Microbial assembly and metabolic restructuring following CO2 injection in subsurface oil reservoir

Carbon dioxide (CO 2 ) injection into oil reservoirs is a well-established and promising strategy for enhancing oil recovery while simultaneously sequestering carbon, whose effectiveness is increasingly recognized to be influenced by microbial-mediated mechanisms and biogeochemical processes. However, the impact of CO 2 -injected and storage on functional microbial communities and their metabolic potential remains poorly understood. The study employed high-throughput sequencing to analyze the composition and diversity of microbial communities in produced water from three production wells with a long-term (> 20 years) history of CO 2 injection and two adjacent water-flooding ones in Xinli Oilfield, China. The results showed that CO 2 -injected wells had significantly higher microbial diversity and distinct metabolic potential relative to the adjacent water-flooding wells. A metabolic difference in the archaeal communities towards hydrogenotrophic and heterotrophic metabolisms, alongside an enrichment of bacterial taxa associated with sulfur and nitrogen cycling was observed. Correlation analysis revealed that microbial differentiation was strong associated with geochemical alteration (especially pH and some inorganic ions), with NH 4 + and S 2− identified as key factors in this process. Metagenomic analysis based on high-quality metagenome-assembled genomes (MAGs) further demonstrated that CO 2 injection led to a different profile of microbial metabolic functions relative to the water-flooding condition, characterized by significantly enhancing hydrogenotrophic methanogenesis, dissimilatory sulfate reduction, and denitrification, while diminishing methylotrophic methanogenesis and Wood-Ljungdahl pathway activity. These findings provide new insights into the microbial mechanisms driving carbon transformation in CO 2 -flooded oil reservoirs.