Study of thermochemical sulfate reduction of different organic matter: Insight from systematic TSR simulation experiments

Abstract A series of thermochemical sulfate reduction (TSR) simulation experiments were carried out involving different organic matter (crude oil, solid bitumen, type II kerogen, type III kerogen) and different sulfate species (anhydrite and MgSO 4 ) to address the chemical and carbon isotopic variations of the hydrocarbon, H 2 S and CO 2 . The increase of main peak carbons of residual saturated hydrocarbon and the decrease of gaseous hydrocarbons from control treatment to corresponding sulfate treatments suggest that TSR promote the consumption of both liquid and gaseous hydrocarbons. The δ 13 C 1 -3 values generally shift positively from control treatment to sulfate treatments and the (δ 13 C ethane -δ 13 C methane ) values in sulfate treatments are higher than that in control treatments. The variation of δ 13 C 1 -3 in TSR is controlled by the isotope fractionation during the generation and consumption of C 1 -3 in TSR. MgSO 4 is more reactive than anhydrite in TSR. A certain amount of H 2 S is incorporated into solid bitumen as the result of secondary alteration. We believe that the reactivity order of different organic matter in TSR is crude oil > solid bitumen > type II kerogen > type III kerogen. The reactivity of organic matter in TSR depends on the hydrocarbon generation kinetics of each organic matter. The dissolution/decomposition and precipitation of carbonate control the yield of CO 2 in sour reservoirs. The negative shift of δ 13 CO 2 with increasing TSR extent is mainly due to the inheritance effect of carbon isotope from hydrocarbons. Inorganic CO 2 sourced from the thermal decomposition or acid dissolution of carbonate mineral impose significant influence on δ 13 CO 2 . The H 2 S yields decrease with CH 4 yields and increase with δ 13 CH 4 value, the δ 13 CH 4 values increase with residual amount of gaseous alkane (1- H 2 S/(residual alkane + H 2 S)), suggesting that methane acted as reactant in TSR. The (δ 13 CO 2 -δ 13 CH 4 ) values decrease significantly with increasing temperature, and the δ 13 CO 2 is even more negative than δ 13 CH 4 in 450 °C MgSO 4 treatments involving type II and type Ⅲ kerogen. Accordingly, methane acted as a predominant reactant in 450 °C sulfate treatments in our experiment. It is possible for natural gas with high gas dryness to experience methane-dominated TSR in geological reservoirs.