Metagenomic analysis of compositions and metabolic potential of microbial communities in production water from CO2-and water- flooded petroleum reservoirs

CO2 enhanced oil recovery (CO2-EOR) is one of the common and effective ways for carbon capture, utilization and storage(CCUS) in China. The injection of CO2 into petroleum reservoirs may influence subsurface environments and further affect microorganisms in oil reservoirs. However, the current knowledge about the impact of CO2 flooding operation on microbial communities and their metabolic functions in oil reservoirs is still limited. In this study, the compositions and metabolic potential of microbial communities in production water from CO2-and water-flooded oil reservoirs in Jilin oilfield were investigated by using a metagenomic approach. Comparative analyses indicated that the microbial community compositions in CO2-flooded oil reservoir samples (GQ43 and GH46) were significantly different from those in water-flooded ones (WQ21 and WH71), with lower microbial diversity. The difference analysis (p<0.05) showed that Pseudomonas, Stutzerimonas, Marinobacterium,Pseudomonadaceae, Methanosarcina and Archaeoglobus were dominant in the former, while Azonexus, Sulfurospirillum, Candidatus Woesearchaeota, Candidatus Methanofastidiosa and Nanoarchaeota predominated in the latter. According to the high-quality metagenome-assembled genomes (MAGs) obtained, some members identified in the CO2-flooded oil reservoir samples might be involved in aerobic alkane biodegradation (Stutzerimonas and Hyphomonas), activated hydrocarbon utilization (Archaeoglobus and Magnetospirillum), fatty acid degradation (Stutzerimonas and Halomonas), fermentative metabolism (Stutzerimonas, Acidaminobacter, Fusibacter, Magnetospirillum, Shewanella, Halodesulfovibrio, Pseudodesulfovibrio and Halomonas), carbon fixation (Methanosarcina and Halodesulfovibrio)and syntrophic methanogenesis (Methanosarcina), simultaneously accompanied by dissimilatory sulfate reduction, thiosulfate reduction and denitrification. Whereas, a series of MAGs recovered from the water-flooded oil reservoir samples might be responsible for fumarate addition of aromatic hydrocarbons, activated hydrocarbon utilization, acetogenesis, reductive citrate cycle, dissimilatory nitrate reduction and sulfur metabolism (dissimilatory sulfate reduction, thiosulfate reduction and sulfur oxidation). These results contribute a broad and deep understanding of microbial communities and their roles in petroleum reservoirs especially affected by CO2 flooding operation, and provide the basic biological information for CCUS.