Products and distinguishing criteria of bacterial and thermochemical sulfate reduction

Bacterial and thermochemical sulfate reduction apparently occur in two mutually exclusive thermal regimes, i.e., low-temperature diagenetic environments with0 < T < 60–80°C and high-temperature diagenetic environments with80–100 < T < 150–200°C, respectively. The major reaction products and by-products are identical in both thermal regimes and include altered and oxidized hydrocarbons (originally mainly crude oil, gas condensate, and/or methane), hydrogen sulfide, base and transition metal sulfides, elemental sulfur, and carbonates (mainly calcite and dolomite). The mere presence of the above reaction products and by-products does not discriminate between the low- and high-temperature diagenetic environments. However, petrographic, isotopic and compositional data of these products and by-products may permit identification of a bacterial versus a thermochemical origin. Regarding the inorganic phases, the carbon isotope ratios of the carbonates, sulfur isotope ratios of elemental sulfur and sulfides, and fluid inclusion data appear to yield the best discriminating geochemical criteria. Among the organic phases, reservoir bitumen and gas condensates display a number of useful isotopic and compositional (chromatographic) criteria. The most reliable approach for discriminating bacterial versus thermochemical sulfate reduction is to combine as many of these criteria as possible. These criteria can be used in exploration or deposits of hydrocarbons, sour gas, elemental sulfur, and certain metal sulfides.