Hydrothermal experiments involving methane and sulfate: Insights into carbon isotope fractionation of methane during thermochemical sulfate reduction

To understand the isotope fractionation of methane (CH4) during thermochemical sulfate reduction (TSR), isothermal pyrolysis of CH4/N-2 gas involving gypsum + MgCl2 and MgSO4 solutions were conducted in a gold-tube system. CH4 was oxidized to generate H2S and CO2 via TSR in the hydrothermal experiments with sulfate. The delta C-13(1) becomes more enriched with the conversion of CH4. However, there is no evident enrichment of H-2 for CH4 during TSR under hydrothermal conditions. Ab initio quantum calculations based on Density Functional Theory (DFT) demonstrate that the kinetic isotope effect (KIE) rather than the thermodynamic equilibrium isotope effect (EIE) governs the C-13 fractionation during methane TSR. The kinetic C-13 fractionation factor (alpha) for TSR of CH4 involving SO4 and HSO4 is 0.960-0.992 between 0 degrees C and 600 degrees C. Combining these experimental results, a for methane TSR was in the range 0.98-0.99 with an average value of 0.985. Based on these results, the extent of methane TSR for natural gas in the Puguang gas field in the northeastern Sichuan Basin was determined via the evolution of delta C-13(1). Geological extrapolation of the kinetic parameters and KIE suggests that the conversion of CH4 via TSR in the P(2)ch-T(1)f Formations in this gas field is <20%. (C) 2020 Elsevier Ltd. All rights reserved.