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

Abstract

AbstractThermochemical 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 (C9) and methylnaphthalene (MN) in the presence of MgSO4 solution at temperatures of 350 °C, 360 °C, and 370 °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., δ13C1 < δ13C2 < δ13C3 and δ2H–C1 < δ2H–C2 < δ2H–C3. Compared with the δ13C1, δ13C2 and δ13C3 increased in a much wider range as heating continued. Carbon and hydrogen isotopes of alkane gas produced by TSR became heavier with increasing gas souring index. Values for δ13C1–δ13C2 and δ2H–C1– δ2H–C2 typically decreased as oil and C9 underwent thermal cracking. Comparative experiments using C9 in the presence of MgSO4 produced partially reversed carbon isotope series (δ13C1 > δ13C2), which, for the first time, confirmed the ability of TSR to cause isotopic reversal from pyrolysis. The residual heavy alkanes gradually became 13C-enriched during TSR, which increased δ13C2 values and changed the partially reversed isotope sequence to a positive sequence (δ13C1 < δ13C2). The discovery of a partial reversal of the carbon isotope series of alkane gases through pyrolysis will further deepen the understanding of TSR-altered natural gas.