We are focusing on in-situ microbial conversion of CO2 that was injected into depleted oil reservoirs for CCS into CH4 by oil-degrading and H2-producing bacteria and hydrogenotrophic methanogens. Those bacteria are concerned to be affected under high CO 2 condition, which causes pH reduction of the brine; therefore, we estimated the possibility of this microbial conversion process under high partial pressure of CO2 in this study by investigating the following: - The habitations of above bacteria in a high/low CO2 content reservoir - The pH reduction of brine under high partial pressure of CO 2 - Productivities of H2 and CH4 by indigenous bacteria of the high CO2 content reservoirs Thermotoga sp. and Thermoanaerobacter sp. whose productivities of hydrogen from crude oil had been shown by Fujiwara et al.1) were detected as a dominant in the high CO2 content reservoir. Moreover, Methanobacterium sp. and Methanothermobacter sp. that are well known as hydrogenotrophic methanogens were also detected from the reservoir. These results indicate that the effective microorganisms for this microbial conversion process can inhabit reservoirs that stored with CO2 for CCS dominantly. The pH of the brine including bicarbonate more than 0.1 mol/L can be maintained at 7.0 to 8.0, which was suitable pH for many kinds of microorganisms, even under high CO2 partial pressure such as 5.0 MPa, suggesting that this microbial conversion process can be applied easily on reservoirs whose brine is abundant in bicarbonate. Enrichment culture experiments of the brine were carried out under high CO2 partial pressure (3.0 MPa) at 75°C. Both H2 and CH4 production were found in not only the brine having the pH buffering action but also the brine whose pH was reduced significantly under high CO2 partial pressure; therefore, the microbial conversion process may be also expected in normal reservoirs that have poor acid neutralizing capacity.