Geochemical characteristics of Longmaxi Formation shale gas in the Weiyuan area, Sichuan Basin, China

Recently, shale-gas exploration in the Lower Silurian has been conducted in the Weiyuan area, southwestern Sichuan Basin, which has a special symbolic significance for the development of shale gas in China. According to the first systematic analysis of its geochemical characteristics, methane accounts for the vast majority (95.7%) of the Longmaxi Formation shale gas in the Weiyuan area, with extremely low wetness (C2-3/C1-3%) of 0.57%. Non-hydrocarbon gases consist of small amounts of N2 and inorganic-induced CO2. The δ13C-CH4 values of shale gas range from −36.9‰ to −34.0‰, and the δ13C-C2H6 values range from −42.8‰ to −37.5‰, indicating that the gases originate from sapropelic kerogen. A reversal in carbon-isotopic composition, i.e., δ13C-CH4 > δ13C-C2H6 > δ13C-C3H8, results from combined secondary effects such as secondary cracking and diffusion under extremely high thermal maturity. Geologic features of organic-rich shale and secondary effects at the overmature stage together lead to differences in geochemical characteristics of shale gas in the Weiyuan and Changning and Fuling (CF) areas, which can be used to indicate the distribution of geochemical parameters and predict shale-gas production.