Evidence for in situ methanogenic oil degradation in the Dagang oil field

► A sulfate-free thermophilic oil reservoir in Dagang, China has been studied. ► Indigenous microbial communities include high numbers of methanogenic Archaea. ► Geochemical data are consistent with in situ biogenic methane production from oil. ► Indigenous microbiota can produce CH4 from 13C labelled n-C16 or 2-methylnaphthalene in vitro. ► Polyaromatic fractions of oil may be an important methane source.