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 MgSO(4) 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 MgSO(4)) became heavier with increasing carbon number, i.e., δ(13)C(1) < δ(13)C(2) < δ(13)C(3) and δ(2)H–C(1) < δ(2)H–C(2) < δ(2)H–C(3). Compared with the δ(13)C(1), δ(13)C(2) and δ(13)C(3) 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 δ(13)C(1)–δ(13)C(2) and δ(2)H–C(1)– δ(2)H–C(2) typically decreased as oil and C(9) underwent thermal cracking. Comparative experiments using C(9) in the presence of MgSO(4) produced partially reversed carbon isotope series (δ(13)C(1) > δ(13)C(2)), which, for the first time, confirmed the ability of TSR to cause isotopic reversal from pyrolysis. The residual heavy alkanes gradually became (13)C-enriched during TSR, which increased δ(13)C(2) values and changed the partially reversed isotope sequence to a positive sequence (δ(13)C(1) < δ(13)C(2)). 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.