Stable isotope reversal and evolution of gas during the hydrous pyrolysis of continental kerogen in source rocks under supercritical conditions

Abstract Semi-closed hydrous pyrolysis experiments were conducted to investigate the isotopic evolution of shale gas produced from continental organic-rich shales with increasing thermal maturity and prospecting potential. The δ 13 C values of methane, ethane, propane and n-butane became heavier with increasing thermal maturity and showed good relationships with vitrinite reflectance (VR). Gases expelled from type-I, type-II, and type-III kerogens followed the normal carbon sequence ( δ 13 C C1 δ 13 C C2 δ 13 C C3 δ 13 C nC4 ) and the hydrogen isotopic sequence ( δ 2 H C1 δ 2 H C2 δ 2 H C3 δ 2 H nC4 δ 2 H iC4 ) among the alkanes at VRs lower than 2.14% R o , 2.42% R o , and 1.87% R o respectively. The isotopically reversed gases ( δ 13 C C1 δ 13 C C2 δ 13 C nC4 δ 13 C C3 ) were observed in type-I kerogen with a VRs above 2.14% R o and in type-II kerogen with a VR above 2.42% R o . In type-III kerogen, isotopically reversed gases ( δ 13 C C1 δ 13 C nC4 δ 13 C C3 δ 13 C C2 ) were observed with a VR above 2.94% R o . These results suggest four stages in the stable carbon isotope reversal of ethane, propane, and n-butane with increasing maturity. The isotopically reversed gases ( δ 13 C C1 δ 13 C C2 δ 13 C nC4 δ 13 C C3 ) may represent lower maturity and higher productivity shale gas than the isotopically reversed gases ( δ 13 C C1 δ 13 C nC4 δ 13 C C3 δ 13 C C2 ), indicating that continental type-I kerogen has the highest potential productivity for shale gas, and continental type-III kerogen has the lowest. Stable hydrogen isotope composition did not respond to the stable carbon isotope reversal in isotopically reversed gases. We suggest that indigenous generation and mixing may be the dominant mechanisms responsible for the stable carbon isotope reversal, and adsorption/desorption during hydrocarbon expulsion may promote the stable isotope reversal under semi-closed conditions. Overall, these results suggest that this study is an important contribution to continental shale gas exploration.