The effects of selected minerals on laboratory simulated thermochemical sulfate reduction

Improved understanding of the reservoir mineral effects on the occurrence of thermochemical sulfate reduction (TSR) and possible mechanisms is critical for petroleum exploration and production. A series of hydrous pyrolysis experiments were conducted with and without the presence of montmorillonite, illite and quartz (silicates), calcite and dolomite (carbonates) at 360 °C and 50 MPa for 12 h, 48 h and 312 h. Gaseous compounds, C7-8 light hydrocarbons, n-C9+ and organic sulfur compounds as well as compound-specific δ34S analysis were conducted subsequently. The results show that with the heating time increasing or TSR advancing, n-C9+ decrease gradually accompanied by the increases of TSR products, H2S, alkylbenzenes, benzothiophenes (BTs) and dibenzothiophenes (DBTs). The δ34S values of BTs and DBTs progressively approached that of MgSO4 and the difference in their average δ34S values (Δ34SBT-DBT) decreased. Compared to experiments with the absence of minerals, the experiments with silicates, in particular montmorillonite, are depleted in n-C9+ and enriched in H2S, light monoaromatics and organosulfur compounds and smaller Δ34SBT-DBT. Experiments with carbonates demonstrated the opposite trend. This indicates that silicates, in particular montmorillonite, accelerated TSR rates while carbonates reduced them. Under the experimental conditions, the exchange cations within montmorillonite and on the surface of clays and talc precipitation can increase the concentrations of H+ and hence HSO4− and then catalyze the generation of isoalkanes, which are prone to be oxidized in TSR. This can reduce the activation energy required for the occurrence of TSR. It might therefore be possible for TSR to occur at relatively low temperatures in silicate reservoirs and source rocks in natural environments. Carbonate dissolution can reduce the concentrations of H+ and HSO4−, and hence reduce the TSR rates. This study demonstrates the importance of minerals in regulating the TSR occurrence and suggests that TSR can occur in siliciclasitic as well as carbonate formations in nature. Therefore, the significance of minerals should be considered in the evaluation of TSR and sour gas risk in TSR-altered hydrocarbon accumulations in different geological contexts.