Kinetic controls on thermochemical sulfate reduction as a source of sedimentary H2S

Laboratory experiments with aqueous ammonium sulfate in the presence of H2S and toluene are reported which show measurable SO42- reduction in 4 to 30 days at 175-250 degrees C. Reduction rates increase with both increasing temperature and H2S pressure but reduction was not measurable on our experimental timescale without H2S initially present. An activation energy of 96 (+/-16) kJ/mole was estimated from the data. These results (and other published studies) indicate that thermochemical sulfate reduction (TSR) is difficult to document below 200 degrees C on a laboratory time scale unless Sigma S (i.e. SO42- + H2S)is initially very high (>.5M) and pH is low (<2). Even though these conditions may fall outside the range found in nature, extrapolation of kinetic data to more typical natural values predicts that the formation of high-H2S natural gas and sulfide mineral deposits of the Mississippi Valley Type at 100-200 degrees C can readily occur at geologically reasonable rates.