Geochemical characteristics and geological significance of desorbed shale gas in Longmaxi Formation, Jiaoshiba area

This study is focused on the carbon isotope reversal of shale gas in Silurian Longmaxi Formation, Sichuan Basin. Core desorption test is conducted on the shale in Longmaxi Formation, Jiaoshiba area, so as to analyze its gas components and carbon isotope compositions. The results indicate that the desorbed gas is obviously wetter than the wellhead gas in component, and the carbon isotope of the former is significantly heavier; the carbon isotope values of each component became greater with the increase of desorption time. In terms of different samples, the maximum variation of δ13C1 value is from 12.3‰ to 23.9‰, while that of δ13C2 is only from 0.8‰ to 2.3‰, i. e., the carbon isotope of methane presents more obvious variations than heavy hydrocarbon, which is consistent with the results of previous shale core desorption experiments. It is concluded that δ13C1 value observed in core desorption process has more obvious variations than δ13C2 value, which is not caused by the difference in diffusion rate, but mainly results from the differences in desorption stages, i. e., ethane is in its early desorption stage while methane in the late desorption stage. In the production process, the differences in the component phase of alkane hydrocarbon gas and those in desorption stage caused by adsorption-desorption process may be the major reasons for complete reversal of the carbon isotopes of shale gas in Longmaxi Formation, Sichuan Basin. However, it cannot be denied that the mixing of kerogen pyrolysis gas and crude oil cracking gas make a partial or even a major contribution to the reversal of carbon isotopes in shale gas. © 2016, Editorial Office of ACTA PETROLEI SINICA. All right reserved.