Characterization of chemical and carbon isotopic compositions of gases during thermochemical sulfate reduction and implications for gas origin and content

For identifying the occurrence and extent of thermochemical sulfate reduction (TSR) reaction of natural gas and better understanding the chemical and carbon isotopic variations in natural gas reservoirs, high-pressure hydro-pyrolysis with a special designed apparatus was performed using natural gas and various amounts of MgSO(4)·7H(2)O at up to 360 °C. The yields, chemical and isotopic compositions of the gases produced during TSR and thermal cracking were measured. As the extent of TSR reaction increased, the concentrations of CH(4), CO(2) and H(2)S increased in a nonlinear way, while those of C(2)H(6) and C(3)H(8) decreased. According to the variation of gas content, the TSR reaction of alkane gases can be divided into an uncatalyzed and a catalyzed stage, which is different from previous studies that treated the TSR reaction of alkane gases as a non-autocatalytic reduction process. As the concentration of MgSO(4)·7H(2)O increased, the rate of TSR reaction with hydrocarbon gases increased. The concentrations of HSO(4)(−) and volume of aqueous phase could be responsible for the different TSR reaction rates in the catalyzed stage. The co-variation of ln(C(1)/C(2)) and ln(C(2)/C(3)) could be related to the TSR reaction of alkane gases. Our study provides clues for understanding the compositional variations in natural conditions.