The water samples collected from two non water-flooded oil fields contained a variety of organic acids, sulfate-reducing bacteria (SRB) and little sulfate. Acetate and propionate were the major components of organic acids. Over 6 weeks of artificial souring experiment, a maximum of 3 mM of sulfide was produced when oil field water was mixed with seawater at 25 degrees C. Propionate was completely consumed under soured conditions. This indicated that the propionate-consuming SRB underwent souring in this experiment. Significant cell growth was confirmed at 25 degrees C with no relation to souring. The dominant SRB species were shifted from Desulfomicrobium thermophilum to Desulfobacter vibrioformis and uncultured Desulfobacter.