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

Abstract

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 MgSO4) to address the chemical and carbon isotopic variations of the hydrocarbon, H2S and CO2. 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 δ13C1-3 values generally shift positively from control treatment to sulfate treatments and the (δ13Cethane-δ13Cmethane) values in sulfate treatments are higher than that in control treatments. The variation of δ13C1-3 in TSR is controlled by the isotope fractionation during the generation and consumption of C1-3 in TSR. MgSO4 is more reactive than anhydrite in TSR. A certain amount of H2S 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 CO2 in sour reservoirs. The negative shift of δ13CO2 with increasing TSR extent is mainly due to the inheritance effect of carbon isotope from hydrocarbons. Inorganic CO2 sourced from the thermal decomposition or acid dissolution of carbonate mineral impose significant influence on δ13CO2. The H2S yields decrease with CH4 yields and increase with δ13CH4 value, the δ13CH4 values increase with residual amount of gaseous alkane (1- H2S/(residual alkane + H2S)), suggesting that methane acted as reactant in TSR. The (δ13CO2-δ13CH4) values decrease significantly with increasing temperature, and the δ13CO2 is even more negative than δ13CH4 in 450 °C MgSO4 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.