Clumped isotope effects of thermogenic methane formation: Insights from pyrolysis of hydrocarbons

Methane clumped isotope analysis is a tool used to constrain the formation or equilibration temperatures of methane, differentiate methane thermogenic, microbial ‘abiotic’ origins. Geothermometry applications are based on temperature dependence relative abundances multiply-substituted isotopologues in thermodynamic equilibrium, whereas assignments biogenicity ‘abiogenicity’ rely kinetic effects associated with synthesis, which disturb away from expected equilibrium proportions. However, processes thermogenesis during post-generation storage thermogenic gas may cause isotopic disequilibrium, confounding thermometry leading ‘false positive’ identifications abiogenic gases. Non-equilibrated compositions have been observed gases including unconventional oil-associated and coal pyrolysis experiments. The disequilibria might be caused by expressed migration (including extraction), irreversible chemical processes, such as breaking carbon–carbon bonds an alkyl precursor. In this study, we performed controlled experiments at 400 °C n-octadecane (C 18 H 38 ). We characterized chemistry, compound-specific carbon hydrogen products. found that ? 13 CH 3 D values (anomalies stochastic distribution isotopes) appear relatively close experimental temperature, 12 2 30–40‰ lower than for equilibrium. large deficit can explained assembling atoms affected two distinct into molecule, previously referred ‘combinatorial effect’. present model describes full systematics, anomalous deficits, product methane. Finally, propose signatures natural where non-equilibrium composition signature onset catagenetic production. Our also ways disappears further maturation drives through exchange. findings demonstrate depletion not unique putative abiotic specifically, it generated pyrolytic chemistry.