Stable hydrogen and carbon isotopic ratios of coal-derived gases from the Turpan-Hami Basin, NW China

Abstract

Natural gases from the Qiudong and Hongtai gas fields in the Taibei sag in the Turpan-Hami Basin of China were analyzed for stable hydrogen and carbon isotopic characteristics, to investigate the origin of the gas and the gas-source correlation. Natural gases from the two gas fields are dominated by alkane gases with methane content varying from 81.61% to 92.57%. Dryness ranges from 0.76 to 0.93, indicating an abundance of wet gas. The carbon and hydrogen isotopic compositions of the methane vary from −39.8‰ to −42.6‰ and from −263‰ to −279‰ in the Qiudong gas field, respectively, and from −37.7‰ to −39.0‰ and from −250‰ to −257‰ in the Hongtai gas field, respectively. All gases have a characteristic carbon and hydrogen isotopic distribution pattern among the C1–C5 alkanes, in which δ13CCH4<δ13CC2H6<δ13CC3H8<δ13CC4H10<δ13CC5H12 and δ2HCH4<δ2HC2H6<δ2HC3H8. Both the stable carbon and hydrogen isotopic compositions of the gases and the hydrogen isotopic value of the formation water suggest that gases in the two gas fields are coal-derived without admixtures with gases of other origin. According to the calculation with empirical δ13CCH4–Ro% relationship, the thermal maturity (vitrinite reflectance Ro%) of the source rocks is up to 0.76% in the Qiudong area and up to 0.86% in the Hongtai area, which are in the oil window and approaching the start of major thermogenesis at the top of the maturity range. Since the gases in the Hongtai gas field are relatively more enriched in 13C and 2H, combined with the local geological thermal evolution history, it is concluded that the gases in the two gas fields were derived from the Lower Jurassic Badaowan source rocks. The heavier hydrogen and carbon isotopes of the gases from the Hongtai gas field are interpreted as a product of higher thermal maturity of the source rocks. This is in consistent with the thermal maturity of the local source rocks. The modern thermal maturity of the top of the Badaowan Formation is around 0.8% in the Qiudong area and around 1.0% in the Hongtai area. The combined use of both carbon and hydrogen isotopes is of high value in providing information on gas origin and gas-source correlation, which will be very useful for future gas exploration.