Geochemical Characteristics and Sources of Natural Gas from the South Margin of Junggar Basin

Natural gas distributes widely in all the three anticline belts in the south margin of Junggar basin.It is mainly composed of hydrocarbons,in which methane is dominated,often with nitrogen and carbon dioxide.The gas is typically thermogenic,and mainly coal-derived from Middle-Lower Jurassic mature-over mature coal-measure source rocks.The natural gas in the first and second anticline belts,and part of the third anticline belt(Dushanzi anticline of Lower Paleogene and Hutubi anticline) is typical high-over mature coal-derived,while that in the other part of the third anticline belt is typical mature coal-derived.The coal-derived gas in the south margin of Junggar basin shows partially reversal of carbon isotopes,mainly due to the mixing of the same type gas with different sources(different strata of Jurassic),and partially due to the bacterial oxidation(in the Gumudi anticline) or the mixing with oil-associated gas(in the Qigu and Anjihai anticlines).The oil-associated gas of this area is only in the Cretaceous Kayindike oil field and Paleogene Dushanzi oil gas field,and sourced from mature Cretaceous sapropelic source rocks and high-over mature Permian sapropelic source rocks respectively.The Qigu coal-derived gas of the Permian and Triassic is mainly derived from the Lower Jurassic Badaowan Formation coal-measure source rocks deep-seated in the footwall of the fault.while that of the Jurassic is mainly derived from the Middle Jurassic Xishanyao Formation and Lower Jurassic Sangonghe Formation source rocks buried shallower with relatively lower maturity and therefore has relatively lower carbon isotope value and methane content.