Unusual 13C depletion in the central carbon of propane in the gases of Longmaxi shale in the Sichuan Basin

Abstract

Abiotically synthesized hydrocarbons are significant in the fields of astrobiology, biosphere, energy, environment, and geology. Abiotic hydrocarbons have never been unambiguously proven to occur in common sediments partly because identifying these components in crustal settings is challenging as they share similar characteristics with hydrocarbons of biotic origin. In this study, we measured intramolecular carbon isotopes of 10 propane samples from Longmaxi shale in the Sichuan Basin, China, and found distinct 13C-depletion in the central position carbon with its δ13C decreased by −8.0 to −7.0‰ (Vienna Pee Dee Belemnite) with respect to terminal carbons. The δ13C value of central carbon (H3C–#CH2-CH3) in propane is equal to the second methyl (H3C–#CH3) in ethane possibly resulting from surface catalytic polymerization reactions of methane. Our novel findings indicate that the abiotic synthesis of C2+ hydrocarbons by the surface catalyzed polymerization of methane occurs in commercial gas reservoirs and suggest that this process may be more common than previously thought in methane-rich scenarios on Earth and other planets.