Abstract
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 (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.
Highlights
Thermochemical sulfate reduction (TSR) is common in marine carbonate gas reservoirs, leading to complicated isotope characteristics of TSR-altered gas
Pyrolysis of TSR was carried out using different organic matter, and the characteristics of carbon and hydrogen isotopes, as well as the composition and yields of the reaction products were analysed
The following main conclusions can be drawn: (1) The carbon and hydrogen isotopic composition of alkane gas generally becomes heavier with increasing carbon number, i.e., δ13C1 < δ13C2 < δ13C3 and δ2H–C1 < δ2H–C2 < δ2H–C3
Summary
Thermochemical sulfate reduction (TSR) is common in marine carbonate gas reservoirs, leading to complicated isotope characteristics of TSR-altered gas. Pan et al.[12] conducted a hightemperature, long-time, step-by-step thermal cracking simulation with organic matter and F e2O3, MgSO4, and a mixture of both They found that C H4 was produced by TSR in the presence of heavy hydrocarbons and that isotope fractionation became more pronounced with increasing carbon number. This is the first study that considered the impact of the presence of water during TSR alteration on the hydrogen isotope fractionation of alkane gas. The effects of thermal cracking and TSR on the fractionation of carbon and hydrogen isotopes of alkane gas were studied to explore the isotopic evolution of carbon and hydrogen caused by TSR, aiming to provide experimental evidence to better understand the isotopic composition and variation of gases in H 2S-bearing reservoirs
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