Characterizing Thermochemical Sulphate Reduction: Towards Making Predictions of Sour Gas in Sedimentary Basins

Worden, Richard H.¹ (1) University of Liverpool, Liverpool, United Kingdom

Thermochemical Sulphate Reduction (TSR) involves the reaction between petroleum fluids and sulphate minerals in reservoirs and leads to the growth of carbonate minerals and large quantities of H2S in the remaining petroleum. TSR involves a large number of steps including mineral dissolution, the redox process, transport of reactants and growth of mineral reaction products. It has not been possible to predict the occurrence of TSR and so risk H2S occurrence in prospects. The number of TSR case studies remains limited and the Permian-Triassic Khuff Formation in the Middle East remains one of the better studied systems. A review of the data shows that oil-TSR occurs at temperatures >110ºC while gas-TSR occurs at >140ºC showing that the minimum temperature appears to sensitive to the type of petroleum in the reservoir. Experimental determinations of TSR redox rates have merely shown that the redox step is unlikely to be rate-limiting since the determined half life of reactants is of the order of 100 to 100,000 years whereas fields have contained co-existing sulphate minerals and petroleum for minerals of years without reacting. Sulphur isotope data from sulphate and H2S surprisingly show no fractionation suggesting that the delivery of sulphate (dissolution of anhydrite) into the reactive milieu may be rate limiting. Furthermore, partly reacted anhydrite nodules commonly develop an armour plate of calcite that physically isolates remaining anhydrite from petroleum. The rate limiting steps in TSR probably varies as reaction proceeds but, despite vast experimental efforts, is unlikely to be the redox step.