Geochemistry and Origins of Natural Gases in the Central Junggar Basin, Northwest China

The southwestern Junggar Basin in northwest China is a significant target of basin's hydrocarbon exploration and exploitation at present. It is petroliferous mainly in oil production. However, natural gas should have good prospects because multiple sets of gas-prone source rocks are developed. Thus, in order to expand the field of hydrocarbon exploration (natural gas in particular), origins of the gases were discussed in this paper based on relatively comprehensive analyses of gas geochemistry, which include components, carbon isotopes and light hydrocarbons of gas and biomarkers of associated condensates. The results indicate two typical genetic types of gases. The first type is the coal-type and oil-type gases sourced from Permian lacustrine mudstones in the Shawan sag. It is distributed mainly in the Chepaizi area, whose most distinctive geochemical feature is the θ13C2 value (ranging from −30.29‰ to −25.09‰ with an average of −27.03‰.) The gas exploration potential is good. By contrast, the second type of gas is the coal-type gas sourced from Jurassic coal-bearing rocks in the southern basin. It is distributed mainly in the western area of the southern basin, with a few in the southern part of the Chepaizi area. θ13C2 value of the gases ranges from −27.14‰ to −21.74‰ with an average of −24.81‰, sharply heavier than that of the first type of gas. Gas exploration potential is fairly good, mainly being controlled by source-rock maturity.