Carbon and hydrogen isotopes of the wet gases produced by gamma-ray-induced polymerisation of methane: Insights into radiogenic mechanism and natural gas formation

Abstract

The 60Co gamma ray radiolysis of methane was undertaken at room temperature and at a low proportion of methane conversion to observe initial polymerisation reactions in the formation of C2─C5 saturated hydrocarbon gases. The gamma ray radiolysis product follows a Schulz-Flory distribution model for the straight-chained C2+ alkane gases consistent with chain propagation by addition of C1 intermediates. The high relative concentration of branched isomers suggests the total product distribution is likely controlled by propagation reactions involving mainly radical intermediates. In particular, the pentane isomers show a relative dominance of the branched isomers with sub-equal amounts of iso-pentane and neo-pentane. Carbon isotopes of the C2─C5 wet gases are all depleted in 13C relative to δ13C methane reactant with δ13C C2 < δ13C C3 and carbon isotope reversals in the C3─C5 gases while only ethane and neo-pentane are depleted in 2H compared to δ2H of the starting methane. As such, these molecular and isotopic features show similar characteristics to abiogenic wet gases found in terrestrial settings.