Stable carbon and hydrogen isotopic characteristics of natural gas from Taibei sag, Turpan-Hami Basin, NW China

Abstract

Turpan-Hami Basin is a major petroliferous basin in China. To date the natural gas exploration is concentrated in the Taibei sag. The origin and source of natural gas in the Taibei sag has long been controversial. To further investigate the origin and source of the natural gas in the Taibei sag, combined with previous studies and the local geological backgrounds, this study collected 23 gas samples from the Baka, Qiuling, Shanshan and Wenmi oil fields in the Taibei sag and analyzed the sample composition, stable carbon and hydrogen isotopes of all the gas samples. The results show that, gases from the four oil fields in the Taibei sag are dominated by hydrocarbon gas and belong to wet gas. Methane accounts for 65.84% to 97.94%, the content of heavy hydrocarbon (C2-5) can be up to 34.98%, while the content of nonhydrocarbon (CO2, N2) is trace. The δ13C1 value is –44.9‰ to –40.4‰, δ13C2 is –28.2‰ to –24.9‰, δ13C3 is –27.1‰ to –18.0‰ and δ13C4 is –26.7‰ to –22.1; while the variation of δD1 is not significant from –272‰ to –252‰, δD2 is –236‰ to –200‰ and δD3 is –222‰ to –174‰. Methane and its homologues (C2-5) are characterized by normal stable carbon and hydrogen isotopic distribution pattern, i.e., with the increase of carbon number, methane and its homologues become more and more enriched in 13C or D (δ13C1<δ13C2<δ13C3<δ13C4<δ13C5, δD1<δD2<δD3), which is consistent with the carbon and hydrogen isotopic features of typical thermogenic gas. All these results show that the natural gases in the four oil fields are coal-derived gas with low maturity (Ro averaged at 0.7%), and are sourced from the Middle-Lower Jurassic coal measure. The hydrogen isotopic data of natural gas are affected by both thermal maturity and the water medium of the environment where source rocks are formed. The hydrogen isotopic data indicate that the source rocks are formed in terrestrial limnetic facies with freshwater. Natural gases from Well Ba23 and Well Ke19 experienced biodegradation in the late stage. Key words: Turpan-Hami Basin, Taibei sag, Jurassic, carbon isotope, hydrogen isotope, coal-derived gas, low mature gas