Genesis and Origins of Natural Gas in Eastern Belt around Penyijingxi Sag in Junggar Basin, NW China

Abstract: The Junggar basin is sandwiched between the Siberian plate, the Kazakhstan plate and the Tarim plate, and is an important part of the Central Asian orogenic belt. Based on the comprehensive analysis of the characteristics of the natural gas composition，carbon isotope, light hydrocarbons and source rocks in the eastern belt around Penyijingxi Sag, Junggar Basin, i.e., our studied area, the genesis and origins of natural gas in this area are discussed. The natural gases in the eastern belt around Penyijingxi Sag, are dominated by alkane gases, and have relatively low contents of heavy hydrocarbons and non-hydrocarbons. Methane is dominant in alkane gas, with volume fraction varies from 70.36% to 93.34%. In non-hydrocarbon gas, the volume fraction of nitrogen varies from 0.69% to 11.95%, and the volume fraction of carbon dioxide varies from 0 to 1.49%. The values of δ¹³methane(C₁), δ¹³ethane(C₂), δ¹³propane(C₃) and δ¹³butane(C₄) of natural gas are in the ranges from −45.57‰ to −31.19‰, −31.69‰ to −24.66‰, −28.76‰ to −23.56‰, −27.96‰ to −23.64‰, respectively. The overall carbon isotopic composition of the alkanes shows a trend as δ¹³C₁ < δ¹³C₂ < δ¹³C₃ < δ¹³C₄, and all δ¹³C₁ values are ≤ -30‰, which are typical of gases of organic origin. The methane and ethane isotopic compositions and the characteristics of light hydrocarbons show that the natural gases in the studied area are dominated by coal-type gas and contain a small amount of oil-type gas. Specifically, the coal-type gas is from the mature to highly mature source rocks of the Lower Urho Formation, and the oil-type gas is from the mature to highly mature source rocks of the Fengcheng Formation. Analysis of gas migration parameters show that, while there was no significant lateral migration of natural gas in the studied area, natural gases once migrated vertically and resulted in the mixing of oil- and coal-type gases as well as the mixing of alkane gases of the same genetic type formed at different stages, which should be the cause of observed reversed carbon isotopic series. The diffusion and migration of carboniferous oil and gas after reservoir formation have led to differences in gas geochemical characteristics among gas wells in this area, which may provide important information for oil and gas exploration in the central Junggar Basin.

Keywords: Junggar Basin; geochemistry; natural gas genesis; carbon isotopes; light hydrocarbons