鄂尔多斯盆地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 Ordos Basin

鄂尔多斯盆地古生界天然气地球化学特征与成因复杂,下古生界气源存在争议。为更好地把握盆地古生界天然气的整体地球化学特征和变化规律,在系统收集、整理和解析前人资料基础上,对全盆地700余个天然气样品的地球化学资料进行了综合分析,并结合对天然气成源、成烃和成藏等关键要素的分析,探讨了天然气的成因。结果表明,鄂尔多斯盆地上古生界天然气主体为受成熟度控制的煤型气,由东北向西南方向成熟度升高,盆地南部上古生界地层存在上、下古生界天然气的混合。下古生界天然气以海相碳酸盐岩层系油型气为主,其中上组合具有上古生界煤型气—下古生界油型气两源复合成藏特征,中—下组合天然气是相对独立的油型气藏,以地质条件和有机质热演化程度为基础受不同程度的次生改造作用,造成了其碳同位素组成的显著异常。研究证明了盆地下古生界存在包括碳酸盐岩在内的海相烃源的规模贡献,在盆地的不同位置,上、下古生界天然气存在不同程度的混合。

The geochemical characteristics and genesis of Paleozoic natural gas in the Ordos Basin are complex,with some disputes on the gas source of Lower Paleozoic.In order to better grasp the overall geochemical charac⁃teristics and variation laws of Paleozoic natural gas in the basin,the geochemical data of more than 700 natural gas samples from the whole basin are comprehensively analyzed based on the systematic collection,collation and analysis of previous data,and the genesis of natural gas is explored by combining the analysis of key elements such as natural gas formation,hydrocarbon formation and reservoir formation.The results show that the main body of Upper Paleozoic natural gas is coal⁃based gas controlled by maturity,with maturity increasing from north⁃east to southwest,and the southern part of the basin has a mixture of Upper and Lower Paleozoic gas.The Lower Paleozoic gas is dominated by oil⁃type gas from marine carbonate formations,of which the upper combination is characterized by a two⁃source composite reservoir of Upper Paleozoic coal⁃type gas and Lower Paleozoic oil⁃type gas;the middle-lower combination is a relatively independent oil⁃type gas reservoir,subject to different degrees of secondary transformation based on geological conditions and the degree of thermal evolution of organic matter,resulting in significant anomalies in its carbon isotope composition.The research has proved that marine hydrocarbon sources including carbonate rocks exist in the Lower Paleozoic of the basin,and the natural gas in the Upper and Lower Paleozoic has mixed to varying degrees in different locations of the basin.