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, 76 μmol CH4 per day and gram of oil added.

The potential for aerobic and methanogenic oil biodegradation processes was investigated in an oil reservoir undergoing secondary and tertiary oil production in the Gangxi area of Dagang, China. In this area a highly biodegraded basically sulfate-free and methane-rich oil reservoir was investigated. Laboratory degradation experiments under aerobic and methanogenic conditions were performed using production waters from the reservoir. In the anaerobic experiments, methane was produced at a rate of 76 ± 6 μmol day−1 g−1 oil added. Compositional changes of the oil within 90 days of aerobic degradation and about 200 days of methanogenic were similar to those observed in the field: n-alkanes were completely depleted and long-chained linear monoaromatic hydrocarbons were preferentially removed over PAH, steranes, hopanes and triaromatic steroids. Among the alkylated PAH, those with 2-methyl substituents were preferentially removed. Regarding the biomarkers, only slight differences between experiments were observed: under aerobic conditions diasteranes were more readily degraded than regular steranes; under methanogenic conditions a relative depletion of 17α-trisnorhopane versus 18α was observed. Reservoir microbiota are highly induced and capable of mineralizing oil under both conditions. In addition, both methanogenic and facultative anaerobic populations have been identified in water samples from the reservoir used as inoculum. However we speculate that the production mode of recirculating process water has contributed to the enrichment of a highly methanogenic oil-degrading community, and that aerobic biodegradation would only play a marginal role in the overall transformation of oil.