Abiotic synthesis has been hypothesized as a mechanism for occurrences of hydrocarbon gases with atypical molecular and isotopic compositions. This paper provides biotic interpretations of these atypical compositions, as follows: (1) microbial CH4 oxidation and CO2-sourced methanogenesis may enrich 13C in hydrothermal CH4; (2) microbial hydrocarbon generation using serpentinization-derived H2 may deplete deuterium in hydrocarbons; (3) three processes may cause isotopic reversal with the carbon number in biotic hydrocarbons (the decrease of kinetic isotope effect (KIE) with the increase of carbon number during alkane biodegradation, inverse KIE during the thermal decomposition of higher alkanes and isotopic fractionation during gas diffusion in rock samples); (4) random scission of long alkyl chains may form the exponential distribution of alkane abundance with respect to carbon number (the ‘Schulz–Flory distribution’); (5) isotopic compositions are often not equilibrated; even if they are, the equilibrium temperatures are not necessarily the same as the temperature of hydrocarbon generation. Case studies demonstrate that previously proposed abiotic hydrocarbon gases in continental serpentinite-hosted seepages, continental or oceanic hydrothermal vents, volcanic emissions, gas fields in volcanic reservoirs and fluid inclusions in alkaline or granitic rocks were formed by various biotic processes. The occurrence of abiotic hydrocarbon gas with decisive evidence is limited to fluid inclusions in some mantle minerals.
Supplementary material: Expressions for isotopic fractionation of CH4 and model details of the Xujiaweizi gas are available at https://doi.org/10.6084/m9.figshare.c.5660278