Microbial community composition of a hydrocarbon reservoir 40 years after a CO(2) enhanced oil recovery flood

Injecting CO(2) into depleted oil reservoirs to extract additional crude oil is a common enhanced oil recovery (CO(2)-EOR) technique. However, little is known about how in situ microbial communities may be impacted by CO(2) flooding, or if any permanent microbiological changes occur after flooding has ceased. Formation water was collected from an oil field that was flooded for CO(2)-EOR in the 1980s, including samples from areas affected by or outside of the flood region, to determine the impacts of CO(2)-EOR on reservoir microbial communities. Archaea, specifically methanogens, were more abundant than bacteria in all samples, while identified bacteria exhibited much greater diversity than the archaea. Microbial communities in CO(2)-impacted and non-impacted samples did not significantly differ (ANOSIM: Statistic R = -0.2597, significance = 0.769). However, several low abundance bacteria were found to be significantly associated with the CO(2)-affected group; very few of these species are known to metabolize CO(2) or are associated with CO(2)-rich habitats. Although this study had limitations, on a broad scale, either the CO(2) flood did not impact the microbial community composition of the target formation, or microbial communities in affected wells may have reverted back to pre-injection conditions over the ca. 40 years since the CO(2)-EOR.