Coupling noble gas and alkane gas isotopes to constrain normally pressured shale gas expulsion in SE Sichuan Basin, China

The molecular and isotopic compositions of shale gases exhibit substantial differences under different storage conditions. Gas geochemistry is widely used when evaluating gas accumulation and expulsion in petroleum systems. Gas geochemical characteristics can provide important references for determining the enrichment mechanism of shale gas reservoirs and predicting shale gas production capacity in different regions. In tectonically stable regions with similar reservoir formation and evolution histories, shale gas reservoirs are expected to exhibit favorable storage conditions with only relatively small variations in gas geochemical characteristics. In tectonically active regions, shale gas preservation conditions are expected to be more variable. In this study, we systematically analyzed the stable isotope signatures (δ¹³C and δD) of alkane gases (CH₄, C₂H₆, and C₃H₈), along with noble gas compositions and isotopic signatures, of normally pressured Wufeng‒Longmaxi marine shale gas samples comprising a continuous pressure coefficient series from a structurally active region at the transition between an orogenic belt and the southeastern (SE) Sichuan Basin, China. The relationships between noble gas contents, isotopic signatures, and shale gas yields were evaluated, and a mechanism for normally pressured shale gas accumulation and expulsion was presented. The δ¹³C and δD data suggest that the normally pressured shale gas originated from late-mature thermogenic generation, equivalent to shale gas from other production areas in the inner Sichuan Basin. Gas dryness ratios [C₁/(C₂ + C₃)] exhibit negative relationships with δ¹³C₁ and δ¹³C₂. Normally pressured shale gas yields exhibit a negative correlation with δ¹³C and a positive correlation with [C₁/(C₂ + C₃)], suggesting differences in shale gas accumulation and expulsion across the studied region related to changes in the pressure coefficient. Noble gas isotope data suggest that the normally pressured Longmaxi shale gas received a substantial contribution of crust-derived He. Coupling noble gas and stable C/H isotope data reveals that the abundance of He and Ar, along with the δ¹³C signatures of alkane gases, is affected by the abundance of shale gas during the accumulation and expulsion process. The noble gas and stable isotope distribution trends presented herein can be used to evaluate Wufeng‒Longmaxi’s normally pressured shale gas accumulation and expulsion in complex structural areas of the southeastern Sichuan Basin. Better preservation conditions accompanying lower tectonic activity will normally result in higher shale gas production and a lower concentration of noble gases. The above findings show that gas geochemical characteristics could be used as effective evaluation indicators for determining shale gas accumulation mechanisms in tectonically active regions.

1 Introduction

Gas geochemical characteristics are important tools for studying the formation mechanism of natural gas reservoirs. The fractionation of alkane gas carbon isotopes in natural gas, owing to mass transport, is widely used to identify genetic types of natural gas, as well as oil and gas source correlation and alteration processes (Schoell, 1988; Whiticar, 1996; Xia and Tang, 2012). Owing to chemical inertness, noble gases are not impacted by secondary chemical processes, no matter whether the chemical reactions are inorganic or organic (Ozima and Podosek, 2017; Byrne et al., 2018). Noble gas isotope data have been widely used as important gas tracers for a variety of subsurface fluid transport processes in petroleum systems (Zhou et al., 2012; Burnard et al., 2013; Wang et al., 2013; Byrne et al., 2017; Byrne et al., 2020; Zhang et al., 2019).

Shale gas is considered a notable discovery in terms of natural gas exploration in the Sichuan Basin, given its great resource potential (Dai et al., 2014; Zou et al., 2015; Dong et al., 2016). Multiple exploration breakthroughs have been achieved in the Sichuan Basin, with the discovery of several very large shale gas fields, including Fuling, Weiyuan, Weirong, and Zhaotong (Jiang et al., 2020; Ma et al., 2020; Qiu et al., 2020; Zhao et al., 2020; Zou et al., 2021). The aforementioned successful shale gas fields have all targeted deep and high/over-pressured shale layers at depths >2000 m and pressure coefficients >1.3. In contrast, orogenic processes have caused Wufeng‒Longmaxi shale to be markedly uplifted in Sichuan Basin–margin transitional regions. Hence, shale layers in the southeastern (SE) Sichuan Basin are now characterized by low burial depths (<2000 m) and low pressure coefficients (<1.3), leading to the formation of normally pressured shale gas.

Shale gas exploration in the United States began in normally pressured organic-rich shale layers (Guo et al., 2015; Hu et al., 2015). Before 2004, the main shale gas fields, such as the Marcellus shale gas field in the Appalachian Basin and Ohio shale gas field, were dominated by normally pressured shale gas, characterized by low-pressure or normal-pressure conditions (Lancaster et al., 1989; Curtis, 2002; Montgomery et al., 2006; Pollastro et al., 2007; Darabi et al., 2012; Meng et al., 2023). The typical characteristics of normally pressured shale gas in the United States are a continuous distribution of organic-rich shales, low thermal evolution, and a stable tectonic environment (Guo, 2016, Guo et al., 2020; Jiang et al., 2022). The thermal maturity of normally pressured shale gas in the United States is normally <1.5%, with an adsorbed shale gas content >50%. In contrast to low-maturity normally pressured shale gas in the United States, normally pressured shale gas exploration in China has achieved notable success in highly mature shales (Guo et al., 2020). Normally pressured organic-rich shale layers are widely distributed along the margins of the Sichuan Basin. Currently, the Nanchuan‒Wulong regions, located in the southeastern Sichuan Basin, are the only successfully developed normally pressured shale gas fields in this basin. Compared with high-/over-pressured shale gas reservoirs, normally pressured shale gas reservoirs have the characteristics of weak formation energy, complex ground stress fields, low single-well production, high drilling and production costs, a wide distribution area, a large total resource, and low resource abundance (Guo et al., 2020; He, 2021). Under existing technical and economic conditions, commercial development of normally pressured shale gas faces considerable challenges. Previous studies have revealed tectonic deformation, such as folding, to be the key factor in controlling the quality of Wufeng‒Longmaxi shale gas preservation conditions. More intense strain results in more free gas migrating to pressure-relief areas, such as eroded areas, permeable faults, and fracture zones. Meanwhile, owing to pressure reduction and desorption, the adsorbed gas will be converted into free gas and also dissipate, resulting in the further reduction of shale gas content or potentially no gas at all. Therefore, the intensity of the tectonic strain controls the quality of preservation conditions, affecting the gas content of shale and the proportion of free gas, and determines the enrichment of shale gas reservoirs (He et al., 2019; He, 2021).

Geological studies suggest that multi-stage tectonic movement has occurred in the southeastern Sichuan Basin, causing shale gas preservation conditions to be degraded (He et al., 2019). Gas geochemistry has been widely used to provide important information in the evaluation of shale gas type, origin, evolution, migration, and accumulation (Dai et al., 2014; Cao et al., 2018; Cao et al., 2020; Chen et al., 2020; Liu et al., 2021; Wang et al., 2022). Wang et al. (2022) studied the chemical compositions and isotopic signatures of alkane gases, He, and Ar associated with Lower Paleozoic shale gas in the southern Sichuan Basin. Cao et al. (2020) analyzed shale gas origin and evolution using the stable carbon isotope signature (δ¹³C) of alkane gases and the geochemistry of noble gases. Liu et al. (2021) used noble gas and C/H isotopic analyses to evaluate reservoir compartmentalization in the Wufeng‒Longmaxi organic-rich shale and determined the geologic causation mechanism (e.g., tectonic activity). Most of the aforementioned research has focused on well-developed high-/over-pressured shale reservoirs; however, the accumulation and expulsion mechanisms of normally pressured shale gas reservoirs have received much less attention.

In this study, normally pressured shale gas samples from various structural positions across the southeastern Sichuan Basin were collected, and their alkane gas (methane (CH₄), ethane (C₂H₆), and propane (C₃H₈)) stable isotope (δ¹³C and δD) and noble gas isotope signatures were analyzed. The main objectives of this study were to 1) reveal the coupling relationship between hydrocarbon gas isotope signatures and shale gas yield; 2) understand the origin of noble gas from normally pressured shale gas reservoirs; 3) determine the accumulation and expulsion mechanisms of normally pressured shale gas in the tectonically complex margin of the southeastern Sichuan Basin; and 4) provide insights into the enrichment mechanism of shale gas reservoirs and the prediction of shale gas production capacity in other structurally complex regions of the world.

2 Geological setting

The Sichuan Basin is located in southwest China, northwest of the Yangtze metaplatform; tectonically, it is an important gas-producing sedimentary basin in China. Wufeng‒Longmaxi organic-rich shales are widely distributed across the Sichuan Basin and surrounding regions and are key exploration and evaluation targets for shale gas reservoirs (Dai et al., 2014; Gao et al., 2014; Zhao et al., 2019). Our study area is located in the southeastern Sichuan Basin and comprises a basin-margin transition belt and an extra-basin complex fold belt (Figure 1).

Figure 1

Figure 1. Location and degree of geologic deformation of the study area in the southeastern Sichuan Basin.

The basin-margin transition belt is located between the Sichuan Basin and a complex fold belt lying exterior to the basin; it shows a higher level of tectonic deformation compared with most parts of the Sichuan Basin, but deformation is weaker than that outside of the basin. The region has experienced multiple tectonic phases, including during the Caledonian, Hercynian, Indosinian, and Yanshan‒Himalayan periods, with the latter having the strongest impact. This has laid the foundation for the current structural pattern, dominated by northeast‒southwest-trending synclines and anticlines. From east to west, the basin-margin transition belt includes seven tectonic units: Shimen slope, Shiqiao fault depression, Pingqiao anticline, Yuanjiagou syncline, Dongsheng anticline, Shentongba syncline, and Yangchungou anticline. Wufeng‒Longmaxi organic-rich shales in this basin-margin transition belt are buried under high- to normal-pressure conditions, with pressure coefficients ranging between 1.10 and 1.35.

The extra-basin complex fold belt exhibits an increased degree of uplift-related erosion, with the Longmaxi Formation being almost completely denuded in anticlinal regions. Hence, Longmaxi organic-rich shales mainly occur in residual synclines, such as the Wulong, Sangzeping, and Daozhen synclines. The present burial depth of Longmaxi organic-rich shales in these residual synclines ranges from 1,000 to 3,500 m (He et al., 2019), where they are buried under typical normal-pressure conditions with pressure coefficients ranging from 0.95 to 1.10.

3 Sampling and analytical methodology

3.1 Shale gas samples

Eighteen shale gas samples from the southeastern Sichuan Basin were collected using steel cylinder gas containers with a maximum gas pressure capacity of 15 MPa and a maximum volume of 1,000 mL. To avoid atmospheric contamination, the containers were flushed with wellhead gas at least 10 times before gas collection. The studied shale gas samples were collected from the sweet shale gas layer (Wufeng‒Longmaxi Formation) based on a continuous pressure coefficient series moving from the intra-basin and basin-margin transition belt toward the basin exterior; specifically, they were collected from Pingqiao (PQ), Dongsheng (DS), Jinfo (JF), Wulong (WL), and Pengshui (PS). The pressure coefficients for the Long-1 shale gas from these regions were 1.35 (PQ), 1.20–1.35 (DS), 1.12–1.20 (JF), 1.08 (WL), and 0.98 (PS), respectively.

3.2 Analytical methodology

The chemical compositions of non-hydrocarbon gases (CO₂, H₂, H₂S, N₂, and He) and hydrocarbon gases were examined. Non-hydrocarbon molecular compositions were analyzed using a gas composition mass spectrometer (MAT-271). Hydrocarbon gas compositions were analyzed using a gas chromatograph (GC, Agilent 6890N) equipped with an FID detector. To differentiate each of the individual hydrocarbon gas components (e.g., CH₄, C₂H₆, and C₃H₈), an aluminum oxide capillary column with a length of 50 m and a diameter of 0.53 mm was used. The oven temperature setting for GC analysis was set at 30°C for 10 min, followed by an increase to a target temperature of 180°C at a heating rate of 10°C/min, and then maintained at the target temperature for 20–30 min.

An Agilent 6890N GC equipped with an isotope ratio mass spectrometer (IRMS, Delta plus XP) was used to determine the δ¹³C signatures of individual hydrocarbon gas components. The operating temperature for δ¹³C analysis was set at 30°C for 3 min, followed by an increase to a target temperature of 250°C at a heating rate of 10°C/min, and then maintained at the target temperature for 50 min. To calculate the δ¹³C values of hydrocarbon gases, three pulses of standard pure CO₂ gas were injected into the GC‒IRMS instrument. The overall analytical precision was ±3‰.

A MAT-253 IRMS coupled with an HP6890 GC instrument was used to analyze shale gas H isotopes. CH₄ and C₂H₆ components were chromatographically distinguished using a fused silica capillary column (HP-PLOT Q, 30 m × 0.32 mm × 20 μm). The operating GC temperature for H isotope analysis was set at 40°C for 5 min, followed by an increase to 80°C at a heating rate of 5°C/min, a further increase to 140°C at a heating rate of 10°C/min, and a final increase to a target temperature of 260°C at a heating rate of 30°C/min (Xing et al., 2021). The pyrolysis-oven temperature was set at 1,450°C, and a standard H₂ sample was used as the reference gas. The accuracy of the H isotope analysis was approximately ±3‰. Hydrogen isotope data are reported in δ notation (δD, ‰) relative to Vienna Standard Mean Ocean Water (VSMOW = 0.0‰). An internal standard was measured to calibrate the δD data for the studied shale gas samples (Cao et al., 2020).

Abundances and isotopic compositions of noble gases (He, Ne, and Ar) were measured using a noble gas isotope mass spectrometer (Noblesse SFT). The shale gas sample container was first connected to the purification vacuum system. After the system reached vacuum, the gas sample was introduced into a gas pipette with a volume of 8.5 cm³. According to the pressure and temperature variations in the gas pipette before and after the gas sample introduction, the absolute amount of gas sample introduced was calculated. Then, 1.5 cm³ of the gas sample was further introduced to the online noble gas isotope mass spectrometer for noble gas purification. Details of the analytical procedure for noble gas abundance and isotope composition determination can be found in Cao et al. (2018).

4 Results

4.1 Shale gas yields

Shale gas yields from the basin-margin transition belt and extra-basin complex fold belt vary markedly. Considering that drilling wells in different structural zones are currently in different stages of production, daily shale gas production values for the first 500 production days were compared herein (Table 1; Figure 2). Compared with the highly pressured Jiaoshiba shale gas production region (e.g., 20.3 × 10⁴ m³/day for well JY1), daily shale gas production values in the study area were much lower. Shale gas production in the PQ area was the highest, ranging from 4.19 × 10⁴ to 6.55 × 10⁴ m³/day (average: 5.55 × 10⁴ m³/day). The average daily shale gas production values for the DS and JF areas were 3.32 × 10⁴ and 3.40 × 10⁴ m³/day, respectively. Shale gas production in the extra-basin complex fold belt was much lower, with average values of 2.15 × 10⁴ m³/day for the WL area and 0.77 × 10⁴ m³/day for the PS area. In general, daily shale gas production exhibited a decreasing trend with a decreasing pressure coefficient across the study area.

Table 1

Table 1. Chemical and carbon isotope (δ¹³C) compositions of normally pressured shale gas samples in the southeastern Sichuan Basin.

Figure 2

Figure 2. Variation in gas composition in normally pressured Longmaxi shale gas from the southeastern Sichuan Basin.

4.2 Major gas compositions

Shale gas compositions from 18 wells within the Wufeng‒Longmaxi Formation in the southeastern Sichuan Basin are shown in Table 1. All samples were dominated by CH₄, with minor amounts of C₂H₆ and non-hydrocarbon gases (e.g., N₂, CO₂, and He). The CH₄ content ranged from 95.99% to 98.8%, with an average of 97.2%; the C₂H₆ content ranged from 0.28% to 0.99%, with an average of 0.48%; and the C₃H₈ content in all shale gas samples was very low, with an average of 0.01%. Non-hydrocarbon gases in the studied shale gas samples mainly consisted of N₂ (0.76%–3.11%, average: 1.39%) and CO₂ (0.01%–0.321%, average: 0.16%). The non-hydrocarbon gases He and Ar occurred in trace amounts in the shale gas samples and were detected using more precise noble gas mass spectrometry (the results are presented in Section 4.4).

4.3 Carbon and hydrogen isotope signatures of alkane gases

The δ¹³C signatures of alkane gases varied among shale gas samples collected from different areas. The δ¹³C₁ values ranged from −36.5‰ to −29.6‰, with average values of −34.3‰, −33.8‰, −33.1‰, −30.7‰, and −30.8‰ for the PQ, DS, JF, WL, and PS areas, respectively. The δ¹³C₂ values ranged from −38.2‰ to −33.4‰, with average values of −37.1‰, −37.4‰, −36.2‰, −30.7‰, and −33.4‰ for the PQ, DS, JF, WL, and PS areas, respectively. The δ¹³C signatures of alkane gases exhibited an increasing trend with a decreasing pressure coefficient. In contrast to the δ¹³C data, the δD signatures of alkane gases showed no evident pattern of change with a decreasing pressure coefficient, with δDC₁ ranging from −162.2‰ to −165.4‰ and δDC₂ ranging from −166.1‰ to −135.1‰.

4.4 Noble gas compositions and isotopes

He, Ne, and Ar concentrations and isotopic data from the 18 studied Longmaxi shale gas samples are presented in Table 2. The He concentration ranged from 383.6 to 976.9 ppm, with an average of 550.3 ppm. The He concentration varied among shale gas samples collected from different areas, showing an increasing trend with a decreasing pressure coefficient moving from the intra-basin toward the basin exterior. The average He contents of shale gas from the PQ, DS, JF, WL, and PS areas were 551.0, 446.2, 444.8, 530.5, and 954.3 ppm, respectively. Ne and Ar concentrations had ranges of 0.01–0.278 and 26–96 ppm, respectively.

Table 2

Table 2. Noble gas composition and isotopic data from normally pressured shale gas samples in the southeastern Sichuan Basin.

³He/⁴He ratios ranged from 0.0045 to 0.013 Ra, where Ra is the atmospheric ratio of (1.384 ± 0.013) × 10^–6 (Ozima and Podosek, 2017; Mishima et al., 2018). ²⁰Ne/²²Ne ratios (9.3–11.7, average: 10.6) were higher than the atmospheric value (9.80; Sarda et al., 1988) and lower than the mantle value (12.2; Ballentine et al., 2005). ⁴He/²⁰Ne ratios ranged from 2,425.8 to 75,716.2, much higher than the atmospheric value of 0.288 (Kipfer et al., 2002). Most ²¹Ne/²²Ne ratios (0.028–0.039, average: 0.033) were higher than the atmospheric value (0.02959; Gyore et al., 2019), indicating a mixed source of two end-members: crustal (0.03–0.70; Ozima and Podosek, 2017) and atmospheric Ne. The ⁴⁰Ar/³⁶Ar ratios of the studied shale gas samples varied from 730.1 to 1,648.2, with an average of 1,286.7; these ratios are much higher than those of air, indicating no apparent noble gas contamination from the atmosphere during shale gas sampling and analysis.

5 Discussion

5.1 Coupling relationship between alkane gas isotopes and shale gas yield

The origin of Longmaxi shale gas has been widely discussed in previous studies (Hao et al., 2013; Dai et al., 2014; Wu et al., 2017; Cao et al., 2018; Liu et al., 2021). Methane is the dominant component in the high-/over-pressured Longmaxi shale gas produced (Dai et al., 2014; Liu et al., 2021). Normally pressured shale gas samples in the studied southeastern margin of the Sichuan Basin show similar characteristics, with high CH₄ abundance and high gas dryness ratios [C₁/(C₂ + C₃)] (94.3–339.7, avg. 216.8). The δDC₁ values of normally pressured shale gas have a similar distribution range to those of Wufeng‒Longmaxi shale gas from Weiyuan and Changning, as reported by Dai et al. (2014). Plots of δ¹³C₁ versus [C₁/(C₂ + C₃)] (Figure 3A) and δ¹³C₁ versus δDC₁ (Figure 3B) both suggest that the Longmaxi shale gas is of late-mature thermogenic origin. A kinetic isotope effect resulting from differences in the energy required to cleave ¹²C\¹²C and ¹²C\¹³C bonds in shale gas precursors has been reported (Tang et al., 2000). Such a kinetic isotope effect results in a more positive δ¹³C composition in hydrocarbons at higher maturity (Xia et al., 2013).

Figure 3

Figure 3. Gas molecular and stable isotope composition plots of normally pressured Longmaxi shale gas from the southeastern Sichuan Basin, along with other shales. (A) [C₁/(C₂ + C₃)] vs δ¹³C₁. (B) δ¹³C₁ vs δDC₁. (C) Isotopic composition of C₂H₆ exhibiting a reversal trend with increasing maturity. Fayetteville and Barnett shale gas data and genetic diagrams are adapted from Milkov et al. (2020), while previously studied shale gas data are adapted from Dai et al. (2014). Abbreviations: SM, secondary microbial; EMT, early-mature thermogenic gas; OA, oil-associated (mid-mature) thermogenic gas; LMT, late-mature thermogenic gas.

The δ¹³C₁ and δ¹³C₂ values of our normally pressured shale gas samples are plotted in Figure 3C, together with those of Barnett (Milkov et al., 2020), Fayetteville (Milkov et al., 2020), and Wufeng‒Longmaxi (Dai et al., 2014) shale gas samples studied previously, in which the dashed line represents the relationship δ¹³C₁ = δ¹³C₂. Above the dashed line, a normal δ¹³C trend exists, characterized by δ¹³C₁ < δ¹³C₂. The turning point in this normal trend area has been suggested to represent the start of secondary hydrocarbon cracking and mixing of gas (primary and secondary cracking), which would likely cause a δ¹³C reversal among alkane gases (Tilley and Muehlenbachs, 2013; Xia et al., 2013). Shale gas with a normal δ¹³C trend is mostly related to the low‒middle stage of maturity. As maturity increases, a reversal in the trend (δ¹³C₁ > δ¹³C₂) can be observed. A further turning point in this reversed trend area has been suggested to represent a post-reversal stage at an extremely high or over-mature stage of evolution (Dai et al., 2014). This was confirmed by Xia et al. (2013) when applying a δ¹³C correlated with the maturity model and by Wu et al. (2017) when studying even higher-maturity Lower Cambrian Niutitang shale gas.

For our normally pressured shale gas, δ¹³C and δD data suggest that the shale gas originated from late-mature thermogenic generation, equivalent to shale gas in other production areas of the inner Sichuan Basin. The [C₁/(C₂ + C₃)] data exhibit a negative relationship with δ¹³C₁ and δ¹³C₂ (Figure 4). Shale gas from the basin-margin transition belt (areas PQ, DS, and JF) is characterized by higher [C₁/(C₂ + C₃)] and lighter δ¹³C compared with shale gas samples from the extra-basin complex fold belt (areas WL and PS). The slope trend of decreasing [C₁/(C₂ + C₃)] with heavier δ¹³C matches the decreasing trend of the pressure coefficient for the selected shale gas samples. Meanwhile, daily shale gas yields for the selected production wells exhibit a negative correlation with δ¹³C and a positive correlation with [C₁/(C₂ + C₃)] (Figure 5), indicating that shale gas production wells in the basin-margin transition belt and extra-basin complex fold belt have completely different shale gas yields and gas geochemistry characteristics. The PS, DS, and JF areas have better gas production than WL and PS. The reason for this phenomenon may be that shale gas expulsion efficiency and gas source supply are different in different structural belts.

Figure 4

Figure 4. Correlations between the dryness coefficient (C₁/C_2‒3) and δ¹³C. (A) C₁/C_2‒3 vs δ¹³C₁. (B) C₁/C_2‒3 vs δ¹³C₂.

Figure 5

Figure 5. Correlations between daily shale gas production and hydrocarbon gas geochemistry characteristics. Daily shale gas production vs δ¹³C₁ (A), δ¹³C₂ (B), and dryness coefficient (C₁/C_2‒3) (C).

Meng et al. (2022) studied the CH₄ and C₂H₆ δ¹³C changes in the Triassic Yanchang Formation from the Ordos Basin using pressure coring to avoid the problem of lost gas. During shale gas desorption, both δ¹³C₁ and δ¹³C₂ values became heavier, and gas dryness coefficients (C₁/C_1‒5) decreased with increasing desorption rates. Smaller molecules (e.g., CH₄) desorb more rapidly than longer-chain hydrocarbon gases. For normally pressured shale gas, variations in gas geochemistry characteristics provide evidence for the gas accumulation and expulsion processes during geological time. The WL and PS areas within the extra-basin complex fold belt experienced earlier uplift compared with the PQ, DS, and JF areas within the basin-margin transition belt (Figure 6, modified after Teng et al., 2020). Although shale layers in the WL and PS areas have similar top and bottom cap-rock conditions to those in the inner basin area (e.g., Fulin shale gas field), their relatively poor self-sealing capacity and low displacement pressure mean that an episodic hydrocarbon expulsion process is more likely to occur. More oil and gas escape from these shale layers during the peak hydrocarbon generation period, reducing the potential source supply for later shale gas generation during higher maturity stages. During expulsion, hydrocarbons with a lighter δ¹³C escape first, leaving the residual hydrocarbons richer in ¹³C. As the pressure coefficient decreases, the hydrocarbon expulsion efficiency increases and residual gas abundance is reduced. Therefore, the PS shale gas samples are characterized by the heaviest δ¹³C values, lowest gas dryness coefficients, and lowest daily gas yields among all studied shale gas samples.

Figure 6

Figure 6. Burial history, thermal history, and paleo-pressure of the Wufeng‒Longmaxi Formation in the (A) basin-margin transition belt and (B) extra-basin complex fold belt.

The H atom is much smaller than the C atom; hence, H isotope fractionation occurs more readily during shale gas expulsion or desorption. Notably, unlike δ¹³C, the δD values of CH₄ and C₂H₆ show little variation with changes in pressure coefficient across all studied shale gas samples. A previous study suggested that δD values in alkane gases desorbed from pressure coring remained stable with increasing gas desorption rates (Meng et al., 2022).

5.2 Origin of noble gases in a normally pressured shale gas system

In subsurface fluid systems, noble gases can have atmospheric, mantle, or radiogenic origins, with distinct isotopic signatures associated with each component (Ballentine et al., 2005). According to its origin, there are three types of He: atmospheric, mantle-derived, and crust-derived. Helium has two isotopes: primordial ³He and ⁴He sourced from the radioactive decay of ²³⁸U, ²³⁵U, and ²³²Th over geological time (Graham, 2002). The origin of He is usually determined using the ratio of the isotopic abundances of ³He and ⁴He, i.e., R = ³He/⁴He. Atmospheric He is ubiquitous in the air, showing no industrial significance, owing to its low concentration. The R value for atmospheric helium is 1.4 × 10⁶, normally referred to as Ra. Mantle-derived He refers to He of deep magmatic origin, including the original ³He captured in the early stage of the formation of the Earth, and ⁴He produced by radioactive decay, with a high R ratio of approximately 8 Ra. Crust-derived He is mainly sourced from α radioactive decay of U and Th in ancient sedimentary rocks, metamorphic rocks, migmatites, and granites. This crust-derived He, with a low R ratio of approximately 0.02 Ra, is an important source of He input in natural gas reservoirs (Oxburgh et al., 1986; Ballentine and Lollar, 2002; Kennedy and van Soest, 2005; Wang et al., 2020). As shown in a plot of R/Ra vs ⁴He/²⁰Ne (Figure 7A), the ³He/⁴He ratios (R/Ra) of our normally pressured shale gas samples range from 0.0045 to 0.013 Ra, all plotted within the crust end-member region. A plot of CH₄/³He vs R/Ra (Figure 7B) also indicates that the studied shale gas samples lie in the crustal field, suggesting that the Longmaxi shale gas received a substantial contribution of crust-derived He; this is consistent with the Weiyuan and Changning shale gas samples studied by Cao et al. (2020).

Figure 7

Figure 7. (A) Plot of ³He/⁴He (R/Ra) vs ⁴He/²⁰Ne showing mixing lines between the atmosphere and upper mantle and between the atmosphere and crust. Ratios of the atmosphere, continental crust, and upper mantle end-members are obtained from Ozima and Podosek (2017), Ballentine and Burnard (2002), and Graham (2002). (B) Plot of CH₄/³He vs ³He/⁴He (R/Ra) demonstrating a crustal origin for noble gases.

Argon has three isotopes, namely, ⁴⁰Ar, ³⁸Ar, and ³⁶Ar, with ⁴⁰Ar mainly sourced from the radioactive decay of K, and ³⁸Ar and ³⁶Ar of primordial origin (Ballentine and Burnard, 2002). The ⁴⁰Ar/³⁶Ar ratios of our shale gas samples are relatively high (Figure 8; ranging from 1,194.3 to 4,604.5), indicating a radiogenic origin consistent with that suggested by the He isotope data. The ⁴⁰Ar/³⁶Ar ratios of our shale gas samples exhibit similar values to those of Jiaoshiba shale gas but are higher than those of Changning shale gas samples from the southern Sichuan Basin (Wang et al., 2022).

Figure 8

Figure 8. Plots of (A) ³He/⁴He (R/Ra) vs ⁴⁰Ar/³⁶Ar and (B) ⁴⁰Ar/³⁶Ar vs 1/³⁶Ar.

5.3 Shale gas accumulation and expulsion reflected in noble gas isotope signatures

Compared with conventional natural gas reservoirs, the enrichment and preservation of noble gas and the controlling factors of the formation of He-rich natural gas reservoirs show some similarities and also differences. Regions with high hydrocarbon generation intensity that are conducive to the formation of large oil and gas reservoirs may not be conducive to the formation of He-rich natural gas reservoirs. However, tectonic uplift and denudation areas with relatively low hydrocarbon generation intensity may be favorable for forming He-rich natural gas. The same applies to shale gas reservoirs, in which self-generation and self-storage occur. Excessive hydrocarbon generation intensity greatly dilutes the concentration of He. Therefore, for self-generated and self-stored He-rich shale gas reservoirs, despite the high content of U and Th in shale, it is difficult to achieve commercial development of He, owing to the dilution effect of hydrocarbon gases (Wang et al., 2020). The He concentration of Wufeng‒Longmaxi shale gas is relatively low in the high-yield Jiaoshiba and Changning areas, with average concentrations of 205 and 376 ppm, respectively. However, in extra-basin areas that are structurally complex, the He concentration can reach 941 ppm (Wang et al., 2022).

The spatial distributions of He and Ar concentrations have been suggested to be closely related to the accumulation and dissipation of shale gas (Wang et al., 2022). Owing to mixing and dilution, the concentration of He decreases with an increase in the shale gas content. Compared with shale gas development areas in a structurally stable basin, such as Changning and Jiaoshiba, the preservation conditions of shale gas in structurally complex areas outside the basin, such as Pengshui in southeastern Chongqing, are relatively poor. At the over-mature evolution stage, owing to the continuous decay of U and Th releasing ⁴He, higher He contents are found in structurally complex areas outside the basin compared with inner basin areas with high shale gas production.

As shown in Figure 9, the He and Ar concentrations of shale gas samples from the basin-margin transition belt (PQ, DS, and JF areas) are lower than those of the extra-basin complex fold belt (WL and PS areas). The He and Ar concentrations of PS shale gas samples in this study are consistent with previous data (He: 716–941 ppm, Ar: 42–56 ppm; Wang et al., 2022). The contents of ⁴He and ⁴⁰Ar show a weak positive correlation (Figure 10A), indicating a radiogenic contribution for both noble gases.

Figure 9

Figure 9. Noble gas abundances and gas yield distribution for the normally pressured shale gas studied. (A) He abundance. (B) Ar abundance.

Figure 10

Figure 10. Plots of (A) ⁴He vs ⁴⁰Ar content and (B) ⁴He vs gas yield.

The He content exhibits a negative correlation with daily shale gas yield for the studied shale gas wells (Figure 10B). High-gas yield wells located in structural regions with higher in situ stress and higher pressure coefficients have low concentrations of He and Ar. As an unconventional gas, shale gas has the characteristics of self-generation and self-storage. A large amount of gas can be generated during the hydrocarbon gas generation stage. However, the abundance of noble gas (He and Ar) source elements, such as U, Th, and K, is very low in geological bodies, with an extremely long half-life and a very low rate of He or Ar generation (1 m³ km−3 year−¹). Furthermore, there is no peak of concentrated noble gas generation during the generation and storage processes. After the termination of the hydrocarbon generation process, the concentrations of He and Ar will inevitably be diluted with increasing CH₄ concentrations (Li et al., 2022).

The correlations between He and δ¹³C are shown in Figure 11. The ⁴He content exhibits positive correlations with both δ¹³C₁ and δ¹³C₂, with WL and PS shale gas samples having the highest ⁴He contents and heaviest δ¹³C signatures. The PQ and DS shale gas samples have the lowest ⁴He contents and the lightest δ¹³C signatures, with JF shale gas samples having intermediate values.

Figure 11

Figure 11. Correlations between ⁴He abundance and δ¹³C. (A) ⁴He vs δ¹³C₁. (B) ⁴He vs δ¹³C₂.

The above coupled noble gas and stable isotope analyses reveal that the concentrations of He and Ar and δ¹³C signatures of alkane gases are closely related to the accumulation and expulsion of shale gas. Generally, with an increasing pressure coefficient, shale gas preservation conditions become better, and high shale gas yields are attained, which will dilute the concentration of noble gases. Furthermore, better preservation conditions equate to less shale gas escaping during the gas generation period, causing δ¹³C values to remain closer to their initial values. In contrast, shale gas preserved under poor conditions loses lighter hydrocarbons and isotopes, causing decreasing pressure coefficients and residual gases rich in heavy components. For shale gas reservoirs characterized by a wide range of pressure coefficients, such as the normally pressured Marcellus (pressure coefficient between 0.9 and 1.4; Chalmers et al., 2012) and highly pressured Haynesville (pressure coefficient between 1.61 and 2.07; Hammes et al., 2011) shale gas fields in North America, shale gas production exhibits a similar positive correlation with an increasing pressure coefficient to that found herein. Therefore, gas geochemical data may also provide effective indicators for shale gas production evaluation in these regions. Compared with tectonically stable regions, which exhibit fewer differences in gas geochemical characteristics, the noble gas and stable isotope distribution trends presented herein for tectonically active regions will be more effective in evaluating shale gas accumulation and expulsion in shale gas reservoirs worldwide.

6 Conclusion

On the basis of our systematic study of the noble gas and stable isotope characteristics of normally pressured shale gas samples, which are part of a continuous pressure coefficient series from the southeastern Sichuan Basin, our preliminary conclusions are as follows:

(1) Stable δ¹³C and δD data revealed that the shale gas originated from late-mature thermogenic generation, similar to shale gas from other production areas within the inner Sichuan Basin. As the pressure coefficient decreased, the hydrocarbon expulsion efficiency increased and residual gas abundance decreased. Shale gas from the basin-margin transition belt (PQ, DS, and JF areas) was characterized by a higher dryness ratio and a lighter δ¹³C signature than shale gas from the extra-basin complex fold belt (WL and PS areas).

(2) Noble gas isotopes were used to reveal the origin of noble gas in the normally pressured shale gas system studied. The ³He/⁴He ratios (R/Ra) ranged from 0.0045 to 0.013 Ra, and the ⁴⁰Ar/³⁶Ar ratios were relatively high, suggesting that the shale gas was sourced from a typically radiogenic origin.

(3) High-gas-yield wells located in structural areas with higher in situ stress and higher pressure coefficients had low concentrations of He and Ar. The He content showed positive correlations with δ¹³C₁ and δ¹³C₂ values. Shale gas samples from the WL and PS areas were characterized by the highest He contents and heaviest δ¹³C signatures. Our coupled noble gas and δ¹³C and δD data revealed that the concentrations of He and Ar and δ¹³C signatures of alkane gases are closely related to the accumulation and expulsion of shale gas.

(4) In tectonically stable regions, shale gas reservoirs normally exhibit favorable conditions with only relatively small differences in gas geochemistry. In tectonically active regions, shale gas geochemical characteristics exhibit substantial differences related to changes in structure. Better preservation conditions accompanied by lower-intensity tectonic movement normally result in higher shale gas production and a lower concentration of noble gases. The above results indicate that gas geochemical characteristics can be used as effective evaluation indicators for determining shale gas accumulation mechanisms in structurally active regions.

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

JL: writing–original draft and writing–review and editing. YW: writing–original draft and writing–review and editing. DF: project administration and writing–review and editing. ZW: writing–review and editing. CW: writing–original draft.

Funding

The author(s) declare that financial support was received for the research, authorship, and/or publication of this article. This work was conducted as part of a project funded by Sinopec East China Oil and Gas Company with grant no. 34600355-23-FW2099-0051.

Acknowledgments

The authors are grateful to Sinopec East China Oil and Gas Company for access to samples and associated data. They are sincerely grateful to the Chief Editor, Valerio Acocella, and the reviewers who provided valuable comments.

Conflict of interest

Authors JL, YW, and DF were employed by Sinopec East China Oil and Gas 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.

References

Ballentine, C. J., and Burnard, P. G. (2002). Production, release and transport of noble gases in the continental crust. Rev. Mineral. Geochem. 47, 481–538. doi:10.2138/rmg.2002.47.12

CrossRef Full Text | Google Scholar

Ballentine, C. J., and Lollar, B. S. (2002). Regional groundwater focusing of nitrogen and noble gases into the Hugoton-Panhandle giant gas field, USA. Geochem. Cosmochim. Acta 66, 2483–2497. doi:10.1016/S0016-7037(02)00850-5

CrossRef Full Text | Google Scholar

Ballentine, C. J., Marty, B., and Sherwood, L. B. (2005). Neon isotopes constrain convection and volatile origin in the Earth's mantle. Nature 433, 33–38. doi:10.1038/nature03182

PubMed Abstract | CrossRef Full Text | Google Scholar

Burnard, P., Zimmermann, L., and Sano, Y. (2013). “The noble gases as geochemical tracers: history and background,” in The noble gases as geochemical tracers. Advances in isotope geochemistry. Editor P. Burnard (Berlin, Heidelberg: Springer), 1–15. doi:10.1007/978-3-642-28836-4_1

CrossRef Full Text | Google Scholar

Byrne, D. J., Barry, P. H., Lawson, M., and Ballentine, C. (2018). Determining gas expulsion vs retention during hydrocarbon generation in the Eagle Ford Shale using noble gases. Geochim. Cosmochim. Acta 241, 240–254. doi:10.1016/j.gca.2018.08.042

CrossRef Full Text | Google Scholar

Byrne, D. J., Barry, P. H., Lawson, M., and Ballentine, C. (2020). The use of noble gas isotopes to constrain subsurface fluid flow and hydrocarbon migration in the East Texas Basin. Geochim. Cosmochim. Acta 268, 186–208. doi:10.1016/j.gca.2019.10.001

CrossRef Full Text | Google Scholar

Byrne, D. J., Barry, P. H., Lawson, M., and Ballentine, C. J. (2017). Noble gases in conventional and unconventional petroleum systems. Geol. Soc. Lond., Spec. Publ. 468, 127–149. doi:10.1144/sp468.5

CrossRef Full Text | Google Scholar

Cao, C., Zhang, M., Li, L., Wang, Y., Li, Z., Du, L., et al. (2020). Tracing the sources and evolution processes of shale gas by coupling stable (C, H) and noble gas isotopic compositions: cases from Weiyuan and Changning in Sichuan Basin, China. J. Nat. Gas. Sci. Eng. 78, 103304. doi:10.1016/j.jngse.2020.103304

CrossRef Full Text | Google Scholar

Cao, C., Zhang, M., Tang, Q., Yang, Y., Lv, Z., Zhang, T., et al. (2018). Noble gas isotopic variations and geological implication of Longmaxi shale gas in Sichuan Basin, China. China. Mar. Petrol. Geol. 89, 38–46. doi:10.1016/j.marpetgeo.2017.01.022

CrossRef Full Text | Google Scholar

Chalmers, G., Bustin, R., and Power, I. (2012). Characterization of gas shale pore systems by porosimetry, pycnometry, surface area, and field emission scanning electron microscopy/transmission electron microscopy image analyses: examples from the Barnett, Woodford, Haynesville, Marcellus, and Doig units. Aapg Bull. 96, 1099–1119. doi:10.1306/10171111052

CrossRef Full Text | Google Scholar

Chen, Z., Chen, L., Wang, G., Zou, C., Jiang, S., Si, Z., et al. (2020). Applying isotopic geochemical proxy for gas content prediction of Longmaxi shale in the Sichuan Basin, China. China. Mar. Petrol. Geol. 116, 104329. doi:10.1016/j.marpetgeo.2020.104329

CrossRef Full Text | Google Scholar

Curtis, J. B. (2002). Fractured shale-gas systems. AAPG Bull. 86, 1921–1938. doi:10.1306/61EEDDBE-173E-11D7-8645000102C1865D

CrossRef Full Text | Google Scholar

Dai, J., Zou, C., Liao, S., Dong, D., Ni, Y., and Huang, J. (2014). Geochemistry of the extremely high thermal maturity Longmaxi shale gas, southern Sichuan Basin. Org. Geochem. 74, 3–12. doi:10.1016/j.orggeochem.2014.01.018

CrossRef Full Text | Google Scholar

Darabi, H., Ettehad, A., Javadpour, F., and Sepehrnoori, K. (2012). Gas flow in ultra-tight shale strata. J. Fluid. Mech. 710, 641–658. doi:10.1017/jfm.2012.424

CrossRef Full Text | Google Scholar

Dong, D., Zou, C., Dai, J. X., Huang, S., Zheng, J., Gong, J., et al. (2016). Suggestions on the development strategy of shale gas in China. J. Nat. Gas. Geosci. 1, 413–423. doi:10.1016/j.jnggs.2016.11.011

CrossRef Full Text | Google Scholar

Gao, L., Schimmelmann, A., Tang, Y., and Mastalerz, M. (2014). Isotope rollover in shale gas observed in laboratory pyrolysis experiments: insight to the role of water in thermogenesis of mature gas. Org. Geochem. 68, 95–106. doi:10.1016/j.orggeochem.2014.01.010

CrossRef Full Text | Google Scholar

Graham, D. W. (2002). Noble gas isotope geochemistry of mid-ocean ridge and ocean island basalts: characterization of mantle source reservoirs. Rev. Mineral. Geochem. 47, 247–317. doi:10.2138/rmg.2002.47.8

CrossRef Full Text | Google Scholar

Guo, C., Xu, J., Wu, K., Wei, M., and Liu, S. (2015). Study on gas flow through nano pores of shale gas reservoirs. Fuel 143, 107–117. doi:10.1016/j.fuel.2014.11.032

CrossRef Full Text | Google Scholar

Guo, T. (2016). Key geological issues and main controls on accumulation and enrichment of Chinese shale gas. Petrol. explor. Dev. 43, 349–359. doi:10.1016/S1876-3804(16)30042-8

CrossRef Full Text | Google Scholar

Guo, T., Jiang, S., and Zhang, P. (2020). Progress and direction of exploration and development of normally-pressured shale gas from the periphery of Sichuan Basin. J. Petroleum Geol. Exp. 42, 837–845. doi:10.11781/sysydz202005837

CrossRef Full Text | Google Scholar

Györe, D., Tait, A., Hamilton, D., and Stuart, F. M. (2019). The formation of NeH⁺ in static vacuum mass spectrometers and re-determination of ²¹Ne/²⁰Ne of air. Geochim. Cosmochim. Acta 263, 1–12. doi:10.1016/j.gca.2019.07.059

CrossRef Full Text | Google Scholar

Hammes, U., Hamlin, H., and Ewing, T. (2011). Geologic analysis of the upper jurassic Haynesville shale in east Texas and west Louisiana. J. Biol. Chem. 95, 1643–1666. doi:10.1306/02141110128

CrossRef Full Text | Google Scholar

Hao, F., Zou, H., and Lu, Y. (2013). Mechanisms of shale gas storage: implications for shale gas exploration in China. AAPG Bull. 97, 1325–1346. doi:10.1306/02141312091

CrossRef Full Text | Google Scholar

He, X. (2021). Sweet spot evaluation system and enrichment and high yield influential factors of shale gas in Nanchuan area of eastern Sichuan Basin. Nat. Gas. Ind. 41, 59–71. doi:10.3787/j.issn.1000-0976.2021.01.005

CrossRef Full Text | Google Scholar

He, X., Qi, Y., and He, G. (2019). Further understanding of main controlling factors of normal pressure shale gas enrichment and high yield in the area with complex structure of the southeast area of Chongqing. Reserv. Eval. Dev. 5, 32–39. doi:10.13809/j.cnki.cn32-1825/te.2019.05.004

CrossRef Full Text | Google Scholar

Hu, Q., Liu, X., Gao, Z., Liu, S. G., Zhou, W., and Hu, W. X. (2015). Pore structure and tracer migration behavior of typical American and Chinese shales. Petrol. Sci. 12, 651–663. doi:10.1007/s12182-015-0051-8

CrossRef Full Text | Google Scholar

Jiang, S., Li, C., and Chen, G. (2022). Occurrence of normally-pressured shale gas in China and the United States and their effects on mobility and production: a case study of southeast Sichuan Basin and Appalachia Basin. Petroleum Reserv. Eval. Dev. 12, 399–406. doi:10.13809/j.cnki.cn32-1825/te.2022.03.001

CrossRef Full Text | Google Scholar

Jiang, Z., Song, Y., Tang, X., Li, Z., Wang, X., Wang, G., et al. (2020). Controlling factors of marine shale gas differential enrichment in southern China. Petrol. explor. Dev. 47, 661–673. doi:10.1016/S1876-3804(20)60083-0

CrossRef Full Text | Google Scholar

Kennedy, B. M., and van Soest, M. C. (2005). A helium isotope perspective on the Dixie Valley, Nevada, hydrothermal system. Geothermics 35, 26–43. doi:10.1016/j.geothermics.2005.09.004

CrossRef Full Text | Google Scholar

Kipfer, R., Aeschbach-Gertig, W., Peeteres, F., and Stute, M. (2002). Noble gases in lakes and ground waters. Rev. Mineral. Geochem. 47, 615–700. doi:10.2138/rmg.2002.47.14

CrossRef Full Text | Google Scholar

Lancaster, D. E., Guldry, F. K., Graham, R. L., Curtis, J., Shaw, J., and Blake, T. (1989). A case study of the evaluations completion and testing of a Devonian shale gas well. J. Petrol. Technol. 41, 509–518. doi:10.2118/17037-PA

CrossRef Full Text | Google Scholar

Li, Y., Li, J., and Zhou, J. (2022). Research progress and new views of evaluation of helium resources. J. Sci. Environ. 44, 363–373. doi:10.19814/j.jese.2021.11008

CrossRef Full Text | Google Scholar

Liu, R., Wen, T., Julien, A., Zheng, J., Hao, F., and Jiang, D. (2021). The dichotomy in noble gas signatures linked to tectonic deformation in Wufeng-Longmaxi Shale, Sichuan Basin. Chem. Geol. 581, 120412. doi:10.1016/j.chemgeo.2021.120412

CrossRef Full Text | Google Scholar

Ma, X., Li, X., Liang, F., Wan, Y., Shi, Q., Wang, Y., et al. (2020). Dominating factors on well productivity and development strategies optimization in Weiyuan shale gas play, Sichuan Basin, SW China. Petrol. explor. Dev. 47, 594–602. doi:10.1016/S1876-3804(20)60076-3

CrossRef Full Text | Google Scholar

Meng, M., Zhang, Y., Yuan, B., and Li, Z. (2023). Imbibition behavior of oil-saturated rock: implications for enhanced oil recovery in unconventional reservoirs. Energy and Fuels 37 (18), 13759–13768. doi:10.1021/acs.energyfuels.3c02501

CrossRef Full Text | Google Scholar

Meng, Q., Wang, X., Shi, B., Lei, Y., Zhao, H., Zhao, Q., et al. (2022). The ¹³C-depleted methane in terrigenous shale gas: a case study in the Triassic Yanchang Formation, Ordos Basin. Mar. Petrol. Geol. 141, 105688. doi:10.1016/j.marpetgeo.2022.105688

CrossRef Full Text | Google Scholar

Milkov, A. V., Faiz, M., and Etiope, G. (2020). Geochemistry of shale gases from around the world: composition, origins, isotope reversals and rollovers, and implications for the exploration of shale plays. Org. Geochem. 143, 103997. doi:10.1016/j.orggeochem.2020.103997

CrossRef Full Text | Google Scholar

Mishima, K., Sumino, H., Yamada, T., Ieki, S., Nagakura, N., Otono, H., et al. (2018). Accurate determination of the absolute ³He/⁴He ratio of a synthesized helium standard gas (helium standard of Japan, HESJ): toward revision of the atmospheric ³He/⁴He ratio. Geochem. Geophys. Geosyst. 19, 3995–4005. doi:10.1029/2018GC007554

CrossRef Full Text | Google Scholar

Montgomery, S. L., Jarvie, D. M., Bowker, K. A., and Pollastro, R. M. (2006). Mississippian Barnett Shale, Fort Worth basin, north-central Texas: gas-shale play with multi–trillion cubic foot potential. AAPG Bull. 89, 155–175. doi:10.1306/09170404042

CrossRef Full Text | Google Scholar

Oxburgh, E. R., O’Nions, R. K., and Hill, R. I. (1986). Helium isotopes in sedimentary basins. Nature 324, 632–635. doi:10.1038/324632a0

CrossRef Full Text | Google Scholar

Ozima, M., and Podosek, F. A. (2017). Noble gas geochemistry. 2nd edition. New York: Cambridge University Press. doi:10.1017/cbo9780511545986

CrossRef Full Text | Google Scholar

Pollastro, R. M., Jarvie, D. M., Hill, R. J., and Adams, C. W. (2007). Geologic framework of the mississippian Barnett shale, barnett-paleozoic total petroleum system, bend arch–fort worth basin, Texas. AAPG Bull. 91, 405–436. doi:10.1306/10300606008

CrossRef Full Text | Google Scholar

Qiu, Z., Zou, C., and Wang, H. (2020). Discussion on the characteristics and controlling factors of differential enrichment of Wufeng-Longmaxi formations shale gas in south China. Nat. Gas. Geosci. 31, 163–175. doi:10.1016/j.jnggs.2020.05.004

CrossRef Full Text | Google Scholar

Sarda, P., Staudacher, T., and Allegre, C. J. (1988). Neon isotopes in submarine basalts. Earth Planet. Sci. Lett. 91, 73–88. doi:10.1016/0012-821X(88)90152-5

CrossRef Full Text | Google Scholar

Schoell, M. (1988). Multiple origins of methane in the Earth. Chem. Geol. 71 (1), 1–10. doi:10.1016/0009-2541(88)90101-5

CrossRef Full Text | Google Scholar

Tang, Y., Perry, J. K., Jenden, P. D., and Schoell, M. (2000). Mathematical modeling of stable carbon isotope ratios in natural gases. Geochimica Cosmochimica Acta 64, 2673–2687. doi:10.1016/s0016-7037(00)00377-x

CrossRef Full Text | Google Scholar

Teng, G., Tao, C., and Hu, G. (2020). Effect of hydrocarbon expulsion efficiency on shale gas formation and enrichment. Pet. Geol. Exp. 42, 325–334. doi:10.11781/sysydz202003325

CrossRef Full Text | Google Scholar

Tilley, B., and Muehlenbachs, K. (2013). Isotope reversals and universal stages and trends of gas maturation in sealed, self-contained petroleum systems. Chem. Geol. 339, 194–204. doi:10.1016/j.chemgeo.2012.08.002

CrossRef Full Text | Google Scholar

Wang, P., Shen, Z., Liu, S., Lv, Z., Zhu, T., and Gong, Y. (2013). Geochemical characteristics of noble gases in natural gas and their application in tracing natural gas migration in the middle part of the western Sichuan Depression, China. Petrol. Sci. 10, 327–335. doi:10.1007/s12182-013-0281-6

CrossRef Full Text | Google Scholar

Wang, X., Liu, W., Li, X., Liu, Q., Tao, C., and Xu, Y. (2020). Radiogenic helium concentration and isotope variations in crustal gas pools from Sichuan Basin, China. Appl. Geochem. 117, 104586. doi:10.1016/j.apgeochem.2020.104586

CrossRef Full Text | Google Scholar

Wang, X., Liu, W., Li, X., Tao, C., Borjigin, T., Liu, P., et al. (2022). Application of noble gas geochemistry to the quantitative study of the accumulation and expulsion of lower Paleozoic shale gas in southern China. Appl. Geochem. 146, 105446. doi:10.1016/j.apgeochem.2022.105446

CrossRef Full Text | Google Scholar

Whiticar, M. J. (1996). Stable isotope geochemistry of coals, humic kerogens and related natural gases. Int. J. Coal Geol. 32, 191–215. doi:10.1016/S0166-5162(96)00042-0

CrossRef Full Text | Google Scholar

Wu, C. J., Tuo, J. C., Zhang, L. F., Zhang, M., Li, J., Liu, Y., et al. (2017). Pore characteristics differences between clay-rich and clay-poor shales of the Lower Cambrian Niutitang Formation in the Northern Guizhou area, and insights into shale gas storage mechanisms. Int. J. Coal Geol. 178, 13–25. doi:10.1016/j.coal.2017.04.009

CrossRef Full Text | Google Scholar

Xia, X., Chen, J., Braun, R., and Tang, Y. (2013). Isotopic reversals with respect to maturity trends due to mixing of primary and secondary products in source rocks. Chem. Geol. 339, 205–212. doi:10.1016/j.chemgeo.2012.07.025

CrossRef Full Text | Google Scholar

Xia, X. Y., and Tang, Y. C. (2012). Isotope fractionation of methane during natural gas flow with coupled diffusion and adsorption/desorption. Geochem. Cosmochim. Acta 77, 489–503. doi:10.1016/j.gca.2011.10.014

CrossRef Full Text | Google Scholar

Xing, L., Li, Z., Li, L., and Liu, Y. (2021). Application of chromium catalysis technology to compound-specific hydrogen isotope analysis of natural gas samples. Talanta 239, 123133. doi:10.1016/j.talanta.2021.123133

PubMed Abstract | CrossRef Full Text | Google Scholar

Zhang, W., Li, Y., Zhao, F., Han, W., Zhou, J., Holland, G., et al. (2019). Quantifying the helium and hydrocarbon accumulation processes using noble gases in the North Qaidam Basin, China. Chem. Geol. 523, 368–379. doi:10.1016/j.chemgeo.2019.07.020

CrossRef Full Text | Google Scholar

Zhao, W., Jia, A., and Wei, Y. (2020). Progress in shale gas exploration in China and prospects for future development. China Pet. Explor. 25, 31–44. doi:10.3969/j.issn.1672-7703.2020.01.004

CrossRef Full Text | Google Scholar

Zhao, W., Zhang, S., He, K., Zeng, H., Hu, G., Zhang, B., et al. (2019). Origin of conventional and shale gas in Sinian–lower Paleozoic strata in the Sichuan Basin: relayed gas generation from liquid hydrocarbon cracking. AAPG Bull. 103, 1265–1296. doi:10.1306/11151817334

CrossRef Full Text | Google Scholar

Zhou, Z., Ballentine, C. J., Schoell, M., and Stevens, S. H. (2012). Identifying and quantifying natural CO₂ sequestration processes over geological timescales: the Jackson Dome CO₂ Deposit, USA. Geochim. Cosmochim. Acta 86, 257–275. doi:10.1016/j.gca.2012.02.028

CrossRef Full Text | Google Scholar

Zou, C., Yang, Z., Dai, J., Dong, D., Zhang, B., Wang, Y., et al. (2015). The characteristics and significance of conventional and unconventional Sinian–Silurian gas systems in the Sichuan Basin, central China. Mar. Petrol. Geol. 64, 386–402. doi:10.1016/j.marpetgeo.2015.03.005

CrossRef Full Text | Google Scholar

Zou, C., Zhao, Q., and Wang, H. (2021). Theory and technology of unconventional oil and gas exploration and development helps China increase oil and gas reserves and production. Petroleum Sci. Technol. Forum. 40, 72–79. doi:10.3969/j.issn.1002-302x.2021.03.007

CrossRef Full Text | Google Scholar