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Microbiology of Anaerobic Hydrocarbon Degradation in Petroleum Reservoirs

Final Report Summary - MICROBEOIL (Microbiology of Anaerobic Hydrocarbon Degradation in Petroleum Reservoirs)

The broad objective of MICROBEOIL has been to gain an improved understanding of the subsurface microbiology of heavy oil reservoirs where anaerobic hydrocarbon degradation is a dominant biogeochemical process.

MICROBEOIL has resulted in the first ever publication describing the microbiology of an unconventional heavy oil field - the tar sands in Canada. This research combined petroleum geochemistry with molecular microbiology and was able to demonstrate that a certain class of bacteria are abundant and important in low temperature heavy oil reservoirs. Epsilonproteobacteria are famous for being able to oxidise sulfur compounds, and MICROBEOIL's combined geochemical-biological analyses revealed that these organisms appear to be catalysing the oxidation of reduced sulfur compounds in crude oil in aquifers associated with heavy oil. This biogeochemical activity was shown to occur at the oil-water contact in the reservoir, in agreement with previous research on anaerobic hydrocarbon biodegradation in subsurface reservoirs.

The removal of reduced sulfur compounds in crude oil is an important process hence the inferred epsilon proteobacterial activity may be considered akin to in situ biorefining. One of the many challenges associated with producing heavy oil such as is found in these Athabasca tar sands is that the concentration of sulfur is elevated in heavy oils. This increases the cost of refining heavy oils into a low-sulfur usable product. MICROBEOIL's results suggest that microbes that are associated with the subsurface 'petroleum microbiome' may contribute to the desulfurisation of crude oil in situ. With society's reliance on petroleum energy at an all-time high and given important decisions being considered regarding the role of tar sands as an energy source in the years ahead, the results of MICROBEOIL will have an impact and contribute to decisions about the further development of heavy oil reservoirs.

The discovery described above raised an important issue that had to be addressed - the veracity of discovering such a massive dominance of Epsilonproteobacteria had to be addressed through using a variety of complementary analytical approaches. In the past, several studies have commented and concluded that the occurrence of Epsilonproteobacteria in petroleum-associated subsurface habitats may have been an artefact of the chosen methods (selectivity of a limited set of PCR primers). By using multiple and overlapping analytical PCR approaches MICROBEOIL has debunked this idea and provided solid evidence of the occurrence of these bacteria and their putative roles in oil reservoirs. To put this in context a meta-analysis of published microbial community analyses from oil reservoirs was performed, revealing patterns related to different bacterial taxa in subsurface oil reservoirs of different temperature. Epsilonproteobacteria are the dominant bacterial taxa in low temperature oil fields, whereas Firmicutes are dominant in high temperature fields.

The meta-analysis and high temperature firmicutes association has led to new hypotheses and research directions, linked to those in the MICROBEOIL proposal, involving microcosm enrichment culture experiments where River Tyne sediments are incubated at various temperatures with and without crude oil. This is a useful and convenient experimental analogue for understanding important processes relevant to subsurface petroleum reservoirs (anaerobic hydrocarbon degradation; sulfate reduction at high temperature). In particular, intriguing preliminary results pointing to sulfate-reducing Firmicutes from the subsurface has led to new hypotheses and research proposals that follow on from MICROBEOIL. These proposals have been successful in securing significant funding from UK research councils. The legacy of the MICROBEOIL Marie Curie International Incoming Fellowship (IIF) project will therefore include several more years of research on deep biosphere and petroleum microbiology questions for > 5 early career scientist.