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

Geochemical Characteristics of Ordovician Subsalt Gas Reservoir and Their Significance for Reunderstanding the Gas Source of Jingbian Gasfield,East Ordos Basin

在鄂尔多斯盆地东部龙探1井盐下马五7储层获得了低产天然气。测试资料显示,其甲烷含量为96.871%,C2+重烃组分含量仅为2.396%,具高演化干气特征;非烃组分含量很低,应属储层中的正常天然气;甲烷的碳同位素组成偏轻,δ13C1值为-39.26‰,代表了奥陶系原生过成熟油型气的特征;C2+重烃组分的单体烃碳同位素显著偏重,分布于-19.27‰～-23.78‰之间,与高演化的煤成气相似。仅从空间配置关系判断,上古生界煤成气混入的可能性很小,因而,次生因素——TSR反应等是造成重烃组分碳同位素偏重的主要原因。以龙探1井马五7天然气为油型气端元,甲烷碳同位素组成作为主要指标,进行靖边气田奥陶系天然气主要气源的判识,得出了靖边气田奥陶系风化壳天然气的主要气源应是上古生界煤系的认识。

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 compo- nents, 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 identi- fied, 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.