Geochemical characteristics of shale gas and its response to thermal maturity (Ro) in the Longmaxi formation, Dingshan area, Southeast Sichuan

The composition and carbon isotope distribution of shale gas from the Longmaxi Formation in the Dingshan area were measured, and their responses to thermal maturity (Ro) were analyzed. The results show that the shale gas is mainly composed of methane (97.98-98.99%), ethane, propane and nonhydrocarbon gases (N-2 and CO2) and is an organic high temperature oil-type cracked gas. The wetness value [(C-2+C-3)/(C-1+C-2+C-3) x 100%] ranges from 0.39% to 0.74%. The early (Ro > 1.3%) residual kerogen and crude oil cracking gas was mixed with the late (Ro > 2.0%) secondary cracking gas, resulting in the full inversion of the carbon isotope sequence (C-13(1)>C-13(2)>C-13(3)).