Non-methane hydrocarbons in the earth are predominantly sourced from the thermal degradation of organic matter (thermogenesis). Abiogenic polymerization has been proposed as an alternative mechanism of hydrocarbon creation in Precambrian shields, based on differences in carbon and hydrogen isotopic compositions between co-existing compounds. However, inverse isotopic signatures have been observed in thermogenic systems, suggesting this evidence is ambiguous. Prior work suggested low values of Δ 13 C 2 H 6 (a measure of the departure from stochastic abundances of 13 C 13 CH 6 ) in ethane recovered from Kidd Creek formation waters are consistent with formation via abiotic polymerization (Taguchi et al., 2022). We have developed dual clumped isotopic measurements (Δ 13 C 2 H 6 and Δ 12 C 13 CH 5 D) in ethane to investigate the sources and processes involved in the creation and destruction of terrestrial ethane from petroleum fields, fracture fluids from the Canadian Shield, and other sources. Our results suggest that ethanes created from all studied formation pathways and not subjected to subsequent intense heating are characterized by disequilibrated intermolecular and intramolecular isotope signatures. In low maturity thermogenic gases, both kinetic isotope effects and source inheritance can impart strongly disequilibrated Δ 12 C 13 CH 5 D signatures in product ethane, resulting in at least two distinct and recognizable thermogenic end members. Low Δ 13 C 2 H 6 and Δ 12 C 13 CH 5 D signatures at Kidd Creek on the Canadian Shield are distinct from thermogenic ethanes and consistent with radical-mediated polymerization, favoring interpretation of abiotic origins. Δ 12 C 13 CH 5 D values for thermogenic ethanes approach equilibrium with increasing thermal maturity, consistent with suggestions from other intramolecular and intermolecular isotopic signatures in hydrocarbon systems that suggest partial equilibration through ‘metathetic’ chemical cycles (e.g., radical reaction networks) or interaction with solid catalysts.
Anadu et al. (Sun,) studied this question.