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 with 0 < T < 60-80 degrees C and high-temperature diagene tic environments with 80-100 < T < 150-200 degrees C, respectively. Th e major reaction products and by-products are identical in both therma l regimes and include altered and oxidized hydrocarbons (originally ma inly crude oil, gas condensate, and/or methane), hydrogen sulfide, bas e and transition metal sulfides, elemental sulfur, and carbonates (mai nly calcite and dolomite). The mere presence of the above reaction pro ducts and by-products does not discriminate between the low- and high- temperature diagenetic environments. However, petrographic, isotopic a nd compositional data of these products and by-products may permit ide ntification of a bacterial versus a thermochemical origin. Regarding t he inorganic phases, the carbon isotope ratios of the carbonates, sulf ur isotope ratios of elemental sulfur and sulfides, and fluid inclusio n data appear to yield the best discriminating geochemical criteria. A mong the organic phases, reservoir bitumen and gas condensates display a number of useful isotopic and compositional (chromatographic) crite ria. The most reliable approach for discriminating bacterial versus th ermochemical sulfate reduction is to combine as many of these criteria as possible. These criteria can be used in exploration for deposits o f hydrocarbons, sour gas, elemental sulfur, and certain metal sulfides .