Potential for aerobic and methanogenic oil biodegradation in a water flooded oil field (Dagang oil field)

•Gangxi oil exhibits evidences of heavy biodegradation.•Anaerobic (methanogenic) and facultative aerobic microbiota have been identified.•Gangxi microbiota can degrade oil under aerobic and methanogenic conditions.•In vitro oil biodegradation patterns resemble those observed in situ.•Gangxi microbiota can produce, in vitro, 76μmol CH4 per day and gram of oil added.