Carbon-initiated sulfate reduction in a closed hydrous system: implications for the formation of H2S in deeply buried petroleum reservoirs

Sulfate reduction by activated carbon (AC), a model compound for solid bitumen (pyrobitumen) without gas generative potential, was studied experimentally for the first time. The simulation experiments show that hydrogen sulfide (H 2 S) is produced from hydrothermal reduction of calcium sulfate (CaSO 4 ) by AC at elevated temperatures. This result demonstrates that water can also act as a hydrogen source for H 2 S generation in addition to hydrocarbons. The formation mechanism of H 2 S may undergo a three-stage sequential process with calcium sulfide (CaS) as the unstable intermediate. The reactivity of carbon is thermodynamically higher than that of methane at ≥ 150 o C but lower than that of ethane or propane in sedimentary basins. Carbon-initiated sulfate reduction follows a zero–order reaction with an apparent activation energy E a of 172.36 kJ/mol. When extrapolated to the temperature range of typical oil and gas reservoirs (100–200 o C), the order of the rate constant is determined as propane > ethane > solid bitumen > methane in the TSR of CaSO 4 . Although solid bitumen contributes little to hydrocarbon gas generation, H 2 S derived from carbon-initiated sulfate reduction may act as an indispensable catalyst for the consequent alteration of methane by TSR in deep buried gas reservoirs with high dryness coefficient.