Hydrogen isotopic characteristics of shale gases

• Published in: Journal of Asian earth sciences Vol. 257; p. 105838
• Main Authors: Ni, Yunyan, Dong, Dazhong, Yao, Limiao, Chen, Jianping, Sui, Jianli, Wang, Fei, Liu, Fei, Li, Jian, Guo, Jinhao, Liu, Dan, Gao, Jinliang
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01-11-2023
• Subjects: Hydrogen isotope; Isotopic reversal; Shale gas; Sichuan Basin; Wufeng-Longmaxi; Sichuan Basin; Hydrogen isotope; Wufeng-Longmaxi; Isotopic reversal; Shale gas
• ISSN: 1367-9120; 1878-5786
• DOI: 10.1016/j.jseaes.2023.105838

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•Taiyang, Jiaoshiba and Weirong shale gases show 12‰ variation in δ2H of methane.•Carbon isotopic reversal occurs in Taiyang, Jiaoshiba and Weirong shale gases.•Hydrogen isotopic reversal only occurs in Taiyang and Jiaoshiba shale gases.•Gas δ2H is greatly affected by water salinity, causing delay of isotope change.
Sichuan Basin is important commercial shale gas producing basin in China, where shale gas explorations were mainly targeted at the Upper Ordovician Wufeng-Lower Silurian Longmaxi formations with burial depth mostly of 2000 ∼ 3500 m. This study performed a detailed investigation about the δ2H features of shale gases from the Wufeng-Longmaxi shale with burial depth of <2250 m (Taiyang), 2250 ∼ 3500 m (Jiaoshiba) and >3500 m (Weirong). The average δ2H1 and δ2H2 values are of −135‰ and −146‰, −137‰ and −157‰, −137‰ and −124‰, respectively in Taiyang, Jiaoshiba and Weirong shale gas fields. The average δ2H1 values are nearly the same in the three gas fields, while average δ13C1 values increase from −35.2‰ to −28.5‰ as from Weirong to Taiyang. When wetness >8%, δ2H1 values increase with decreasing wetness rapidly implying dominant influences from maturity, when wetness <8%, the δ2H1 variation is small and mainly in the range of (−140 ± 15)‰, indicating a strong influence from cogenetic water. All gases are characterized by a trend of δ13C1 > δ13C2 > δ13C3, and gases in Taiyang and Jiaoshiba are characterized by δ2H1 > δ2H2, while gases in Weirong are dominated by δ2H1 < δ2H2. Generally, with decreasing gas wetness, ethane first become more enriched in D and 13C, then become more depleted in D and 13C, and finally become more enriched in D and 13C again. The turning point of wetness from rollover to post-rollover zones is 1.2% for the δ13C2 while 0.8% for the δ2H2. Such delay in δ2H2 also indicates the strong influences from the cogenetic water medium.