Shale gas geochemistry in the Sichuan Basin, China
Introduction
Shale gas refers to natural gas occurring in dark organic-rich shale systems with very low porosity and permeability, which has advantages such as large resource potential and long production life (Curtis, 2002; Zou et al., 2016, Zou et al., 2019; Milkov, 2021). As a hotspot of unconventional resource exploration in the world, the United States surpassed Russia in natural gas production for the first time in 2012, greatly promoting the country's energy independence (Melikoglu, 2014; Milkov et al., 2020). After more than a decade of efforts, China has evaluated onshore shale gas resources, selected favorable areas, and identified approximately 110 × 1012 m3 of geological resources (Dai et al., 2016; Liu et al., 2018, Liu et al., 2019; Zou et al., 2016, Zou et al., 2019), making China the second largest shale gas producing region in the world (Zou et al., 2019).
Currently, China's shale gas exploration rights cover an area of >17 × 104 km2, including five National Demonstration Areas (NDAs) set up by the National Energy Administration, namely the Changning, Zhaotong, Fuling and Weiyuan areas in the Sichuan Basin of South China, where the producing strata are the marine-hosted shales of the Late Ordovician–Early Silurian Wufeng (O3w) and Longmaxi (S1l) formations (hereinafter referred to as Longmaxi Formation) (Zou et al., 2016, Zou et al., 2019; Fig. 1), which are the most abundant and favorable targets at present (Dai et al., 2016; Guo and Zeng, 2017; Feng et al., 2018, Feng et al., 2019; Liu et al., 2018, Liu et al., 2019; Zou et al., 2019; Feng et al., 2020; Wang et al., 2021), and the Yanchang area in the Ordos Basin of North China, aiming at the lacustrine-hosted shale of the Middle to Late Triassic Yanchang Formation (T3y) (Dai et al., 2016; Zou et al., 2019).
Shale gas in North America is derived from biogenic (New Albany shale, Osborn and McIntosh, 2010; Strąpoć et al., 2010), and thermogenic causes (Queenston shale, Jenden et al., 1993; Marcellus shale, Osborn and McIntosh, 2010; Conasauga shale, Pashin et al., 2012; Barnett and Fayetteville shale, Zumberge et al., 2012; Horn River shale, Tilley et al., 2011; Tilley and Muehlenbachs, 2013). The majority of shale gas in China is thermogenic, and its formation usually goes through two processes: primary cracking of kerogen and secondary cracking of retained oil and wet gases (Tissot, 1987; Galimov, 2006; Dai et al., 2014; Liu et al., 2018; Milkov, 2021).
Shale gas has special geochemical characteristics, and there are universal carbon isotope anomalies (rollover and reversal phenomena) in shale gas with high and over-maturity (Zumberge et al., 2012; Hao and Zou, 2013; Tilley and Muehlenbachs, 2013; Dai et al., 2014, Dai et al., 2016; Feng et al., 2016a; Liu et al., 2018; Milkov and Etiope, 2018, Milkov and Etiope, 2019; Liu et al., 2021a, Liu et al., 2021b; Milkov, 2021), that is, the distribution of carbon isotope composition is not the characteristics of primary organic gas (δ13C1 < δ13C2 < δ13C3) but is partially reversed (δ13C1 > δ13C2; δ13C2 > δ13C3), and there is even a negative carbon isotope series (δ13C1 > δ13C2 > δ13C3). These abnormalities are often associated with the high yield and overpressure of shale gas (Ferworn et al., 2008; Zumberge et al., 2012; Hao et al., 2013; Milkov, 2021), or related to the sealing ability of the system (Tilley et al., 2011; Tilley and Muehlenbachs, 2013). In addition, due to the characteristics of self-generation and self-storage, shale gas can better reflect the geochemical characteristics generated under geological pressure and temperature, which is an important supplement to the traditional method of gas source identification, and is also significant for the prediction of shale gas production capacity (Dai et al., 2014; Dai et al., 2016; Liu et al., 2018; Zhang et al., 2018; Feng et al., 2020).
The success of the “shale gas revolution” in a sense promotes the study of shale gas geochemistry. As early as 1990, Johnson and Rice (1990) studied the gas source types in the Piceane Basin and used the gas occurring in the thin interbeddings of fine siltstone in the Mancos shale to represent the Mancos shale gas (Johnson and Rice, 1990; Curtis, 2002). Jenden et al. (1993) reported the geochemical characteristics of shale gas produced from Marcellus, Queenston and Upper Devonian organic-rich shales in the Appalachian Basin and were the first to observe the phenomenon of carbon isotope reversal in over-mature shale gas. The record of obtaining industrial gas flow in shale strata in China can be traced back to 1966, when Well Wei 5 in the Weiyuan area obtained gas from the Cambrian shale (Dai et al., 1992). However, not enough attention was paid to it until 2014 when Dai et al. (2014) analyzed the geochemical characteristics of the Silurian shale gas in the Sichuan Basin for the first time. Due to the relatively old age, high maturity, and complex geological conditions of the Longmaxi Formation, it is necessary to systematically summarize the special geochemical characteristics and enrichment rules of shale gas during the evolution process and establish a practical exploration and evaluation system.
In this review, a large amount of published shale gas geochemical data were compiled including data from the Silurian (Longmaxi Formation), specifically from the Changning-Zhaotong area in the southern periphery of the Sichuan Basin (Dai et al., 2014; Dai et al., 2016; Liu et al., 2018; Feng et al., 2019), Weiyuan area (Feng et al., 2018), and Fuling area (Liu et al., 2018, Liu et al., 2019; Feng et al., 2020), and the Triassic Yanchang Formation in the Ordos Basin (Dai et al., 2016) and geochemical data of typical shale gas in North America. The geochemical data included gas composition, carbon and hydrogen isotope composition, light hydrocarbon characteristics, and noble gases. The data sources are shown in Table 1, and then the specific data details for each gas sample are presented in the supplementary material.
Section snippets
Organic-rich shales in the Sichuan Basin
The Sichuan Basin is a large diamond-shaped superimposed basin with a sedimentary area of nearly 18 × 104 km2 (Guo and Zeng, 2017; Zou et al., 2019; Feng et al., 2020) (Fig. 1). As the earliest area to develop and utilize natural gas in China, the Sichuan Basin has taken the forefront in shale gas exploration with the successive discovery of natural gas in the Sinian-Jurassic multilayer system (Dai et al., 1992, Dai et al., 2014, Dai et al., 2016; Zou et al., 2019). In addition to the
Chemical composition of alkane gases
The over-mature Longmaxi shale gas is dominated by alkane gas components (>99%), which almost entirely (98.7%) consist of methane (CH4). Specifically, the average CH4 contents of the gases from the Changning-Zhaotong, Fuling, and Weiyuan areas are 98.8%, 98.7%, and 98.2%, respectively, and the average contents of C2H6 are all about 0.5% (Dai et al., 2014, Dai et al., 2016; Liu et al., 2018, Liu et al., 2019; Feng et al., 2018, Feng et al., 2019, Feng et al., 2020). The C3H8 content in various
Carbon isotope composition of alkane gas
Dai et al. (1992) and Rooney et al. (1995) argued that the carbon isotope composition of methane is closely related to maturity, which mainly depends on the extent of C2H6 (and higher hydrocarbons) conversion and the initial proportion of the alkane gas (Hao and Zou, 2013; Xia and Gao, 2017). The δ13C1 values of the Longmaxi shale gas in each NDA are generally heavy and are distributed in the order of maturity (Dai et al., 2016a; Feng et al., 2019, Feng et al., 2020). Specifically, they range
Noble gases
Noble gases are chemically inert and have different isotope compositions in the atmosphere, crust, and mantle (Ozima and Podosek, 2002), which can be used to distinguish between organic and inorganic sources of natural gas (Dai et al., 1992; Galimov, 2006), to trace the transport processes of hydrocarbons (Prinzhofer et al., 2010), and to indicate exchange reactions associated with atmospheric water or crustal source fluids or deep high-temperature mantle fluids (Prinzhofer et al., 2010). Some
Mechanism of geochemical anomalies
Shale gas tends to be in a relatively closed system during the generation and accumulation process, and the abnormal distribution of carbon isotope composition does not occur during normal oil cracking (Curtis, 2002; Martini et al., 2003, Martini et al., 2008; Jarvie et al., 2007). From the above, geochemical anomalies of Longmaxi shale gas mainly refer to abnormally enriched δ13C1 values and negative carbon isotopes series. Xia et al. (2013) suggested that isotopic reversal results from the
Appropriate maturity of shale gas
Well-confined shale systems also portend high production potential, and overpressurized formations are conducive to horizontal fracturing in the development stage (Ferworn et al., 2008; Zumberge et al., 2012; Hao et al., 2013). In terms of the gas generation process, shale gas with a positive carbon isotope series is relatively unlikely to be highly productive, because it tends to be less mature, more humid, and fails to reach peak gas generation. Jarvie et al. (2007) suggested that the gas
Conclusion
The over-mature Longmaxi shale gas is dominated by alkane gas components (> 99%), which almost entirely (98.7%) consist of methane (CH4). The distinctive characteristics of high CH4 and low C2+ contents result in extremely low wetness. The Longmaxi shale gas belongs to late-mature thermogenic gases (LMT). Moreover, its alkane gas is in the post-rollover stage, and the evolutionary trend of carbon‑hydrogen isotope composition is no longer obvious, but the samples are still sequentially
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this manuscript, and all authors approve the manuscript for publication.
Acknowledgments
We are grateful to Prof. Jinxing Dai and Caineng Zou, from the Research Institute of Petroleum Exploration & Development (PetroChina), for their helpful comments and contributions that greatly improve the manuscript. The authors benefited greatly from the constructive reviews this paper by the anonymous reviewers. This research was supported by the National Natural Science Foundation Project of China (No. 41821002, 42072167), and Shandong Provincial Major-Type Grant for Research and Development
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