Stable carbon isotopic composition of light hydrocarbons and n-alkanes of condensates in the Tarim Basin, NW China

Abstract

The carbon isotope ratios of individual light hydrocarbons and the n-alkanes of twenty-one condensates from the Tarim Basin, as well as 47 condensates and oils from other petroliferous basins (Ordos Basin, Sichuan Basin, Turpan-Harmi Basin, Qiongdongnan Basin, Beibu Gulf Basin and Bohai Bay Basin) in China, were analyzed. We investigated the oil–oil correlation, the effects of gas washing and maturity, as well as the distinguishing parameters of humic and sapropelic condensates, and have come to the following conclusion. The carbon isotopic patterns of condensates and oils in the Ordovician strata of Tarim Basin are very similar, indicating they originate from the same type of source rocks. The condensates from Dawanqi oil field and Yinan 2, as well as Ti'ergen and Yitikelike gas fields, have similar carbon isotopic patterns. Thus, they probably have originated from the same terrestrial Jurassic source rock. The carbon isotopic patterns of the condensates from the Dabei, Kela 2, and Keshen gas fields are also similar, indicating they are of the same oil family and sourced from the Triassic and Jurassic terrestrial source rock. The carbon isotopic ratios of 2-MP, 3-MP, 3-MH, and nC5-8 are much more susceptible to maturity level than other light hydrocarbons. Gas washing has minor effects on the δ13C compositions of individual light hydrocarbons and n-alkanes, although it causes <2‰ shifts. The δ13C compositions of MCP, CH, MCH, benzene, and toluene can be used as identification parameters for humic and sapropelic condensates. Humic condensates generally have δ13CMCP > −25‰, δ13CCH > −24‰, δ13CMCH > −24‰, δ13Cbenzene > −25‰, and δ13Ctoluene > −24‰, whereas sapropelic condensates mainly have δ13CMCP < −26‰, δ13CCH < −26‰, δ13CMCH < −24‰, δ13Cbenzene < −25‰ and δ13Ctoluene < −24‰. Moreover, the mixing humic and sapropelic condensates usually show intermediate values.