CO2 sequestration into depleted oil reservoirs has been expected to be a method of reducing CO2 emission. We focus on the in-situ conversion of CO2 into CH4 by hydrogenotrophic methanogens which inhabit oil reservoirs universally. This conversion process has not only the potential of reducing CO2 emission but also the potential of reproducing natural gas deposit in reservoirs. The hydrogenotrophic methanogens need not only CO2 but also H2 to produce CH4, therefore, H2 should be supplied to them in reservoirs for this process. It has been reported there are several kinds of bacteria which produce H2 by degrading parts of crude oil in reservoirs. In this study, the potential of this conversion process was evaluated by investigating the habitation of these bacteria in a high CO2 content oilfield. Reservoir brine samples were extracted in a high CO2 content oilfield. Concentration of CO2 and CH4 in the gases which were produced from reservoirs in the oilfield was 35–60% and 20–40% respectively. Microbial genomic DNAs were extracted from reservoir brine and their nucleotide sequences were analyzed to identify the species inhabiting the brine. Thermotoga naphthophila and Thermoanaerobacter sp. which are well known as oil-degrading and hydrogen-producing bacteria were detected as a dominant in the brine. Moreover, Methanobacterium sp. and Methermicoccus sp. which are well known as hydrogenotrophic methanogens were also detected in the brine. It is assumed that these hydrogenotrophic methanogens live in symbiosis with oil-degrading and hydrogen-producing bacteria and convert CO2 into CH4 in the reservoirs. In addition, culture experiments of the microorganisms which belonged same genus or same family of the above microorganisms were carried out under the reservoir conditions. Both hydrogen production and methane production were observed in the experiments. These results indicate that the in-situ microbial conversion process of CO2 into CH4 has high potential.