Geochemical Characteristics of Ordovician Subsalt Gas Reservoir and Their Significance for Reunderstanding the Gas Source of Jingbian Gasfield, East Ordos Basin
鄂尔多斯盆地东部奥陶系盐下天然气地球化学特征及其对靖边气田气源再认识

Low yield gas was discovered in the subsalt Mawu 7 reservoir at Longtan 1 well in the east of Ordos basin. As revealed by the test data, the gas contains 96.871% of methane, only 2.396% of C2+ heavy hydrocarbon characteristic of dry gas of high maturity, and very low contents of non-hydrocarbon components, indicating that the gas is a normal type in a typical reservoir. The carbon isotope of methane is light type, with δ13C1 being -39.26‰, indicating that the gas is of primitive oilfield type at overmature stage in the Ordovician. The hydrocarbon\|specific carbon isotope of C2+ heavy hydrocarbon components is obviously heavy type, with δ13C1 varying between -19.27‰ and -23.78‰, suggesting that the gas is similar to coal-generated gas of high maturity. Based on the spatial distribution of the gas reservoir, coal-generated gas in upper Paleozoic is less likely to be involved in the formation of the salt gas, as a result, secondary reactions like TSR reaction could be the dominating affecting element for formation of the salt gas. When Mawu 7 gas from Longtan 1 well is taken as an end member of oilfield type gas and the carbon isotope of methane as a major index, the major source for the Ordovician gas in the Jingbian gasfield can be identified, and a conclusion can be drawn that the major source for natural gas in the Ordovician weathering crust in Jingbian gasfield is the Upper Paleozoic coal measure.