Geochemical characteristics and genesis of the Upper Paleozoic natural gas in Longdong area, Ordos Basin, NW China

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Abstract
The Upper Paleozoic gases from the Longdong areas in the Ordos Basin are characterized by low contents of heavy hydrocarbon gases, the mean value of wetness as 2%, relatively heavy δ13C1(δ13C1 value -28.3‰), and large variations in δ13C2 +. The distribution pattern of carbon isotopic sequence of natural gas is mainly characterized by the abnormal light δ13C2 + values and carbon isotopic reversal. According to the organic geochemical characteristics of natural gas and the development of gas-source rocks, the upper Paleozoic natural gas in Longdong area belongs to high mature and over-mature “coal-formed gas”. In theory, only the mixture of highly mature gas with extremely low contents of heavy hydrocarbon gases and a small amount of natural gas with higher contents and lighter carbon isotopic composition of heavy hydrocarbon gases can lead to the occurrence of abnormally light δ13C2 + and the carbon isotope reversal between methane and ethane. The mixed gas is probably derived from the thermal cracking of a small amount of residual soluble organic matter in the gas-source rocks and reservoirs during the initial stage of the uplift movement at the end of Early Cretaceous. Key words: Ethane carbon isotope; Inverted sequence of carbon isotopic composition; Natural gas of high maturity; natural gas genesis; Ordos Basin.