Microbial enhance oil recovery utilizes microorganisms and their metabolic products to improve the recovery of crude oil from reservoir rocks. In this study an anaerobic bacterium, Clostridium acetobutylicum was injected into a one-dimensional model reservoir containing a Turkish heavy oil (Raman oil) at 38° C. This injection was followed by water flooding after a suitable shut-in period. Comparison of oil recovery results of pure water flooding runs with experiments in which bacterial concentration and shut-in periods were varied indicated increases in oil recovery of about 12% of the original oil in place. This increase was attributed to changes in the viscosity and pH of the crude oil. © 1992 Springer-Verlag.