Geochemical Characteristics and Origin of Tight Gas in Member Two of the Upper Triassic Xujiahe Formation in Western Sichuan Depression, Sichuan Basin, SW China

Natural gas in the tight sandstone reservoirs in Member two of the Upper Triassic Xujiahe Formation in the Western Sichuan Depression of the Sichuan Basin has complex geochemical characteristics, and the origin and source of the tight gas are revealed based on the geochemical analysis and comparison in this study. The tight gas has the dryness coefficient of 0.950–0.994, which is positively correlated with the CH₄ content. The gaseous alkanes display positive carbon isotopic series, with the δ¹³C₁ and δ¹³C₂ values ranging from −35.6‰ to −30.3‰ and from −29.1‰ to −21.0‰, respectively, and the δD₁ values range from −176‰ to −155‰. Genetic identification based on the carbon and hydrogen isotopic compositions indicates that a small amount of tight gas is typically coal-derived gas, whereas most of the tight-gas samples have experienced mixing by oil-associated gas. Geochemical comparisons suggest that the tight gas displays distinct differences with the typical oil-associated gas in Member four of the Middle Triassic Leikoupo Formation in the Chuanxi gas field and the typical coal-derived gas in Member five of Xujiahe Formation in the Xinchang gas field. It is also apparently different from the typical coal-derived gas in Member two of Xujiahe Formation in both Zhongba and Qiongxi gas fields. Among the tight gas in Member two of the Xujiahe Formation from the Western Sichuan Depression, the coal-derived gas is generated mainly by the humic mudstone in the Upper Triassic Ma’antang and Xiaotangzi formations, with assistance of the humic mudstone in Member two of the Xujiahe Formation, whereas the oil-associated gas is derived from the sapropelic limestone in the Ma’antang Formation.

1 Introduction

Tight-gas reservoirs refer to the tight sandstone fields or traps which accumulate commercial natural gas, and they can be divided into continuous-type and trap-type (Dai et al., 2012a). As one important type of unconventional natural gas, tight gas has made a crucial contribution to the rapid increase of both the yields and reserves of natural gas in China. The proven reserves of tight gas in China reached 3.0109 × 10¹² m³ by the end of 2010, which accounted for 39.2% of the total gas reserves (Dai et al., 2012a).

The Sichuan Basin is one of the important onshore petroliferous basins in China, and the exploration fields include terrigenous and marine strata (Dai et al., 2009; Dai et al., 2012b; Liu et al., 2012; Liu et al., 2013; Liu et al., 2014; Liu et al., 2016), with the former consisting of tight sandstone reservoirs in both the Upper Triassic Xujiahe Formation (T₃x) and Jurassic strata. The Zhongba gas field in Member two of the Xujiahe Formation (T₃x²), located in the western Sichuan Basin, is the first tight-gas field discovered in China (Dai et al., 2014). Several giant tight-gas fields (e.g., Xinchang, Guang’an, Hechuan, and Anyue) with proven reserves exceeding 10 × 10⁹ m³ have been revealed during the gas exploration in the Xujiahe Formation of the Sichuan Basin (Dai et al., 2014).

The Western Sichuan Depression (WSD) is situated in the western Sichuan Basin, and the gas exploration has achieved successive breakthroughs in the Middle Triassic to Jurassic strata (Wu et al., 2017; Wu et al., 2020a; Wu et al., 2020b). The Member two of the Middle Triassic Xujiahe Formation (T₃x²) has attracted wide attention due to the favorable exploration potential. The proven reserves of T₃x² in the Xinchang gas field from WSD are 121.12 × 10⁹ m³ (Dai, 2016), and the Zhongba and Qiongxi gas fields have also been discovered in the adjacent areas (Dai et al., 2012b).

The origin and source of tight gas in different strata of the Sichuan Basin have been extensively studied based on geochemical analysis, and the tight gas in the T₃x and Jurassic strata is considered as typical coal-derived gas and sourced mainly from the T₃x coal-measure source rocks (Dai et al., 2009; Dai et al., 2012b; Ni et al., 2014a; Ni et al., 2014b; Wu et al., 2017; Dai et al., 2018; Wu et al., 2019). The T₃x² gas from WSD displays complex geochemical characteristics, which are significantly different from the typical coal-derived gas from other members of T₃x. The authors intend to conduct geochemical analysis of the main components and stable carbon and hydrogen isotopes of the T₃x² gas from WSD, in order to reveal the genetic types and source of the gas as well as the differences with other reservoirs in the same area.

2 Geological Setting

The Sichuan Basin is developed on the basis of the Upper Yangtze Craton in SW China (Figure 1A), covering an area of 180 × 10³ km². It is the third largest petroliferous basin in China and superimposed by a marine craton and a terrigenous foreland basin. The annual gas yield in 2020 reaches 56.5 × 10⁹ m³, making it become the most productive petroliferous basin in China (Zhang, 2021). It consists of four exploration districts based on the structural units, i.e., Western, Eastern, and Southern gas districts, and Central oil and gas district (Dai et al., 2009).

FIGURE 1

FIGURE 1. The position of the Sichuan Basin in China (A), the location of the depression in the Sichuan Basin (B), and distribution of gas wells and structural units in the Western Sichuan Depression (C).

The WSD is located in the central part of the Western gas district (Figure 1B), covering an area of 10 × 10³ km². It is generally divided into four structural belts (Dayi-Anxian, Xinchang, Zhixinchang, and Zhongjiang-Huilong) and two sags (Zitong and Chengdu) (Figure 1C). The commercial gas wells in the Member two of the Xujiahe Fm. (T₃x²) are mainly situated in the Xinchang and Dayi-Anxian Structural Belts (Figure 1C). The Qiongxi and Zhongba gas fields have been discovered in natural gas exploration to the southwest and north of WSD, respectively (Figure 1B).

There are multiple gas-bearing layers in WSD, and the gas exploration has been conducted in Member four of the Middle Triassic Leikoupo Fm. (T₂l⁴), Upper Triassic Xujiahe Fm. (T₃x), and Jurassic strata (Figure 2). The Xujiahe Fm. is commonly divided into six members bottom-up, i.e., Member 1 (T₃x¹) to Member 6 (T₃x⁶), in which Member six is missing in the WSD due to weathering denudation, and Member one is generally divided into Xiaotangzi (T₃t) and Ma’antang (T₃m) formations (Figure 2). Member two of the Xujiahe Fm. (T₃x²) consists of siltstone interbedded with mudstone, and it is conformably and disconformably contacted with the overlying Member three of T₃x and underlying T₃t, respectively.

FIGURE 2

FIGURE 2. Stratigraphic column of the Western Sichuan Depression, Sichuan Basin, China.

3 Samples and Analytical Methods

The T₃x² tight-gas samples in the WSD of the Sichuan Basin were collected from the wellheads using 5-cm-radius stainless steel cylinders with double valves, and the lines were flushed for 10–15 min in order to remove air contamination at first. A geochemical analysis of the gas samples was conducted at the Wuxi Research Institute of Petroleum Geology, Petroleum Exploration and Production Research Institute, SINOPEC.

The chemical composition of gas samples measured using an Agilent 7890A gas chromatograph (GC) equipped with a flame ionization detector and a thermal conductivity detector. Individual alkane gas components were separated using a capillary column (PLOT Al₂O₃ 50 m × 0.53 mm × 25 μm). Helium was used as the carrier gas with a rate of 40 ml/min and unsplit stream sampling. The GC oven temperature was initially set at 40°C for 5 min, heating at a rate of 10°C/min to a final temperature of 180°C, which was held for 20 min.

The stable carbon isotopic composition of alkane gases was measured on a Finnigan MAT 253 mass spectrometer. The alkane gas components were initially separated using a fused silica capillary column (PLOT Q 30 m × 0.32 mm × 20 μm) with helium carrier gas at the flow rate of 10 ml/min. The gas was injected into the GC in the split injection model with the split ratio of 15:1. The oven temperature was ramped from 40°C to 180°C at a heating rate of 10°C/min, and the final temperature was held for 10 min. Each gas sample was measured in triplicate, and the results were averaged. Stable carbon isotopic values are reported in the δ notation in per mil (‰) relative to VPDB, and the measurement precision is estimated to be ±0.5‰ for δ¹³C.

The stable hydrogen isotopic composition of alkane gases was measured on a Thermo Scientific Delta V Advantage mass spectrometer (GC/TC/IRMS). The alkane gas components were separated on an HP-PLOT Q column (30 m × 0.32 mm × 20 μm) with helium carrier gas at 1.5 ml/min. The gas was injected into the GC in the split injection model with the split ratio of 15:1. The GC oven was initially held at 30°C for 5 min, and then the temperature was programmed to 80°C at 8°C/min then heated to 260°C at 4°C/min where it was held for 10 min. Each gas sample was measured in triplicate, and the results were averaged. The measurement precision is estimated to be ±3‰ for δD with respect to VSMOW.

4 Results

The chemical composition and stable isotopic compositions of the T₃x² tight gas from WSD are listed in Table 1. The previously published data of the T₃x² gases from Zhongba and Qiongxi gas fields (Dai et al., 2012b), T₃x⁵ gas from the Xinchang gas field (Wu et al., 2016), and T₂l⁴ gas from the Chuanxi gas field (Wu et al., 2020b) have also been collected for comparative analysis.

TABLE 1

TABLE 1. Main components and stable carbon and hydrogen isotopic compositions of the T₃x² tight gas from the Western Sichuan Depression.

4.1 Main Components of Natural Gas

The T₃x² gas from WSD is mainly composed of CH₄, and the CH₄ content ranges from 93.03% to 98.14% with an average of 96.49%, whereas the heavy alkane (C₂–C₄) content ranges from 0.59% to 4.92% with an average of 1.72% (Table 1). The T₃x² gas from WSD is dry with the dryness coefficient (C₁/C_1-4) ranging from 0.950 to 0.994 (Table 1), and it displays a positive correlation between the CH₄ content and C₁/C_1-4 ratio (Figure 3A), suggesting the effect of thermal maturity.

FIGURE 3

FIGURE 3. The cross-plots of C₁/C_1-4 versus CH₄% (A) and CO₂% versus N₂% (B). The T₃x² gases of Zhongba and Qiongxi gas fields (Dai et al., 2012b), T₃x⁵ gas from the Xinchang gas field (Wu et al., 2016), and T₂l⁴ gas from the Chuanxi gas field (Wu et al., 2020b) are also plotted for comparative analysis and hereafter the same.

The non-hydrocarbon components of the T₃x² gas from WSD are predominantly CO₂ and N₂, with the contents ranging from 0.82% to 1.94% and from 0% to 0.82%, respectively (Table 1). The T₃x² gas is free of H₂S, and the correlation between CO₂ and N₂ contents is unobservable (Figure 3B).

4.2 Carbon Isotopes of Alkanes

The carbon isotopic value of CH₄ (δ¹³C₁) of T₃x² gas from WSD ranges from −35.6‰ to −30.3‰ with an average of −32.3‰, whereas the δ¹³C₂ value ranges from −29.1‰ to −21.0‰ with an average of −25.5‰ (Table 1). The gaseous alkanes mainly display positive carbon isotopic series (i.e., δ¹³C₁ < δ¹³C₂ < δ¹³C₃), with only a few samples possessing partial reversal between C₂H₆ and C₃H₈ (δ¹³C₂ > δ¹³C₃) (Table 1).

4.3 Hydrogen Isotopes of Alkanes

The hydrogen isotopic value of CH₄ (δD₁) of T₃x² gas from WSD ranges from −176‰ to −155‰ with an average of −163‰, whereas the δD₂ value ranges from −161‰ to −110‰ with an average of −138‰ (Table 1). The gas displays a positive hydrogen isotopic sequence between CH₄ and C₂H₆ (i.e., δD₁ < δD₂) (Table 1).

5 Discussion

5.1 Genetic Types of Natural Gas

Biogenic gas consists of thermogenic gas and bacterial gas according to different mechanisms of gas generation, and the latter mostly has higher C₁/C₂₊₃ ratios and lower δ¹³C₁ values (Bernard et al., 1976). The C₁/C₂₊₃ ratios and δ¹³C₁ values of T₃x² gas from WSD range from 20.7 to 175.8 and from −35.6‰ to −30.3‰, respectively, suggesting the characteristics of thermogenic gas rather than bacterial gas (Figure 4). Thermogenic gas can be divided into coal-derived and oil-associated gases, which are generated by humic (kerogen type III) and sapropelic (kerogen type I/II) organic matters, respectively (Dai, 1992; Rooney et al., 1995; Liu et al., 2019). The Zhongba and Qiongxi T₃x² gases and Xinchang T₃x⁵ gas follow the trend of natural gas from type III kerogen, suggesting the typical characteristics of coal-derived gas, whereas the Chuanxi T₂l⁴ gas shares similar characteristics with natural gas from type II kerogen in the modified Bernard diagram (Figure 4). The T₃x² gas from WSD is mainly plotted between these two types of natural gas in the diagram (Figure 4), suggesting the characteristics of mixing gas, and it seems unlikely to be derived from single-type kerogen.

FIGURE 4

FIGURE 4. The diagram of C₁/C₂₊₃ versus δ¹³C₁ of the T₃x² gas from the Western Sichuan Depression (modified after Bernard et al., 1976).

Coal-derived and oil-associated gases have different evolution trends in the correlation diagram between δ¹³C₂ and δ¹³C₁ values (Rooney et al., 1995), and oil-associated gas displays more negative δ¹³C₂ values than coal-derived gas under similar δ¹³C₁ values (Figure 5A). The Chuanxi T₂l⁴ gas mainly follows the trend of oil-associated gas from type II kerogen in the Delaware/Val Verde Basin, whereas the Zhongba and Qiongxi T₃x² gases and Xinchang T₃x⁵ gas follow the trend of coal-derived gas from type III kerogen in Niger Delta (Figure 5A). Several T₃x² gas samples from WSD are consistent with the coal-derived gas from type III kerogen in the diagram of δ¹³C₂ versus δ¹³C₁ values, whereas the other gas samples display the characteristics of oil-associated or mixing gas (Figure 5A), suggesting the complex features of the T₃x² gas.

FIGURE 5

FIGURE 5. The diagrams of δ¹³C₂ versus δ¹³C₁ (A) and δ¹³C₃ versus δ¹³C₂ (B). The trends representing gases from Type III kerogen in Niger Delta and Type II kerogen in Delaware/Val Verde Basin (A) are after Rooney et al. (1995), and the trend for gas from Type III kerogen in Sacramento Basin (A) are after Jenden and Kaplan (1989). The ranges for coal-derived and oil-associated gases (B) are from Dai (1999).

The δ¹³C₂ and δ¹³C₃ values were considered to inherit the carbon isotopic compositions of original organic matter and thus could be used to identify the coal-derived and oil-associated gases (Dai et al., 2005). The geochemical statistics of natural gas in China indicated that the δ¹³C₂ values of coal-derived and oil-associated gases were commonly higher than −27.5‰ and lower than −29‰, respectively, whereas the δ¹³C₃ values were higher than −25.5‰ and lower than −27.0‰, respectively (Dai, 1999). The Zhongba and Qiongxi T₃x² gases and Xinchang T₃x⁵ gas have consistent δ¹³C₂ and δ¹³C₃ values with typical coal-derived gas, whereas the only two T₂l⁴ gas samples from the Chuanxi gas field display consistent δ¹³C₂ and δ¹³C₃ values with typical oil-associated gas (Figure 5B). Five gas samples in T₃x² reservoirs from WSD with relatively high δ¹³C₂ and δ¹³C₃ values display the characteristics of coal-derived gas, whereas the other gas samples are plotted in the transitional zone between oil-associated and coal-derived gas in Figure 5B, suggesting the characteristics of mixing gases.

Since the δD₁ values are associated with the types and thermal maturity of organic matter and water salinity (Liu et al., 2008; Schoell, 1980; Stahl, 1977), oil-associated gas and coal-derived gas generally follow different maturity trends in the correlation diagram between δD₁ and δ¹³C₂ values (Figure 6) (Wang et al., 2015). The Chuanxi T₂l⁴ gas displays the characteristics of oil-associated gas with δD₁ values higher than −160‰, whereas the Zhongba and Qiongxi T₃x² gases and Xinchang T₃x⁵ gas follow the trend of coal-derived gas in Figure 6. A positive correlation between δD₁ and δ¹³C₂ values is unobservable for the T₃x² gas from WSD, and most gas samples display the characteristics of mixing gas between typical oil-associated and coal-derived gases, with only five gas samples showing the characteristics of coal-derived gas (Figure 6). These indicate that most of the T₃x² gas samples from WSD follow the mixing trend rather than the maturity trend. Therefore, a small amount of the T₃x² gas from WSD is typically coal-derived gas, whereas most of the gas samples have experienced mixing by oil-associated gas.

FIGURE 6

FIGURE 6. Correlation diagram between δD₁ and δ¹³C₂ values of the T₃x² gas from the Western Sichuan Depression (modified after Wang et al., 2015).

The partial reversal of alkane carbon isotopes has been widely studied in China, and it can be attributed to mixing of abiogenic and biogenic gases, mixing of coal-derived and oil-associated gases, mixing of gases from the same types of source rocks with different maturity, or bacterial oxidation (Dai et al., 2004; Wu et al., 2014; Liu et al., 2019). Abiogenic gas or bacterial gas has not been found in WSD, and the mixing of abiogenic and biogenic gases or bacterial oxidation seems unlikely to happen. The mixing of coal-derived gas and oil-associated gas is demonstrated to be common in the T₃x² gas from WSD, and thus it is believed to be the main reason of partial δ¹³C reversal between C₂H₆ and C₃H₈ (δ¹³C₂ > δ¹³C₃). The gas sample from Well JS1 displays a significantly heavier carbon isotope of C₂H₆ than that of C₃H₈ (Table 1). It is mainly caused by the extremely low δ¹³C₃ value (−27.5‰), which suggests the typical characteristics of oil-associated gas (Figure 5B). Therefore, the partial δ¹³C reversal between C₂H₆ and C₃H₈ was attributed to mixing of coal-derived gas and oil-associated gas.

The coal-derived gas is commonly generated by humic organic matter (kerogen type III) through primary cracking of kerogen, whereas the oil-associated gas can be generated by both primary cracking of sapropelic kerogen (type II) and secondary cracking of oil, and the oil cracking gas accounts for nearly 80% of the total gas amount generated by sapropelic organic matter in the high-mature stage (Li et al., 2018). Natural gas in the marine strata of the Sichuan Basin is predominantly composed of oil-cracking gas (Hao et al., 2008; Liu et al., 2012; Liu et al., 2013; Liu et al., 2014). Theoretical and experimental studies indicate that kerogen cracking gas and oil cracking gas follow different evolution trends in the correlation diagram between ln (C₂/C₃) and ln (C₁/C₂) values (Prinzhofer and Huc, 1995; Li et al., 2017). The Chuanxi T₂l⁴ oil-associated gas displays the characteristics of oil cracking gas (Wu et al., 2020b), whereas the coal-derived gas from the Zhongba and Qiongxi T₃x² and Xinchang T₃x⁵ reservoirs primarily follows the trend of kerogen cracking gas (Figure 7). Three gas samples from the T₃x² reservoirs in WSD also follow the trend of kerogen cracking gas (Figure 7), and they have δ¹³C₂ values higher than −24.0‰ and display the characteristics of typical coal-derived gas in the correlation diagram between δD₁ and δ¹³C₂ values (Figure 6). Other gas samples from the T₃x² reservoirs in WSD are distributed closer to the trend line of oil cracking gas in Figure 7; however, they are distinctly different from the oil cracking gas in the Chuanxi T₂l⁴ reservoirs, which suggest that these T₃x² gas samples have experienced mixing by a significant amount of oil cracking gas. Therefore, the T₃x² tight gas from WSD are commonly mixed by oil cracking gas, except a few samples being typically coal-derived gas from kerogen cracking.

FIGURE 7

FIGURE 7. The cross-plot of ln (C₂/C₃) versus ln (C₁/C₂) of the T₃x² gas from the Western Sichuan Depression (modified after Li et al., 2017).

As indicated in Figure 7, the coal-derived gas in the T₃x² reservoirs from WSD seems to be mainly in the high-maturity stage with the maturity ranging from 1.3% to 1.7%, whereas the oil-associated gas reaches the over-mature stage with the maturity higher than 2.0%. The variance of the maturity may be attributed to the regional difference of potential source rocks, which was caused by different burial history and depth.

5.2 Gas—Source Correlation

5.2.1 Geochemical Comparison With the Chuanxi T₂l⁴ Gas

The Chuanxi T₂l⁴ gas is typical dry gas with the dryness coefficient (0.996–0.999) higher than the T₃x² gas from WSD (Figure 3A). The T₂l⁴ gas is a sour gas with the H₂S content ranging from 0.59% to 4.90% (Wu et al., 2020b), and its CO₂ content (4.59%–15.25%) is apparently higher than the T₃x² gas from WSD (Figure 3B). These two gases share similar distribution ranges of δ¹³C₁ values; however, the Chuanxi T₂l⁴ gas has higher C₁/C₂₊₃ ratios (240–820) and thus the characteristics of oil-associated gas in the modified Bernard diagram (Figure 4), whereas the T₃x² gas from WSD displays the characteristics of mixing gas (Figure 4). The Chuanxi T₂l⁴ gas has more negative δ¹³C₂ values (−32.9‰ to −29.6‰) and less negative δD₁ values (−157‰ to −136‰), resulting in different distribution characteristics with the T₃x² gas from WSD in Figure 5A and Figure 6. The larger ln (C₂/C₃), ln (C₁/C₂), and δ¹³C₁-δ¹³C₂ values of the Chuanxi T₂l⁴ gas suggest higher thermal maturity than the T₃x² gas from WSD (Figure 7, Figure 8).

FIGURE 8

FIGURE 8. The diagrams of δ¹³C₁-δ¹³C₂ versus ln (C₁/C₂) of the T₃x² gas from the Western Sichuan Depression (modified after Prinzhofer and Huc, 1995).

The Chuanxi T₂l⁴ gas was demonstrated as typical oil-associated gas and derived mainly from the underlying Upper Permian source rocks (Wu et al., 2020b), and the gas pools were commonly distributed along the deep fault which directly or indirectly connected the reservoirs and the Permian strata (Wu et al., 2020a). The T₃x² gas from WSD is apparently different from typical oil-associated gas (Figure 4, Figure 5A, Figure 6), and the thermal maturity is significantly lower than the Chuanxi T₂l⁴ gas (Figure 7, Figure 8). Therefore, the T₃x² gas from WSD displays few affinities with the Upper Permian source rocks.

5.2.2 Geochemical Comparison With the Xinchang T₃x⁵ Gas

The Xinchang T₃x⁵ gas is mainly wet gas with the dryness coefficient ranging from 0.905 to 0.961 (Wu et al., 2016), which is principally lower than that of the T₃x² gas from WSD (Figure 3A). The CO₂ content ranges from 0.13% to 0.70% and is also lower than that of the T₃x² gas from WSD (Figure 3B). The δ¹³C₁ and C₁/C₂₊₃ values of the Xinchang T₃x⁵ gas, ranging from −40.2‰ to −33.5‰ and from 10.7 to 26.0, respectively, are generally lower than the corresponding values of the T₃x² gas from WSD (Figure 4). This suggests a relatively lower thermal maturity of the Xinchang T₃x⁵ gas. In the modified Bernard diagram (Figure 4), the Xinchang T₃x⁵ gas follows the typical characteristics of coal-derived gas from type III kerogen, and it is different from the transitional or mixing characteristics of the T₃x² gas from WSD. The δ¹³C₂ and δ¹³C₃ values of the Xinchang T₃x⁵ gas, concentrating from −27.3‰ to −23.6‰ and from −23.4‰ to −21.0‰, respectively, display differences with the wide distribution ranges of corresponding values of T₃x² gas from WSD (Figure 5). The Xinchang T₃x⁵ gas has the δD₁ value ranging from −215‰ to −179‰, which is obviously lower than the T₃x² gas from WSD (Figure 6). Moreover, the relatively lower ln (C₂/C₃) and ln (C₁/C₂) values of the Xinchang T₃x⁵ gas indicate its lower thermal maturity than the T₃x² gas from WSD (Figure 7, Figure 8).

The Xinchang T₃x⁵ gas was demonstrated as typical coal-derived gas and mainly generated by the T₃x⁵ source rocks (Wu et al., 2016). The T₃x² gas from WSD displays certain differences with typical coal-derived gas (Figures 4,5, 6), and the thermal maturity is apparently higher than the Xinchang T₃x⁵ gas (Figures 7, 8). Consequently, the T₃x² gas from WSD displays few affinities with the T₃x⁵ source rocks, and it is considered to be derived from source rocks with higher thermal maturity.

5.2.3 Geochemical Comparison With the Zhongba and Qiongxi T₃x² Gases

The Qiongxi T₃x² gas is mainly dry gas with the dryness coefficients between 0.948 and 0.980, whereas the Zhongba T₃x² gas is typical wet gas with the dryness coefficient ranging between 0.885 and 0.921 (Dai et al., 2012b). They are both apparently lower than the coefficient of the T₃x² gas from WSD (Figure 3A). The Qiongxi T₃x² gas has consistent CO₂ content (from 0.92% to 1.67%) with the T₃x² gas from WSD, whereas the Zhongba T₃x² gas displays an obviously lower CO₂ content (from 0.32% to 0.56%) than the T₃x² gas from WSD (Figure 3B).

Compared with the T₃x² gas from WSD, the Qiongxi T₃x² gas displays similar δ¹³C₁ values (ranging from −33.7‰ to −30.8‰) and overall lower C₁/C₂₊₃ ratios (from 19.3 to 50.8), whereas the Zhongba T₃x² gas has apparently lower δ¹³C₁ values (ranging from −36.9‰ to −34.8‰) and C₁/C₂₊₃ ratios (from 9.7 to 12.7) (Figure 4). There are two gases that show the characteristics of coal-derived gas in the modified Bernard diagram (Figure 4). Their δ¹³C₂ values are distributed in the narrow ranges from −24.1‰ to −22.4‰ and from −25.6‰ to −24.0‰, respectively, whereas the δD₁ values range from −159‰ to −157‰ and from −173‰ to −170‰, respectively, with δ¹³C₃ values higher than −24‰ (Dai et al., 2012b). They also follow the typical characteristics of coal-derived gas in Figure 5A,B and Figure 6 and displays certain differences with the T₃x² gas from WSD.

The Qiongxi and Zhongba T₃x² gases were demonstrated to be self-generated and self-accumulated in T₃x, i.e., they were mainly derived from the T₃x¹ coal–measure source rocks with assistance of T₃x² coal–measure source rocks (Zhu et al., 2011; Dai et al., 2012b; Liao et al., 2014). Affected by different largest burial depths, the present thermal maturity of T₃x¹ in the Zhongba gas field is slightly lower than that in the Qiongxi gas field and WSD (Jiang et al., 2012). This is consistent with the maturity difference indicated by the ln (C₂/C₃), ln (C₁/C₂), and δ¹³C₁-δ¹³C₂ values of the T₃x² gases in these areas (Figures 7, 8). Therefore, the geochemical characteristics of the T₃x² gas from WSD match the thermal maturity of the in-situ T₃x¹ and T₃x² source rocks.

5.2.4 Source of the T₃x² Gas From WSD

The above geochemical analysis and genetic identification of natural gas indicate that the T₃x² gas from WSD is mainly coal-derived gas assisted by oil-associated gas, rather than typical coal-derived or oil-associated gas. Therefore, it is unreliable to calculate the thermal maturity by δ¹³C₁ values based on the δ¹³C₁-R_O empirical equations for typical coal-derived or oil-associated gas. According to the thermal maturity comparison indicated by geochemical characteristics of natural gas from different strata, the T₃x² gas from WSD displays apparently higher maturity than the overlying Xinchang T₃x⁵ gas and lower maturity than the underlying Chuanxi T₂l⁴ gas (Figures 7, 8). The T₃x² gas from WSD is inferred to have consistent maturity with the T₃x¹ and T₃x² source rocks.

The T₃x source rocks in the Sichuan Basin were mainly developed in T₃x¹, T₃x³, and T₃x⁵, and a small amount of argillaceous source rocks was developed in T₃x², T₃x⁴, and T₃x⁶ (Dai et al., 2009; Wang et al., 2010; Cai and Yang, 2011; Dai et al., 2012b). T₃x¹ is commonly divided into Xiaotangzi (T₃t) and underlying Ma’antang (T₃m) formations, in which T₃t consists of sandstone interbedded with mudstone and siltstone, and T₃m is composed of silty mudstone at the top and grey limestone at the bottom (Figure 2). The kerogen-type indexes of T₃t and T₃m mudstone are less than 0 and consistent with those of the T₃x² mudstone, displaying the characteristics of typical type III kerogen, whereas the indexes of the T₃m limestone range from 23.75 to 50.5, displaying the characteristics of type II kerogen (Wang et al., 2010; Cai and Yang, 2011). The T₃m limestone from some areas in WSD displays certain hydrocarbon potential with the TOC contents higher than 0.5% (Yang et al., 2012). Therefore, among the T₃x² tight gas from WSD the coal-derived gas is generated mainly by the T₃t and T₃m mudstone with assistance of T₃x² mudstone, whereas the oil-associated gas is derived from the T₃m limestone.

6 Conclusion

The tight gas in Member two of the Upper Triassic Xujiahe Formation in the WSD of the Sichuan Basin is mainly composed of CH₄, with the CH₄ content ranging from 93.03% to 98.14%. The dryness coefficient of the tight gas is 0.950–0.994 and positively correlated with the CH₄ content. The gaseous alkanes display positive carbon isotopic series, with the δ¹³C₁ and δ¹³C₂ values ranging from −35.6‰ to −30.3‰ and from −29.1‰ to −21.0‰, respectively, and the δD₁ values range from −176‰ to −155‰.

According to the genetic identification based on the carbon and hydrogen isotopic compositions, a small amount of the tight gas is typically coal-derived gas, whereas most of the tight-gas samples have experienced mixing by oil-associated gas. Geochemical comparisons indicate that the tight gas displays distinct differences with the typical oil-associated and coal-derived gases in adjacent gas fields. Among the tight gas in Member two of the Xujiahe Formation from the WSD, the coal-derived gas is generated mainly by the humic mudstone in the Upper Triassic Ma’antang and Xiaotangzi formations, with assistance of the humic mudstone in Member two of the Xujiahe Formation, whereas the oil-associated gas is derived from the sapropelic limestone in the Ma’antang Formation.

Data availability statement

The original contributions presented in the study are included in the article/Supplementary Material; further inquiries can be directed to the corresponding author.

Author Contributions

XW: conceptulization, data curation, writing. QL: conceptulization, writing. YC: data curation, methodology. JY: methodology, investigation. HZ: sample collection, investigation. HL: investigation.

Funding

This work was funded by the National Natural Science Foundation of China (Grant Nos. 42172149, 41872122, U20B6001, and 42141021) and Strategic Priority Research Program of the Chinese Academy of Sciences (Class A) (Grant Nos. XDA14010402 and XDA14010404).

Conflict of Interest

HL was employed by Exploration and Production Research Institute, Southwest Branch Company.

The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s Note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors, and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Acknowledgments

The authors appreciate Jinxing Dai, Academician of Chinese Academy of Sciences, for the long-standing guidance of the relevant studies. We also appreciate QL for the valuable discussion and inspiration. The SINOPEC Southwest Branch Company and Key Laboratory for Hydrocarbon Accumulation Mechanism are acknowledged for the assistance on sample collection and geochemical analyses, respectively.

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