13C-enriched carbonate precipitates reveal intense methanogenic oil degradation in the upper Wuerhe Formation, Northwest China

Abstract Microbial methanogenesis from crude oil is an important source of CH4 for gas reservoirs and atmospheric greenhouse gases. However, its petrological records have not been found in natural environments, and the geological conditions under which it may occur remain unclear. Here, we provide the first petrological evidence of Fe(III)-mediated methanogenic hydrocarbon degradation in a deep subsurface oil reservoir in Northwest China. The major findings are as follows: (1) highly positive δ13C values (up to +16‰) of secondary calcite attributed to methanogenesis; (2) paragenetic relation of high-δ13C calcite to biodegraded hydrocarbon; and (3) remarkably high FeO contents (up to 8 wt%) and heavy δ56Fe ratios (up to +0.52‰) in calcite, indicative of microbial Fe(III) reduction. Our study shows that methanogenic hydrocarbon degradation can occur in Fe(III)-reducing environments. This process transformed hydrocarbons into CO2 and CH4, where the former mostly precipitated as Fe-rich calcite (the carbon sink), while the latter, representing an estimated ~1968 Tg, might have escaped into the overburden and atmosphere from the Permian reservoir during the Late Triassic to Early Jurassic, which may have acted as an important CH4 source in changing global climate in the geological past.