Carbon isotope and origin of the hydrocarbon gases in the Junggar Basin, China

Abstract

The genetic type, source and distribution of hydrocarbon gases in the Junggar Basin were clarified through the carbon isotope analysis. Mature to post mature oil-type gas, mature to post mature coal-type gas, transition gas and biogas are identified in the Junggar Basin. Partly reversed order of carbon isotope of hydrocarbon gases in the Junggar Basin are attributed to one or several of the following reasons: mixing of oil-type and coal-type gases, mixing of coal-type gases of different source, mixing of coal-type gases of varied maturity, and microbial action. Three types of coal-type gases in the Junggar Basin are identified. The first type of coal-type gases characterized with high δ13C values of heavy hydrocarbon gases (δ13C2>−26.0‰) are the mature to high mature gases that are generated from Jurassic source rocks. The second type of coal-type gases characterized with low δ13C values of heavy hydrocarbon gases (δ13C2<−26.0‰) and wide maturity range, are generated from one or several source rocks in the Jurassic and the Wuerhe and Jiamuhe Formations of Permian. The third type of coal-type gases characterized with a wide δ13C value of heavy hydrocarbon gases and the high-post maturity are generated from the Carboniferous source rocks.