Microbial community composition of a hydrocarbon reservoir 40 years after a CO₂ enhanced oil recovery flood

Injecting CO₂ into depleted oil reservoirs to extract additional crude oil is a common enhanced oil recovery (CO₂-EOR) technique. However, little is known about how in situ microbial communities may be impacted by CO₂ flooding, or if any permanent microbiological changes occur after flooding has ceased. Formation water was collected from an oil field that was flooded for CO₂-EOR in the 1980s, including samples from areas affected by or outside of the flood region, to determine the impacts of CO₂-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 CO₂-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 CO₂-affected group; very few of these species are known to metabolize CO₂ or are associated with CO₂-rich habitats. Although this study had limitations, on a broad scale, either the CO₂ 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 CO₂-EOR.