Thermochemical Sulfate Reduction by Pyrobitumen: Review and Experiments

It has remained controversial throughout the last three decades whether highly mature solid bitumen derived from petroleum degradation can engage in thermochemical sulfate reduction (TSR)─an organic–inorganic interaction that occurs in sedimentary basins. To investigate the kinetic characteristics of thermal sulfate reduction by thermogenic pyrobitumen, hydropyrolysis experiments with magnesium sulfate (MgSO4) and a model compound, coronene (C24H12), were performed at 300–500 °C under controlled laboratory conditions. The experimental results indicate that detectable amounts of H2S from the designed simulation system were formed at a threshold temperature of 400 °C. Thermodynamically, coronene-initiated sulfate reduction is an exothermic process at 100–220 °C with the reaction heat of 221.0–248.3 kJ/mol hydrocarbon. TSR of coronene is characterized by a first-order reaction with an apparent activation energy of 193.0 kJ/mol. Interactions between C24H12 and MgSO4 are kinetically categorized as a slow TSR system because the high stability of coronene restricts the formation rate of H2S gas relative to rapid TSR systems containing C2+ hydrocarbons. When extrapolated to the temperature range of typical oil and gas reservoirs (100–200 °C), the reaction rate of TSR by coronene is slightly higher than that of TSR involving methane, gypsum, and MgCl2 solutions [He, K.; Zhang, S. C.; Wang, X. M. Hydrothermal experiments involving methane and sulfate: insights into carbon isotope fractionation of methane during thermochemical sulfate reduction, Org. Geochem. 2020, 149, 104107]. Depletion of some organic carbons by TSR may contribute to the increase of S/C ratios in the molecular structure of thermogenic pyrobitumen. In deep carbonate reservoirs of methane-dominated TSR, hydrothermal reduction of sulfates by thermogenic pyrobitumen most likely occurs as a less well-recognized geochemical process.