THERMOCHEMICAL SULFATE REDUCTION IN SEDIMENTARY-BASINS:A REVIEW

Thermochemical sulfate reduction(TSR) has become a hot topic in China after a heavy casualty caused by H_(2)S leakage from natural gas reservoir in East Sichuan Basin.It is frequently observed that methane gas anhydrite mineral H_(2)S gas coexist in the same reservoir.Therefore,reaction of methane with solid anhydrite was proposed to generate H_(2)S under petroleum reservoir conditions(Yue et al.,2003).However,methane is the most stable hydrocarbon under petroleum reservoir conditions.Simulation experiment has shown that it is expected to take 10~(17) years for 10% anhydrite solid to react with methane,the reaction being not significant in terms of geological time.However,our data show that there exists a positive correlative relationship between methane carbon isotope and the degree to sulfate reduction in the Triassic Jianlingjia Formation and Fexiangguan Formation,East Sichuan basin.The relationship suggests that aqueous sulfate may have reacted with dissolved methane and generated H_(2)S.After TSR is initiated by heavier hydrocarbons,natural gas is expected to have much higher partial pressures of methane and much higher concentrations of dissolved methane than heavier hydrocarbons.Relative ~(12)C-rich methane preferentially reacts with sulfate due to a kinetic fractionation,resulting in residual ~(13)C-rich methane.Based on a great number of experimental simulation,we find that organic matter involved in TSR,TSR initiation temperature and sulfur isotope fractionation during TSR are significantly different from case studies,respectively.One possibility among others is some unknown catalysis involved in TSR.Many lines of evidence have shown that H_(2)S or/and elemental sulfur generated during TSR have been incorporated into organic matter,but rare newly generated sulfur-containing organic compound has been separated and identified in nature.