Characterizing the microbiome in petroleum reservoir flooded by different water sources

Petroleum reservoir is an unusual subsurface biosphere, where indigenous microbes lived and evolved for million years. However, continual water injection changed the situation by introduction of new electron acceptors, donors and exogenous microbes. In this study, 16S-rRNA gene sequencing, comparative metagenomics and genomic bins reconstruction were employed to investigate the microbial community and metabolic potential in three typical water-flooded blocks of the Shen84 oil reservoir in Liaohe oil field, China. The results showed significant difference of microbial community compositions and metabolic characteristics existed between the injected water and the produced water/oil mixtures; however, there was considerable uniformity between the produced samples in different blocks. Microbial communities in the produced fluids were dominated by exogenous facultative microbes such as Pseudomonas and Thauera members from Proteobacteria phylum. Metabolic potentials for O₂-dependent hydrocarbon degradation, dissimilarly nitrate reduction, and thiosulfate‑sulfur oxidation were much more abundant, whereas genes involved in dissimilatory sulfate reduction, anaerobic hydrocarbon degradation and methanogenesis were less abundant in the oil reservoir. Statistical analysis indicated the water composition had an obvious influence on microbial community composition and metabolic potential. The water-flooding process accompanied with introduction of nitrate or nitrite, and dissolved oxygen promoted the alteration of microbiome in oil reservoir from slow-growing anaerobic indigenous microbes (such as Thermotoga, Clostridia, and Syntrophobacter) to fast-growing opportunists as Beta- and Gama- Proteobacteria. The findings of this study shed light on the microbial ecology change in water flooded petroleum reservoir.