Crude-oil biodegradation via methanogenesis in subsurface petroleum reservoirs

'Difficult' oil could be a gas More than half of the world's oil inventory consists of biodegraded heavy oil and tar sand deposits. Recovery of oil from these sources is complicated and expensive. Recent findings suggest that anaerobic bacteria may cause this hydrocarbon degradation, but the actual degradation pathway occurring in oil reservoirs remains obscure. Using a combination of laboratory oil degradation experiments and analysis of oilfield samples, it is now shown that the dominant process of subsurface biodegradation is methanogenesis, involving anaerobic degradation of oil hydrocarbons to produce methane. This suggests an alternative way of exploiting these 'difficult' oilfields: by accelerating the natural hydrocarbon degradation process, it may be possible to recover energy as methane, rather than conventionally as oil. Laboratory experiments in microcosms monitoring the hydrocarbon composition of degraded oils are used with carbon isotopic compositions of gas and oil samples taken at wellheads and a Rayleigh isotope fractionation box model to elucidate the mechanisms of hydrocarbon degradation in reservoirs. The data imply a common methanogenic biodegradation mechanism in subsurface degraded oil reservoirs resulting in consistent patterns of hydrocarbon alteration. Biodegradation of crude oil in subsurface petroleum reservoirs has adversely affected the majority of the world’s oil, making recovery and refining of that oil more costly 1 . The prevalent occurrence of biodegradation in shallow subsurface petroleum reservoirs 2 , 3 has been attributed to aerobic bacterial hydrocarbon degradation stimulated by surface recharge of oxygen-bearing meteoric waters 2 . This hypothesis is empirically supported by the likelihood of encountering biodegraded oils at higher levels of degradation in reservoirs near the surface 4 , 5 . More recent findings, however, suggest that anaerobic degradation processes dominate subsurface sedimentary environments 6 , despite slow reaction kinetics and uncertainty as to the actual degradation pathways occurring in oil reservoirs. Here we use laboratory experiments in microcosms monitoring the hydrocarbon composition of degraded oils and generated gases, together with the carbon isotopic compositions of gas and oil samples taken at wellheads and a Rayleigh isotope fractionation box model, to elucidate the probable mechanisms of hydrocarbon degradation in reservoirs. We find that crude-oil hydrocarbon degradation under me