The Taiyang shale gas field is located in the south of Xuyong County, Sichuan Province, and is a part of the Zhaotong National Shale Gas Demonstration Zone (Figure 1). It belongs to the Taiyang anticline, which was a narrow and steep anticlinal structure cut and damaged by strike–slip faults. It has been uplifted continuously since the Late Jurassic, and the uplifted range is about 2,000 m, which resulted in the denudation of the strata above Permian and possible loss of free gas. The black organic-rich siliceous shale and carbonaceous shale of the Wufeng–Longmaxi formations are the main targets of the shale gas field development, with a vertical burial depth (excluding the horizontal portion) of 500–2,250 m and formation pressure coefficient of 1.2–1.47, which belongs to shallow, atmospheric-overpressure shale gas reservoirs. The high-quality reservoirs are 13–18 m thick and stable in distribution, with TOC content of 0.42%–9.05% with an average of 2.92%. The organic matter is dominated by Type II₁, and the equivalent Ro% ranges from 3.0% to 3.9% (average = 3.45%) (Wang et al., 2019; Liang et al., 2021; Zou et al., 2021). The shale porosity is 2.92%–8.01%, with an average of 4.98%. The average brittleness is 72%, the gas content is 3.3–5.51 m³/t, and the average content of clay minerals is 24.7%. By the end of 2020, the accumulative proved reserves had been 1,365 × 10⁸ m³. The completed production capacity was 8 × 10⁸ m³/year, and the annual shale gas output was 4.0 × 10⁸ m³ in 2020.

The Jiaoshiba shale gas field is located in the east of Chongqing and is a part of the Fuling National Shale Gas Demonstration Zone (Figure 1). It has an area of about 575.92 km². The main body the of Jiaoshiba gas field is a box anticlinal structure spreading NE–SW, with faults controlling the east and west sides and large-scale faults developing in the southeast. The uplift was late and large, about 3,000 m, and the limestone of the Triassic Jialingjiang Formation is exposed, but the gas reservoir is well-preserved. The Wufeng–Longmaxi organic-rich shale is the main target layer of the shale gas field. The optimal reservoir section has a burial depth (excluding the horizontal portion) of 2,250–3,500 m. It is stably distributed with a thickness of about 38–40 m and a pressure coefficient of 1.55. It is dominated by Type I organic matter with an average TOC of 3.76%, porosity of 4.87%, brittleness of 62.4%, gas content of 2.96 m³/t, clay mineral content of 31.6%, and equivalent Ro% of 3.1% (2.6%–3.6%) (Dai et al., 2014a; Wang et al., 2019; Zou et al., 2021). By the end of 2020, the accumulative proved reserves had been 6008.14 × 10⁸ m³. The completed production capacity was 100 × 10⁸ m³/year, and the annual shale gas production was 67 × 10⁸ m³ in 2020.

The Weirong shale gas field is located in Neijiang and Zigong counties of Sichuan Province and is a part of the Changning–Weiyuan National Shale Gas Demonstration Zone (Figure 1). The main body of the gas field belongs to the Baimazhen syncline of the low-fold structural belt in southwestern Sichuan. The structure is gentle, and the fault is not developed. Influenced by the uplift evolution of the Sichuan Basin in the Late Cretaceous, the Jurassic was exposed on the surface, and the burial depth (excluding the horizontal portion) of the gas layer was 3,550–3,880 m. The overall preservation condition of the gas layer is good, and the formation pressure is high, that is, 1.94–2.06. The high-quality reservoirs are 25–39 m thick and stable in distribution, with an average TOC of 2.80%, porosity of 6.07%, brittleness of 64%, gas content of 3.15 m³/t, clay mineral content of 34%, and equivalent Ro% of 2.25% (1.8%–2.7%) (Wang et al., 2019; Zou et al., 2021). By the end of 2020, the cumulative proved reserves had been 1,247 × 10⁸ m³. The completed production capacity was 10 × 10⁸ m³/year, and the annual shale gas output was 5.4 × 10⁸ m³ in 2020.

Methane is the dominant composition, which accounts for 98.82% (average, n = 10) in the Taiyang gas samples, 98.13% (average, n = 10) in the Jiaoshiba gas samples, and 96.91% (average, n = 11) in the Weirong gas samples (Table 1). The content of C₂₊ heavy hydrocarbon (mainly C₂H₆ and C₃H₈) is very small, generally less than 1%. It is 0.61% (average, n = 10) in the Taiyang gas samples, 0.58% (average, n = 10) in the Jiaoshiba gas samples, and 0.45% (average, n = 11) in the Weirong gas samples. In general, Taiyang shale gas has the highest content of both methane (98.82%) and C₂₊₃ heavy hydrocarbon gases (0.59%), while Weirong gas samples have the lowest content of both methane (96.91%) and C₂₊₃ heavy hydrocarbon gases (0.44%). All the gases have extremely low gas wetness, which has a value of 0.21%–0.83%, indicating they belong to dry gas. The content of nonhydrocarbon gases such as CO₂ and N₂ is also very less (Table 1). Both the Taiyang and Jiaoshiba gases have a trace amount of CO₂ and N₂, less than 1%. Weirong gas has similar content of N₂ (<1%) but a relatively higher content of CO₂, in which CO₂ is generally more than 1% with an average of 2.05% (average, n = 11) and is up to 6.64% in Well Wei 43-1HF.

Gas samples from the Taiyang, Jiaoshiba, and Weirong gas fields have distinct carbon isotopic values. Gases from the Taiyang gas field have the highest δ¹³C₁ values, −29.5‰ to −27.1‰, with an average of −28.5‰ (n = 10), followed by the gases from the Jiaoshiba gas field, −32.7‰ to −29.0‰, with an average of −30.3‰ (n = 10), and gases from the Weirong gas field have the lowest δ¹³C₁ values, −35.9‰ to −34.1‰, with an average of −35.2‰ (n = 11) (Table 1). The carbon isotopic compositions of ethane and propane are the highest in gas samples from the Taiyang gas field δ¹³C₂: −36.8‰ to −33.7‰, with an average of −35.4‰ (n = 10); δ¹³C₃: −37.6‰ to −34.6‰, with an average of −36.2‰ (n = 10), followed by gas samples from Jiaoshiba gas field [δ¹³C₂: −37.5‰ to −35.5‰, with an average of −36.4‰ (n = 10); δ¹³C₃: −39.5‰ to −37.7‰, with an average of −38.4‰ (n = 10)], and are the lowest in the gas samples from Weirong gas field (δ¹³C₂: −39.7‰ to −38.4‰, with an average of −38.9‰ (n = 11); δ¹³C₃: −42.0‰ to −38.7‰, with an average of −40.8‰ (n = 11) (Table 1).

Carbon isotopic reversal occurs widely among the C₁–C₃ n-alkanes, that is, methane and its homologs get more depleted in ¹³C with increasing carbon number (δ¹³C₁ > δ¹³C₂ > δ¹³C₃) (Figure 3A; Table 1). In general, this is consistent with previous studies of the shale gases from the Changning–Zhaotong, Weiyuan, and Fuling national shale gas demonstration zones (Figure 3B). In contrast, gas samples from the Barnett and New Albany shales mostly have a positive carbon isotopic distribution pattern among the C₁–C₃ alkane gases (δ¹³C₁ < δ¹³C₂ < δ¹³C₃), and those from the Fayetteville shale have similar carbon isotopic reversal (Figure 3C) (Martini et al., 2008; Rodriguez and Philp, 2010; Strą;poć et al., 2010; Zumberge et al., 2012).

Numerical studies have suggested a cut-off δ¹³C₂ value of −28‰ to distinguish the gases sourced from humic and sapropelic organic matter, that is, gases sourced from humic organic matter have δ¹³C₂ > −28‰, while gases sourced from sapropelic organic matter have δ¹³C₂ < −28‰ (Chen et al., 1995; Dai et al., 2009, Dai et al., 2012a; Dai et al., 2014b; Liang et al., 2002; Ni et al., 2014, Ni et al., 2015, Ni et al., 2019b). Gases from Taiyang, Jiaoshiba, and Weirong gas fields have δ¹³C₂ < −33‰, and should source from sapropelic organic matter according to the criterion of δ¹³C₂ value. However, such a distinguishing criterion is effective for gases with normal carbon isotopic distribution patterns which have not undergone secondary alteration but may not be applicable for gases with carbon isotopic reversal (Dai et al., 2013). Therefore, genetic information should be obtained from indexes and not only the δ¹³C₂ values.

“Bernard diagram,” based on the carbon isotopic value of methane and the molecular composition of methane and ethane, was often used to determine the gas origin and later modifications (Bernard et al., 1976). This diagram differentiates the thermogenic and bacterial gases and shows the type of kerogen and later secondary alterations such as mixing between thermogenic and bacterial gases, migration, oxidation, and thermal maturation. Except gases from the New Albany in the Illinois Basin that have mixing effects, all other gases fall in the thermogenic gas area and have no impacts from bacterial gas (Figure 4A). Figure 4B demonstrates that gases from the Weirong gas field are mainly affected by the thermal maturation, but gases from the Jiaoshiba and Taiyang gas fields are likely affected by the migration effects.

In the δ¹³C₁ versus δ¹³C₂ and δ¹³C₂ versus δ¹³C₃ cross-plots, in general, the least mature gases lie in the lower left area and most mature gases lie in the upper right area. Theoretically, gases which have not undergone secondary alteration and are from the same source rocks but with different thermal maturation stages will lie on a straight line. Gases from the Taiyang, Jiaoshiba, and Weirong shale gas fields fall on the bottom right side of the diagram close to the area of sapropelic Type II kerogen and also in the area below the 1:1 line of δ¹³C₁ versus δ¹³C₂ (Figure 5A). This indicates gases from the Taiyang, Jiaoshiba, and Weirong gas fields were sourced from sapropelic organic matter. Figure 5A also shows a very low carbon isotopic composition of ethane, therefore demonstrating a carbon isotopic reversal between methane and ethane (δ¹³C₁ > δ¹³C₂). Similarly, in the plot of δ¹³C₂ versus δ¹³C₃, gases from the three gas fields fall in the area close to that of the sapropelic Type II kerogen (Berner and Faber, 1996) and demonstrate a carbon isotopic reversal between ethane and propane (δ¹³C₂ > δ¹³C₃) (Figure 5B).

Recently, new diagrams of δ¹³C₁ versus δ¹³C₂ and δ¹³C₁ versus Δ (δ¹³C₂–δ¹³C₁) were proposed to separate the shale-sourced and coal-sourced gases (Milkov, 2021). Both the δ¹³C₁ versus δ¹³C₂ and δ¹³C₁ versus Δ (δ¹³C₂–δ¹³C₁) diagrams can distinguish gases of primary microbial, secondary microbial, thermogenic, and abiotic origins, and the thermogenic gas includes mid-mature to late-mature shale-sourced and coal-sourced gases (Figure 6). Gases from the marine facies Taiyang, Jiaoshiba, and Weirong gas fields fall in the area of shale-sourced thermogenic gas toward the late-mature thermal evolution stage, which is similar to the shale gases from the Weiyuan, Fuling, Changning–Zhaotong, Pengshui, and Yongchuan areas. Among the nine sets of source rocks in the Sichuan Basin, source rocks above the Wufeng–Longmaxi shale are impossible to have a contribution. The Qixia and Maokou source rocks are a set of carbonate deposits, the Longtan and Xujiahe source rocks are a set of coal measure deposit, and the Dalong source rocks are locally distributed in northern Sichuan. The Sinian source rocks are mainly distributed in the margins of the Basin and have little contribution. Though the Qiongzhusi shale is widely distributed, the Ordovician deposit is very thick, and the thickness can be up to 600 m. The hot shale in the Wufeng-Longmaxi formations is generally less than 50 m and concentrated in the Wufeng and the Lower Longmaxi formations. Hence, gases in the Taiyang, Jiaoshiba, and Weirong shale gas fields are shale gas reservoirs and sourced in the Wufeng–Longmaxi shale.

When plotting the δ¹³C₁ versus gas wetness, gases from Weirong, Weiyuan, Barnett, and Fayetteville have a good linear relationship, while gases from Taiyang and Jiaoshiba are characterized by an enrichment of ¹³C in methane and thus shift away from the linear maturity trend (Figure 7A). The ¹³C-enriched methane was considered to be a late-stage methane and result from Rayleigh-type isotopic fractionation of ethane (Feng et al., 2020); however, such a mechanism may not satisfy the mass and isotopic balance requirements (Xia and Gao, 2018). In the southern Sichuan Basin, geological conditions are more complex than those in the Appalachian Basin. The Wufeng–Longmaxi shale has experienced very high geo-temperature and is at the over-mature stage at present and has suffered extensive uplifts locally. Considering these complex conditions, more influences should be considered in the southern Sichuan Basin. The offset of the δ¹³C₁ values in Taiyang and Jiaoshiba shale gas fields could be caused either by enrichment of ¹³C in methane or by an increase in wetness. Compared to the Cambrian and Sinian conventional natural gas in the Weiyuan gas field (−32.7‰ to 32.0‰) (Dai, 2003; Wu et al., 2014), the Silurian Wufeng–Longmaxi shale gas in Taiyang and Jiaoshiba areas is clearly more enriched in ¹³C.

At low mature and mature stages, δ¹³C values of CH₄ are much lower than those of C₂H₆ and δ¹³C values of C₂H₆ are much lower than those of C₃H₈, that is, δ¹³C₁ > δ¹³C₂ > δ¹³C₃. When the source rocks reach the oil window with Ro% of 1.5%, the carbon isotopic difference between methane and ethane and between ethane and propane will approach a similar value of −6‰ (Cesar et al., 2020). At this stage, isotopic effects of thermodynamic/equilibrium are predominant controlling factors, and isotopic exchange reactions occur under these conditions (Cesar et al., 2020). When the thermal maturity further increases, isotopic effects of thermodynamic/equilibrium are no longer the predominant controlling factors, and other processes may become more important; thus, carbon isotopic reversal first occurs in ethane and then in propane (Dai et al., 2016a; Cesar et al., 2020). In general, according to the published data, it is clear that with increasing thermal maturity, ethane and propane first become more enriched in ¹³C, then become more depleted in ¹³C, and finally become more enriched in ¹³C again (Ferworn et al., 2008; Burruss and Laughrey, 2010; Rodriguez and Philp, 2010; Zumberge et al., 2012; Hao and Zou, 2013; Tilley and Muehlenbachs, 2013; Dai et al., 2014a; Dai et al., 2016b; Dai et al., 2017; Liu et al., 2018; Ni et al., 2018; Xia and Gao, 2018; Feng et al., 2020) (Figures 7B, C). The Wufeng–Longmaxi shale gases from the Taiyang, Jiaoshiba, and Weirong shale gas fields fall in the post-rollover area, where ethane and propane are still depleted at ¹³C, but the carbon isotopic compositions get to be more positive again (Figures 7B, C).

Among the mechanisms which have been invoked to account for the carbon isotopic depletion in ethane and propane, most of them are related to high temperatures, for example, 1) in situ cracking of C₂₊ gas (Ferworn et al., 2008), in situ cracking of oil or gas (Rodriguez and Philp, 2010), simultaneous cracking of kerogen, oil, and gas in a closed system (Tilley et al., 2011), and cracking of kerogen-entrapped straight-chain aliphatic hydrocarbons (Ni et al., 2018); 2) water reacting with CH₄ to form isotopically light CO₂ and hydrogen and CO₂ and hydrogen, subsequently forming isotopically light ethane and propane (Zumberge et al., 2012); 3) Rayleigh fractionation of ethane and propane involving redox reactions with transition metals and water at temperatures on the order of 250°C–300°C (Burruss and Laughrey, 2010); 4) Diffusive migration during the stage with high maturity (Dai et al., 2016a). As discussed above, Fischer–Tropsch-type synthesis of hydrocarbon from CO₂ and H₂ has contributed to the carbon isotopic depletion in ethane and propane, but due to the limited amount of the reactant, it cannot be the controlling factor. In the southern Sichuan Basin, wet gas cracking cannot explain the depletion of ¹³C in ethane and propane. C₂₊ wet gas cracking at elevated temperatures will increase C₂/C₃ ratio and thus a decrease in gas wetness (Prinzhofer and Huc, 1995). The Wufeng–Longmaxi shale gas is characterized by very low gas wetness, and the content of ethane and propane is generally less than 1. Wet gas cracking of pentanes and butanes will cause enrichment in ¹²C in ethane and propane (Xia et al., 2013); however, under the over-mature dry gas stage, cracking of ethane and propane possibly occurs and the residual ethane and propane will become more enriched in ¹³C due to the kinetic isotopic fractionation effects (Tang et al., 2000). Obviously, this is opposite to the phenomenon of the Wufeng–Longmaxi shale gas.

Actually, Xia and Gao (2018) stated that mechanisms such as mixing of primary and secondary products in source rocks (Rodriguez and Philp, 2010; Ni et al., 2018; Hao and Zou, 2013; Ferworn et al., 2008; Xia et al., 2013) and water-involved reactions (Burruss and Laughrey, 2010; Gao et al., 2014; Zhang et al., 2018b; Zumberge et al., 2012) cannot satisfy the isotopic and mass balance requirements and proposed that the depletion of ¹³C in ethane and propane resulted from the reversible conversion from alkane to alkyl groups. The isotope effect is dependent on the rate and kinetic isotope effects of these reversible free-radical reactions catalyzed by minerals, and the isotopic depletion in ¹³C is consistent with the extent of the decomposition (Xia and Gao, 2018). However, these free-radical reactions may proceed in very different ways in different locations since the free-radical concentrations may vary greatly in different geological environments. In the sandstone and limestone formations, there is no sufficient hydrogen donors, and the reversal reactions will be suppressed and the fractionation will proceed likely in a Rayleigh fractionation mode; while organic-rich shales have abundant hydrogen donor, so the reversible reaction is possible, and the isotopic reversal is commonly found in either conventional and unconventional natural gases at relatively high thermal mature stages (Xia and Gao, 2018). The Upper Ordovician Wufeng–Lower Silurian Longmaxi shale comprises black shale, siliceous shale, calcareous-siliceous shale, and argillaceous-siliceous shale (Zou et al., 2019). It is deposited in a deep water environment and dominated by Type I and II₁ organic matter and typically has TOC of around 3% in the Taiyang, Jiaoshiba, and Weirong areas (Dai et al., 2014a; Liang et al., 2020a; Liang et al., 2020b; Zou et al., 2021). During the Hirnantian Stage glacial interval (Middle and Upper Wufeng Formation), there was a rapid global fall in sea level, while the P₂O₅/TiO₂ ratio reached 0.84 and the southern Sichuan Basin still experienced plankton bloom, high paleo-productivity, and high burial rates of organic matter (Zou et al., 2019). During the Rhuddanian Stage (Lower Longmaxi Formation), sea level increased and plankton bloomed, resulting in the widespread deposition of back shale in deep, oxygen-poor conditions under high-productivity surface waters (Zou et al., 2019). In southern China, Silurian marine source rocks with restored hydrogen index greater than 600 mg/g TOC account for 43%, and samples with restored hydrogen index between 250–600 mg/g TOC account for 57% (Zhang et al., 2013). In addition, the Wufeng–Longmaxi shale reached over 180°C in Late Permian in the Taiyang area and reached over 200°C in Late Jurassic in the Jiaoshiba area (He et al., 2017; Liang et al., 2020b; Pang et al., 2019; Tenger et al., 2017; Xiong et al., 2021). In general, the Wufeng–Longmaxi shale has equivalent vitrinite reflectance Ro% of 2.73%–4.62% (Wang et al., 2019; Zou et al., 2021) and gas wetness between 0.09% and 0.97%. Therefore, the Wufeng–Longmaxi shale is abundant in hydrogen donors and at a high thermal maturation stage with a high conversion rate. When plotting the Wufeng–Longmaxi shale gases into the δ¹³C₂ value versus gas wetness plot of the Barnett shale gases by Xia and Gao (2018), the carbon isotopic fractionation of ethane of the Wufeng–Longmaxi shale gases agrees well with the calculated results (pink line in Figure 12), which mainly fall in the post-rollover stage and indicates high thermal maturity of the gas. As indicated by the reversible free-radical reactions, the isotopic compositions are positive at the stage of low conversion ratios and then become negative with increasing conversion ratios; When the conversion ratios get even higher and the remaining reactant becomes even less, the isotopic compositions become more positive again (Xia and Gao, 2018). As for the Wufeng–Longmaxi shale gas, the extremely low content of ethane and propane (C₂₊₃ < 1%) indicates high conversion ratios, so during the late catagenesis stage, decomposition of ethane and propane is accompanied by the depletion of ¹³C in the residual ethane and propane, but in general, the isotopic composition becomes more positive again and falls in the post-rollover area. During this over-mature stage, reactions of free-radical decomposition are important controlling factors, resulting in the depletion of ¹³C in ethane and propane, which prevail the thermal cracking reactions.