Fractionation of carbon and hydrogen isotopes of TSR-altered gas products under closed system pyrolysis

Thermochemical sulfate reduction (TSR) is common in marine carbonate gas reservoirs, leading to complicated isotope characteristics of TSR-altered gas. This study aims to better understand how TSR affects the geochemical and isotopic compositions of alkanes in pyrolysis products. Pyrolysis of TSR were conducted with crude oil, nonane (C-9) and methylnaphthalene (MN) in the presence of MgSO4 solution at temperatures of 350 degrees C, 360 degrees C, and 370 degrees C for different durations of 4-219 h in a closed system. Results show that carbon and hydrogen isotope compositions of alkane gas resulting from TSR (pyrolysis with crude oil and MgSO4) became heavier with increasing carbon number, i.e., delta C-13(1)delta C-13(2)), which, for the first time, confirmed the ability of TSR to cause isotopic reversal from pyrolysis. The residual heavy alkanes gradually became C-13-enriched during TSR, which increased delta C-13(2) values and changed the partially reversed isotope sequence to a positive sequence (delta C-13(1)