鄂尔多斯盆地南部古生界天然气地球化学特征与成因

Geochemical characteristics and genesis of Paleozoic natural gas in the southern Ordos Basin

鄂尔多斯盆地南部古生界天然气地球化学特征与北部存在较大差异,除了由于烃源岩有机质成熟度升高造成的干燥系数增加、甲烷碳同位素组成较重之外,盆地南部上古生界天然气中CO_(2)气体含量更高、普遍出现甲、乙烷碳同位素组成倒转等现象。以气体地球化学为主要研究手段,在系统对比盆地古生界天然气南北差异的基础上,讨论天然气成因与来源。盆地南部上古生界天然气主体以高过成熟煤型气为主,其中部分地区的部分样品气体地球化学指标体现了下古生界海相烃源特征。盆地南部上古生界天然气整体出现甲、乙烷碳同位素组成倒转现象,这与上古生界与下古生界不同成因类型天然气混合作用有关。具有相对较高的乙烷含量和较轻乙烷碳同位素组成的下古生界油型气不同程度的混合是造成盆地南部上古生界天然气碳同位素组成倒转的主要原因。盆地南部下古生界天然气地球化学指标体现了典型海相烃源特征,同时不排除少量上古生界煤型气混入的可能。

The geochemical characteristics of Paleozoic natural gas in the southern part of the Ordos Basin are significantly different from those in the northern part.In addition to the increase in dryness coefficient caused by the increase in organic matter maturity of the source rocks and the heavier carbon isotope composition of methane,the CO_(2) gas content in the natural gas in the upper Paleozoic in the southern part of the basin is higher,and there is a common phenomenon of methane and ethane carbon isotope composition inversion.This article takes gas geochemistry as the main research method,and based on a systematic comparison of the north-south differences of natural gas in the Paleozoic Basin,discusses the origin and source of natural gas.The natural gas of the Upper Paleozoic in the southern part of the basin is mainly composed of high over mature coal type gas,and some sample gas geochemical indicators in some areas reflect the characteristics of lower paleomarine hydrocarbon sources.The overall inversion of methane and ethane carbon isotope composition in the Upper Paleozoic natural gas in the southern part of the basin is related to the mixing of different types of natural gas.The varying degrees of mixing of Lower Paleozoic oil type gas with relatively high ethane content and lighter ethane carbon isotope composition are the main reasons for the inversion of carbon isotope composition of Upper Paleozoic natural gas in the southern part of the basin.The geochemical indicators of natural gas in the Lower Paleozoic in the southern part of the basin reflect typical marine hydrocarbon source characteristics,and the possibility of a small amount of Upper Paleozoic coal type gas mixing cannot be ruled out.