Enrichment of nitrogen and 13C of methane in natural gases from the Khuff Formation, Saudi Arabia, caused by thermochemical sulfate reduction

Abstract Permian Khuff reservoirs along the east coast of Saudi Arabia and in the Arabian Gulf produce dry sour gas with highly variable nitrogen concentrations. Rough correlations between N2/CH4, CO2/CH4 and H2S/CH4 suggest that non-hydrocarbon gas abundances are controlled by thermochemical sulfate reduction (TSR). In Khuff gases judged to be unaltered by TSR, methane δ13C generally falls between −40‰ and −35‰ VPDB and carbon dioxide δ13C between −3‰ and 0‰ VPDB. As H2S/CH4 increases, methane δ13C increases to as much as −3‰ and carbon dioxide δ13C decreases to as little as −28‰. These changes are interpreted to reflect the oxidation of methane to carbon dioxide. Khuff reservoir temperatures, which locally exceed 150 °C, appear high enough to drive the reduction of sulfate by methane. Anhydrite is abundant in the Khuff and fine grained nodules are commonly rimmed with secondary calcite cement. Some cores contain abundant pyrite, sphalerite and galena. Assuming that nitrogen is inert, loss of methane by TSR should increase N2/CH4 of the residual gas and leave δ15N unaltered. δ15N of Paleozoic gases in Saudi Arabia varies from −7‰ to 1‰ vs. air and supports the TSR hypothesis. N2/CH4 in gases from stacked Khuff reservoirs varies by a factor of 19 yet the variation in δ15N (0.3–0.5‰) is trivial. Because the relative abundance of hydrogen sulfide is not a fully reliable extent of reaction parameter, we have attempted to assess the extent of TSR using plots of methane δ13C versus log(N2/CH4). Observed variations in these parameters can be fitted using simple Rayleigh models with kinetic carbon isotope fractionation factors between 0.98 and 0.99. We calculate that TSR may have destroyed more than 90% of the original methane charge in the most extreme instance. The possibility that methane may be completely destroyed by TSR has important implications for deep gas exploration and the origin of gases rich in nitrogen as well as hydrogen sulfide.

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