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

Abstract

Methane clumped isotope analysis is a tool used to constrain the formation or equilibration temperatures of methane, or to differentiate methane of thermogenic, microbial or ‘abiotic’ origins. Geothermometry applications are based on the temperature dependence of relative abundances of multiply-substituted isotopologues in thermodynamic equilibrium, whereas assignments of biogenicity or ‘abiogenicity’ rely on kinetic isotope effects associated with synthesis, which disturb clumped isotope abundances away from expected equilibrium proportions. However, kinetic processes in thermogenesis or during post-generation storage of thermogenic gas may cause isotopic disequilibrium, confounding thermometry applications or leading to ‘false positive’ identifications of microbial or abiogenic gases. Non-equilibrated clumped isotope compositions have been observed in thermogenic gases including unconventional oil-associated gases and from coal pyrolysis experiments. The isotopic disequilibria might be caused by kinetic isotope effects expressed during gas migration (including extraction), or by irreversible chemical processes, such as breaking carbon–carbon bonds in an alkyl precursor. In this study, we performed controlled pyrolysis experiments at 400 °C on n-octadecane (C₁₈H₃₈). We characterized the gas chemistry, and compound-specific carbon and hydrogen isotope and methane clumped isotope compositions of the gas products. We found that Δ¹³CH₃D values (anomalies relative to a stochastic distribution of isotopes) appear to be relatively close to equilibrium at the experimental temperature, whereas Δ¹²CH₂D₂ values are 30–40‰ lower than expected for equilibrium. The large deficit in Δ¹²CH₂D₂ can be explained by assembling hydrogen atoms affected by two distinct kinetic isotope effects into a methane molecule, previously referred to as a ‘combinatorial effect’. We present a kinetic model that describes the full isotopic systematics, including anomalous Δ¹²CH₂D₂ deficits, of pyrolysis product methane. Finally, we propose a model for the isotope signatures of natural thermogenic methane where the non-equilibrium Δ¹²CH₂D₂ composition is a signature of the onset of catagenetic methane production. Our model also describes ways in which this signature disappears as further maturation drives Δ¹²CH₂D₂ to equilibrium through hydrogen exchange. Our findings demonstrate that anomalous depletion in Δ¹²CH₂D₂ is not a unique signature for microbial or putative abiotic methane, and specifically, it can be generated during pyrolytic chemistry.