Evolution of carbon isotopic compositions for gas generated in semi-closed pyrolysis system: Reflections on the formation of isotopic abnormal gases

Abstract

Carbon isotope anomalies are signals for highly productive shale gas, but are poorly duplicated in completely closed or open pyrolysis systems. In this study, five low-maturity shale samples and one high-maturity shale sample were subjected to semi-closed pyrolysis experiments to investigate whether isotopically anomalous gases could be generated in laboratory simulations of the full maturity range. The results reveal that the first rollover of methane, ethane, and propane δ13C values occurs at the late oil-window stage, and the second rollover of δ13C(C2H6) and δ13C(C3H8) values occurs at the overmature stage. Partial carbon isotopically reversed gases are produced, with isotope compositions evolving from δ13C(C2H6) > δ13C(C3H8) > δ13C(CH4) through δ13C(CH4) > δ13C(C3H8) > δ13C(C2H6) to δ13C(C3H8) > δ13C(CH4) > δ13C(C2H6) as thermal maturity increases, whereas complete carbon isotopically reversed gases are not reproduced. These findings are slightly different from the carbon isotope and chemical characteristics of shale gas under natural conditions, which is universally characterized by full carbon isotope reversal. Compared to closed or open systems, the carbon isotopically anomalous gases are superimposed results of gaseous compounds from each episodic expulsion during semi-closed pyrolysis, illustrating that multiple periods of preservation and loss of gases are more important for carbon isotope reversal than thermal maturity. Carbon isotope fractionation via gas diffusion can be amplified by some large-scale processes of preservation and loss; for example, the uplift of a shale would lead to preferential diffusion of methane rich in 12C, resulting in full isotopic reversal in the residual gas in shales. These processes generally occur under natural conditions, but such factors have yet to be considered in conventional open or closed pyrolysis simulations. This study provides a reasonable reference for the issue of why (full) isotopically reversed gases can barely be produced in conventional pyrolysis experiments.