GAS SOURING BY THERMOCHEMICAL SULFATE REDUCTION AT 140-DEGREES-C

Natural gas in the Permian-Triassic Khuff Formation of Abu Dhabi conta ins variable amounts of H2S. Gas souring occurred through thermochemic al sulfate reduction of anhydrite by hydrocarbon gases, Sour gas is ob served only in reservoirs hotter than a critical reaction temperature: 140 degrees C, Petrographic examination of core from a wide depth ran ge showed that the anhydrite reactant has been replaced by calcite rea ction product only in samples deeper than 4300 m. Gas composition data show that only reservoirs deeper than 4300 m contain large quantities of H2S (i,e,, >10%), At present-day geothermal gradients, 4300 m is e quivalent to 140 degrees C. Fluid inclusion analysis of calcite reacti on product has shown that calcite growth only became significant at te mperatures greater than 140 degrees C, Thus, three independent indicat ors all show that 140 degrees C is the critical temperature above whic h gas souring by thermochemical sulfate reduction begins, The previous ly suggested lower temperature thresholds for other sour gas provinces (80-130 degrees C) derive from gas composition data that may notallow adequately either for the reservoir temperature historJr or for the m igration of gas generated at higher temperatures into present; traps, Comersely, published proposals for higher threshold temperature (180-2 00 degrees C) derive from short duration experimental data that are no t easily extrapolated to geologically realistic temperatures and time scales, Therefore, the temperature of 140 degrees C derived from our s tudy of the Khuff Formation may be the best estimate of temperature re quired for in-situ thermochemical sulfate reduction to produce the hig h H2S concentrations encountered in deep carbonate gas reservoirs