The occurrence of high concentrations (up to 4900 mg/l) of anions of acetic, propionic and butyric acids in co-produced water from oil reservoirs represents a large pool of potential electron donors for bacterial sulphate reduction. Enrichment cultures in defined media, isolated from a variety of oil-field environments demonstrated the wide distribution of acetate- and propionate-utilising sulphate-reducing bacteria (SRB). A propionate-utilising enrichment culture consisting predominantly of SRB, tentatively identified as species of Desulfobulbus, was used to inoculate a pressurised porous rock bioreactor operating under simulated reservoir conditions. Using a flood velocity of 6.3 cm/h with an inlet propionate concentration of 168 mg/l, a sulphide generation rate of 2.5 mu g/ml of rock per hour was achieved at 30 degrees C and 20 MPa. This rate indicates that a sphere of reservoir rock 9.3 m in radius, colonised with propionate-utilising SRB, could produce 50 kg sulphide day. The rates of propionate-driven bacterial sulphate reduction observed in the porous rock bioreactor could sustain the H2S production rates observed from wells in souring reservoirs.