Hydrogen isotopic characteristics of shale gases

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.