Abstract. Sequestration of CO₂ in oil reservoir is one of the feasible options for mitigating atmospheric CO₂ building up. The in situ bioconversion of sequestrated CO₂ to methane by microorganisms inhabiting oil reservoirs is feasible. To evaluate the potential of in situ microbial fixation and conversion of CO₂ into CH₄ in oil reservoirs, a comprehensive molecular survey was performed to reveal microbial communities inhabiting four oil reservoirs with different temperatures by analysis of functional genes involved in the biochemical pathways of CO₂ fixation and CH₄ synthesis (cbbM, cbbL, fthfs, [FeFe]-hydrogenase encoding gene, and mcrA). A rich diversity of these functional genes was found in all the samples with both high and low temperatures and they were affiliated to members of the Proteobacteria (cbbL and cbbM, fthfs), Firmicutes and Actinobacteria (fthfs), uncultured bacteria ([FeFe]-hydrogenase), and Methanomirobiales, Methanobacteriales and Methanosarcinales (mcrA). The predominant methanogens were all identified to be hydrogenotrophic CO₂-reducing physiological types. These results showed that functional microbial communities capable of microbial fixation and bioconversion of CO₂ into methane inhabit widely in oil reservoirs, which is helpful to microbial recycling of sequestrated CO₂ to further new energy in oil reservoirs.