Carbon isotope reversal of desorbed gas in Longmaxi shale of Jiaoshiba area, Sicuhan Basin

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Through analysis of components and carbon isotope compositions of gas desorbed from shale cores, the carbon isotope reversal phenomenon in the shale gas from the Silurian Longmaxi Formation of Jiaoshiba area in Sichuan Basin were well studied. Results showed that compared with the wellhead gas, the desorbed gas from Longmaxi shale had significantly more wet components and more heavy carbon isotope values; carbon isotope values of each component became heavier with the desorption time, δ13C1 values of different samples had maximum positive variations of 12.3–23.9‰, but δ13C2 values only had maximum positive variations of 0.8–2.3‰, indicating carbon isotope values of methane changed more obviously than that of heavy hydrocarbon. The above results were consistent with previous results of shale core desorption experiments carried out by other researchers. Shale gas in strata might have no carbon isotope reversal, and the phenomenon that δ13C1 values changed more significantly than δ13C2 values during the core desorption was not caused by diffusion rate differences among different components, but mainly due to different desorption stages of methane and ethane, i.e., the ethane was in its early desorption stage while the methane was in its later desorption stage; during the production process, phase differences among different components of alkane gas and differences in the desorption stages induced by adsorption, could be the major cause for total reversal of carbon isotopes of shale gas in Longmaxi Formation, but it also could not excluded that mixture of kerogen cracking gas and crude oil cracking gas probably had a partial or more major contribution to the carbon isotope reversal.