys Ocean, and tectonic activities in the Sichuan Basin, in which the sedimentary environment is inheritable [26]. The Xujiahe Formation is divided into the first, second, third, fourth, fifth, and sixth members (henceforth referred to as T₃x¹, T₃x², T₃x³, T₃x⁴, T₃x⁵, and T₃x⁶, respectively) (Figure 1b). Xujiahe source rocks are developed in the T₃x¹, T₃x³, and T₃x⁵. The T₃x¹ source rocks were connected with the Paleo-Tethys Ocean, leading to the increase of liptinites in source rocks [18]. Moreover, much literature suggested that the Paleo-Tethys Ocean was intermittently connected with the Xujiahe water, resulting in higher liptinites of T₃x³ source rocks [24,27]. As the Longmenshan structure was further uplifted during the T₃x⁵ sedimentary period, the OM of T₃x⁵ source rocks was mainly derived from terrestrial inputs with poor OM types and higher humic components [18,28]. In short, the distinct liptinites of Xujiahe coal-bearing source rocks would inevitably lead to the difference in hydrocarbon-generating products owing to different sedimentary environments.

3 Samples and experimental methods

The simulation experiment adopted the Xujiahe low-matured source rocks that were situated in Guan County, Dujiangyan City, Chengdu City, in the Southwest Sichuan (Figures 1a and 2a). As is presented in Table 1, OM maturities were 0.7, 0.68, and 0.65% in the T₃x¹, T₃x³, and T₃x⁵ source rocks, respectively. The total organic carbon content of T₃x¹, T₃x³, and T₃x⁵ source rocks were 1.98, 2.42, and 2.46%, respectively. The OMs were typed III in the T₃x³ and T₃x⁵ source rocks, and the T₃x¹ OM was typed II₂ with higher lipitinites and hydrogen index (90.1 mg/g).

Figure 2

Outcrop location (a) and hydrocarbon-generating thermal simulator (b).

Table 1

Geochemical characteristics of low-matured source rocks in the Xujiahe outcrop

Lithology	Formation	TOC (%)	T _max (°C)	S₁ (mg/g)	S₂ (mg/g)	HI (mg/g)	Ro (%)	Kerogen maceral content (%)				TI values	Type
Lithology	Formation	TOC (%)	T _max (°C)	S₁ (mg/g)	S₂ (mg/g)	HI (mg/g)	Ro (%)	Liptinite	Exinite	Vitrinite	Inertinite	TI values	Type
Mudstone	T₃x¹	1.98	439	0.20	1.78	90.10	0.70	55	—	16	29	14	II₂
Mudstone	T₃x³	2.42	435	0.24	2.17	89.67	0.68	33	—	23	44	−28.25	III
Mudstone	T₃x⁵	2.46	435	0.16	1.89	76.83	0.65	21	—	31	48	−50.25	III

Xujiahe source rocks were crushed to 40–60 mesh before the organic geochemical experiment. The powdered samples were then put in a Soxhlet extractor and extracted with chloroform asphalt “A” for 3 days. The final extract was concentrated in a rotary evaporator and dissolved in n-hexane, and the supernatant was obtained after centrifuging in a centrifuge for 5 min and separated on a silica gel/alumina (3:2) column. Then, 30 mL n-hexane was slowly dropped into the column to obtain the transparent liquid as the saturated hydrocarbon. The saturated hydrocarbons were vaporized at the Agilent 7000A GC-mass spectrometer inlet and carried to the column by the carrier gas. Then, the saturated hydrocarbon of each component was separated in the chromatographic column according to its adsorption capacity, and its content was determined by a hydrogen flame detector. Finally, each component entered the mass spectrometer and was bombarded with an ion source to obtain the content of the corresponding biomarker compound, and the ratio of the geochemical parameters of each sample was calculated according to the peak area in the qualitative analysis software.

The collected samples were powdered into 80 mesh and extracted by chloroform asphalt “A” for 48 h until source rocks became colorless, and then dried in the drying box for 12 h before the thermal simulation. The quantitative characteristics of hydrocarbons in the Xujiahe source rocks were measured by the SRM-II hydrocarbon-generation and expulsion pyrolysis simulator under a closed system at Southwest Petroleum University (Figure 2b). The formation pressure was set to 60 MPa, and the source rock mass ranged from 10 to 300 mg in the equipment. The source rocks were heated from 310 to 610°C at 20°C/h. A total of 11 test points were measured at 310, 340, 370, 400, 430, 460, 490, 520, 550, 580, and 610°C. The Agilent 7000A GC-MS was used to measure the gas components and isotopes at different temperature points, and the volumes of different components were calculated by the ideal gas equation at different temperature points.

4 Results

4.1 Biomarker characteristics of source rocks

The distribution of n-alkanes is complete in the T₃x¹ source rocks, with nC₁₅ as the main peak and Pr/Ph less than 1. Pentacyclic triterpenes are the main terpenes with main C₃₀ hopanes, the high gammacerane content and Ts/Tm close to 1, and the C₂₃ content is high in tricyclic terpenes. The main steranes are regular steranes with C₂₇ and C₂₉, with a low content of diasteranes (Figure 3).

Figure 3

Biomarker compounds in Xujiahe source rocks: (a) T₃x¹, (b) T₃x³, (c) T₃x⁵. (A: 4α-methyl, 24-ethylcholestane; B: 4β-methyl, 24-ethylcholestane; 1: 4α, 23S, 24S-dinosteranes; 2: 4α, 23S, 24R-dinosteranes; 3: 4α, 23R, 24R-dinosteranes; 4: 4α, 23R, 24S-dinosteranes; 5: 4β, 23S, 24S-dinosteranes; 6: 4β, 23S, 24R-dinosteranes; 7: 4β, 23R, 24R-dinosteranes; 8: 4β, 23R, 24S-dinosteranes).

The T₃x³ source rocks have a complete distribution of n-alkanes, with nC₁₅ as the main peak and Pr/Ph close to 1. Pentacyclic triterpenes are characterized by main C₃₀ hopanes, the medium gammacerane content and Ts/Tm close to 1, and the C₂₀ and C₂₁ content is high in tricyclic terpenes. The regular steranes are featured in C₂₇ and C₂₉, with the medium content of diasteranes (Figure 3).

The T₃x⁵ source rocks have a complete distribution of n-alkanes, with nC₁₅ as the main peak and Pr/Ph larger than 1. Pentacyclic triterpenes are characterized by the main C₃₀ hopanes, the low gammacerane content and Ts/Tm less than 1, and the C₂₁ content is high in tricyclic terpenes. The regular steranes are featured in low C₂₇ and high C₂₉, with a high content of diasteranes (Figure 3).

Dinosterane is also known as 4,23,24-trimethyl cholestane. Dinosterane peaks after C₂₉ steranes, with strong M/Z 213 fragments with 8 isomers [29]. Figure 3 shows that the T₃x¹ and T₃x³ source rocks contain high dinosteranes, whereas the T₃x⁵ source rocks do not develop dinosteranes.

4.2 Carbon isotope characteristics of gaseous hydrocarbons

The carbon isotope is an important index of gas source and gas genesis [2]. The content of various alkane gases is too low to be detected in the early and late hydrocarbon-generating stages, resulting in the loss of corresponding carbon isotopes (Table 2). In general, carbon isotope values of methane, ethane, and propane (δ ¹³C₁, δ ¹³C₂ and δ ¹³C₃) range from −41.89 to −28.57‰, −29.31 to −26.78‰, and −26.89 to −25.37‰, respectively, in the T₃x¹ gas (Table 2). The T₃x³ gas carbon isotopes include δ ¹³C₁ (−41.23 to −26.25‰), δ ¹³C₂ (−24.93 to −22.29‰), and δ ¹³C₃ (−23.78 to −21.31‰) (Table 2). The δ ¹³C₁, δ ¹³C₂, and δ ¹³C₃ values of T₃x⁵ gas range from −40.11 to −24.13‰, −24.67 to −21.07‰, and −23.16 to −22.22‰, respectively (Table 2). In short, carbon isotopes of T₃x¹ gas are lighter than those of T₃x³ and T₃x⁵ gases and carbon isotopes of T₃x⁵ gas are heavier than those of T₃x³ gas.

Table 2

Geochemical characteristics of the Xujiahe simulated gas

Formation	Temperature (°C)	δ ¹³C (‰)
Formation	Temperature (°C)	CH₄	C₂H₆	C₃H₈
T₃x¹	310	—	—	—
	340	−39.29	—	—
	370	−40.67	—	—
	400	−41.89	—	—
	430	−37.57	−29.31	−26.89
	460	−34.81	−28.27	−26.45
	490	−34.03	−27.36	−25.86
	520	−33.16	−26.78	−25.37
	550	−32.12	—	—
	580	−30.69	—	—
	610	−28.57	—	—
T₃x³	310	—	—	—
	340	−39.75	—	—
	370	−40.58	—	—
	400	−41.23	—	—
	430	−37.33	−24.93	−23.78
	460	−35.35	−24.32	−22.85
	490	−33.83	−23.52	−21.79
	520	−32.47	−22.49	−21.31
	550	−30.56	—	—
	580	−28.66	—	—
	610	−26.25	—	—
T₃x⁵	310	—	—	—
	340	−38.7	—	—
	370	−39.62	—	—
	400	−40.11	—	—
	430	−37.77	−24.67	−23.16
	460	−35.05	−24.1	−22.63
	490	−33.87	−22.87	−21.46
	520	−32.14	−21.07	−21.22
	550	−29.07	—	—
	580	−27.14	—	—
	610	−24.13	—	—

The carbon isotopes of different alkane gases show the carbon isotope characteristics of organic gas with a positive carbon number series. The carbon isotopes of gas hydrocarbon products show different distribution patterns in Xujiahe source rocks (Figure 4a–c). The δ ¹³C₁ values decrease first and then increase with increased maturity. In contrast, the δ ¹³C₂ and δ ¹³C₃ values increase with maturity, and the increased ratio of the δ ¹³C₂ value is larger than that of the δ ¹³C₃ values (Table 2).

Figure 4

Carbon isotope curves of Xujiahe gas hydrocarbons and organic matter type distribution: (a) T₃x¹, (b) T₃x³, (c) T₃x⁵, and (d) the distribution of δ ¹³C₂ and organic matter type.

5 Discussion

5.1 Sedimentary environments of source rocks

The identification of the Xujiahe continuous sedimentary environments is closely related to the quantitative hydrocarbon-generating characteristics, and biomarkers are commonly used in the study of the evolution of transitional, continental, and marine sedimentary environments [30]. The ratio of pristane (Pr) to phytane (Ph) in alkanes is often used to reflect the sedimentary environments of source rocks. The Pr/Ph less than 1 reflects in the marine sedimentary environment, and the Pr/Ph more than 2 reflects in the swamp sedimentary environment [1]. The gammacerane in pentacyclic triterpenes is mainly derived from tetra-hymenium alcohol, which mainly represents the saline sedimentary environment [31]. Moreover, C₂₁ and C₂₃ tricyclic terpenes are also used to characterize the environment. C₂₁ tricyclic terpenes are mainly presented in freshwater sediments and C₂₃ tricyclic terpenes in saline sediments [30,32,33]. Moreover, dinosteranes are biomarkers in the marine environment [34,35].

The Pr/Ph values lower than 0.8 in the T₃x¹ source rocks indicate that T₃x¹ source rocks are characterized by marine sediments with high reductive water. The Pr/Ph values higher than 1 in the T₃x³ and T₃x⁵ source rocks indicate that the water is in a weak oxidated environment. The gammacerane content decreases from T₃x¹ to T₃x⁵ source rocks, suggesting the body salinity decreases.

Dinosteranes developed when the Paleo-Tethys Ocean was connected to the T₃x¹ and T₃x³ water, while dinosteranes did not develop in the T₃x⁵ source rocks during the stable period of continental deposition, which also proves that algae substances were abundant during the T₃x¹ and T₃x³ sedimentary period (Figure 3) [7].

Briefly, the T₃x¹ and T₃x³ source rocks are mainly characterized by the transitional sedimentary environment, and the T₃x⁵ source rocks are primarily featured in the continental sedimentary environment. During the sedimentary period from T₃x¹ to T₃x⁵, water becomes less salty, the oxygen content gradually increases, the reducibility decreases, and the content of algae substances decreases, leading to II₂ OM of T₃x¹ source rocks and III OM of T₃x³ and T₃x⁵ source rocks.

5.2 Carbon isotopes of alkane gas

The Xujiahe source rocks have a high content of soluble OM owing to the transformation of terrestrial OM by bacteria and algae, and the strong adsorb ability of coal-bearing OM and aromatic-rich products produced by itself makes these petroleum-like substances remain more in coal-bearing source rocks. When the coal-bearing source rocks experience higher thermal evolution, these substances can be used as high-quality parent material for cracking gas, leading to the decrease in gas carbon isotope values. Section 5.1 points out that the different sedimentary environments of Xujiahe source rocks lead to distinct bacteria and algae content. Moreover, the decrease in bacterial and algal substances from T₃x¹ to T₃x⁵ source rocks leads to an increase in carbon isotope values of alkane gas produced by source rocks (Table 2).

5.2.1 Carbon isotopes of ethane and OM types

The δ ¹³C₂ values become heavier with increased maturity, whereas this linear change is less sensitive to maturity (Figure 4a–c). Previous scholars often used δ ¹³C₂ values to identify kerogen types. The kerogen of oil-type gas is lighter, while the kerogen of humic gas is heavier [38,39]. According to the results of the hydrocarbon-generating thermal simulation, the δ ¹³C₂ values correspond to the type of source rocks. The δ ¹³C₂ values produced by T₃x¹ mixed source rocks are relatively light, whereas T₃x³ and T₃x⁵ humic source rocks produce relatively heavier δ ¹³C₂ values (Table 2).

The amount of hydrocarbon produced by source rocks is not enough to detect the ethane content in the early stage of low maturity and the early stage of maturity, based on the limitation of the hydrocarbon-generating thermal simulation experiment. Even if the corresponding ethane content can be detected, the accuracy of the experimental analysis results will be affected due to the low ethane content. The δ ¹³C₂ distribution law cannot be judged due to their relatively low ethane content in the over-matured stage. Moreover, ethane will be decomposed into methane in the over-matured stage with a low ethane content, resulting in a large δ ¹³C₂ measurement error. Therefore, the δ ¹³C₂ content in the late maturity to the high maturity stage can accurately reflect the carbon isotope distribution characteristics of source rocks in the thermal simulation experiment.

The δ ¹³C₂ values of T₃x¹, T₃x³, and T₃x⁵ source rocks differ significantly in the late maturity to the high maturity stage, according to the experimental results. The δ ¹³C₂ values of T₃x³ and T₃x⁵ source rocks range from −24.93 to −21.07‰, whereas δ ¹³C₂ values of T₃x¹ source rocks are between −29.31 and −26.78‰ (Table 2 and Figure 4d). Furthermore, the δ ¹³C₂ values range from −24.93 to −21.07‰ when the OM is typed III, ranging from −29.31 to −26.78‰ in the II₂ OM. Therefore, the parent material types of natural gas can be distinguished according to the δ ¹³C₂ values of natural gas, which lays a foundation for the comparative study of gas sources.

5.2.2 Quantitative relationship between methane carbon isotopes and maturity

The δ ¹³C₁ values from mixed and humic gases decrease at 340–400°C with the lowest values ranging from −41.89 to −40.11‰ (Table 2). Moreover, it cannot determine trends of ethane and propane carbon isotopes due to the low content of ethane and propane gases (Table 2). The decrease of δ ¹³C₁ values in the early maturity stage of source rocks has been found by many scholars in marine source rocks [2,40]. Furthermore, the δ ¹³C₁ values decrease first and then increase under closed, semi-closed, or open systems [3,12]. He et al. [41] believed that the essential reason for the early decrease of δ ¹³C₂ content was that the early heteroatoms of kerogen were more likely to enrich δ ¹²C content and fell off more easily, resulting in the high δ ¹²C and low δ ¹³C values. Tang et al. [42] and Zhang et al. [43] proposed that the difference between δ ¹²C and low δ ¹³C caused the δ ¹³C₁ anomalies by n-octadecane thermal simulation experiments. Briefly, it is common for the δ ¹³C₁ content to decrease in the early-matured stage owing to the complex and heterogeneous kerogen structure and differential fractionation of carbon isotopes. It can be deduced that ethane and propane should have a similar distribution, but the precision of the instrument temporarily hindered the study. Therefore, the δ ¹³C₁ variation is universal in the Xujiahe source rocks.

The δ ¹³C₁ content has a certain linear relationship with the maturity, which can be used to reflect the thermal maturity stage of natural gas formation and judge gas source. Moreover, previous scholars established the empirical formulas for humic gas, oil-type gas, and mixed gas based on the δ ¹³C₁ statistical data in the different basins [44,45]. However, the regression equation of δ ¹³C and Ro has some errors when determining the gas source accurately owing to the OM components, thermal evolution degree, and limited statistical data of source rocks. Therefore, the regression equation of δ ¹³C and Ro was established in the mixed and humic gases by thermal simulation experiment (Figure 5) as follows:

δ 13 C 1 = 11.21 ln Ro – 39.72 ( R 2 = 0.97 ) ( humic gas )

δ 13 C 1 = 8.07 ln Ro – 38.3 ( R 2 = 0.95 ) ( mixed gas )

Figure 5

Relationship between δ ¹³C₁ and Ro in different types of natural gas: (a) mixed gas and (b) humic gas.

The results show that the fitting coefficients of the linear equation of mixed and humic gases are high in the matured to over-matured stage, indicating the validity of the regression equation. However, some errors occur in judging the thermal maturity at the early-matured stage owing to the complex origin of early abnormal δ ¹³C₁.

5.2.3 Carbon isotope inversion

The natural gas δ ¹³C₂ and δ ¹³C₃ values of T₃x⁵ source rocks show slight inversion at 500°C in the late stage of high maturity (Table 2). The carbon isotopes of organic gas show positive carbon isotope distribution, meaning that the carbon isotopes of alkanes become heavier with increased carbon numbers [1]. However, carbon isotope anomalies (e.g., negative carbon isotope series and carbon isotope inversion), have been found in the high-matured to over-matured stage of natural gas globally, which shows that the carbon isotope inversion of natural gas is not an experimental error, but exists in the evolution of source rocks [40,46]. He et al. [47] and Gao et al. [3] also found ethane carbon isotope inversion in thermal simulation experiments. Many scholars found carbon isotope inversion of natural gas in the study of humic and oil-type gases [38,48,49]. All these studies indicate that carbon isotope inversion is a normal phenomenon at the high-matured or over-matured stage.

The causes of carbon isotope inversion mainly include the formation of alkane gases with reconstituted light carbon isotopes and the mixing of primary kerogen and retained hydrocarbon-cracking gas [40]. It is questionable that the alkane gas that recombines hydrogen and carbon dioxide to form light carbon isotopes causes the carbon isotope inversion of heavy hydrocarbon gas, because of sufficient catalysts. More evidence are needed on whether sufficient catalysts occur in realistic geological conditions. Moreover, the carbon isotope inversion of natural gas cannot be produced in the later stage, if the retained hydrocarbon is cracked into gas in the early-matured stage. These studies do not explain the formation of carbon isotope inversion. Peng et al. [4] found that the light carbon isotopes of heavy hydrocarbons formed by methyl recombination were an important reason for the abnormal carbon isotopes of heavy hydrocarbons in the high thermal evolution stage through the study of coal samples. This research result also explains the reason for carbon isotope inversion in coal-bearing source rocks. However, carbon isotope inversion occurs at 550°C in coal samples, whereas carbon isotopes reverse at 520°C in coal-bearing source rocks. This difference may be caused by the distinct OM components between coal samples and coal-bearing source rocks.

6 Conclusions

The distinguishable hydrocarbon gases are developed with different OMs of source rocks in the Xujiahe continuous sedimentary environment.

The T₃x¹ and T₃x³ source rocks are primarily characterized by the transitional sedimentary environment, and the T₃x⁵ source rocks are primarily featured in the continental sedimentary environment. The bacteria and algae decreased from T₃x¹ to T₃x⁵ source rocks, and its corresponding carbon isotope values of alkanes increased successively, leading to the II₂ OM in the T₃x¹ source rocks and the III OM in the T₃x³ and T₃x⁵ source rocks.
The δ ¹³C₁ values decrease first and then increase. The quantitative relationship is presented at δ ¹³C₁ = 12.21 ln Ro – 39.72 (humic gas) and δ ¹³C₁ = 8.07 ln Ro – 38.3 (mixed gas). The δ ¹³C₂ values range from −24.93 to −21.07‰ in the T₃x¹ and T₃x³ source rocks, ranging from −29.31 to −26.78‰ in the T₃x⁵ source rocks, which can effectively distinguish the OM types by δ ¹³C₂ values. The lighter heavy hydrocarbon carbon isotopes formed by the methyl recombination lead to the abnormal carbon isotopes of heavy hydrocarbons in the high thermal evolution stage.

Acknowledgements

The authors declare that they have no financial and personal relationships with other people or organizations that can inappropriately influence their work, there is no professional or other personal interest of any nature or kind in any product, service, and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled “Hydrocarbon-generating characteristics of Xujiahe coal-bearing source rocks in the continuous sedimentary environment of the Southwest Sichuan.”

Funding information: The article is supported by the funding of “Natural Science Foundation of China (42072185).”
Author contributions: Tao Yang: Conceptualization. Kun Xu: Writing original draft, software, experiment. Jungang Lu: Data processing, data curation methodology. Bai Liu: Investigation. Jianbo Liao: Writing, reviewing and editing.
Conflict of interest: Authors state no conflict of interest.
Data availability statement: The data that support the findings of this study are available from the corresponding author.

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Received: 2023-11-13

Revised: 2024-10-08

Accepted: 2024-12-11

Published Online: 2025-02-10

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

https://doi.org/10.1515/geo-2022-0754

Keywords for this article

hydrocarbon-generating characteristics; coal-bearing source rocks; sedimentary environments; carbon isotope; Xujiahe Formation; Sichuan Basin

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