鄂尔多斯盆地中东部下古生界天然气地球化学特征差异性机理

Geochemical characteristics of natural gas in Upper Middle and Lower Paleozoic assemblages in the Middle East,Ordos Basin

鄂尔多斯盆地的天然气勘探进程与油气地球化学研究进展关系密切,通过对鄂尔多斯盆地中东部地区一批古生界天然气样品的地球化学系统分析,结合前人有关地球化学数据的搜集整理,对古生界天然气进行了地质—地球化学相关分析,着重对产于下古生界奥陶系的天然气组分、碳氢同位素组成等进行分层、分区对比,进一步明确奥陶系各层位天然气的主要来源。结果显示不同层系天然气地球化学特征与成因存在明显差异,奥陶系上组合天然气甲、乙烷碳同位素组成相对略重,且存在部分δ^(13)C_(CH_(4))>δ^(13)C_(C_(2)H_(6))的倒转,δD_(CH_(4))值大多低于-180‰;中—下组合天然气碳同位素组成相对偏轻,δD_(CH_(4))值主体高于-180‰;综合认为上组合是上古生界石炭系—二叠系煤型气与下古生界油型气复合,油型气有很大贡献,中—下组合天然气来自奥陶系海相烃源岩自生自储或更深更古老烃源。天然气地球化学特征证明了下古生界具有生烃能力,存在形成规模油气的烃源岩,具有成为未来勘探重点的潜力。

The exploration process of natural gas in Ordos Basin is closely related to the research progress of oil and gas geochemistry.In this paper,through the analysis of the geochemical system of a group of Paleozoic natural gas in the central and eastern Ordos Basin,combined with the collection and collation of previous geochemical data,the geological and geochemical correlation analysis of Paleozoic natural gas is carried out.The main source of natural gas in the Lower Paleozoic is further defined by the stratification and regional comparison of natural gas composition and hydrocarbon isotope composition in the Ordovician system.The results show that there are obvious differences in the geochemical characteristics and genesis of natural gas in different strata.The carbon isotopic composition of methane and ethane in the Upper Ordovician is relatively heavier,and carbon isotopic reversal exists between some methane and ethane,i.e.,δ^(13)C_(CH_(4))>δ^(13)C_(C_(2)H_(6))is reversed,and the δD_(CH_(4))value is mostly lower than−180‰.The carbon isotope composition of middle-lower combination gas is relatively light,and the δD_(CH_(4))value is mainly heavier than−180‰.It is considered that the upper assemblage is the combination of Upper Paleozoic Carboniferous and Permian coal-type gas and lower Paleozoic oil-type gas,and the oil-type gas makes a great contribution.The middle-lower assemblage gas comes from the Ordovician marine source rocks,which are self-generated and self-stored or from deeper and older sources.Natural gas geochemistry has proved that the Lower Paleozoic has hydrocarbon generation capacity,there are source rocks to form large-scale oil and gas,and it has the potential to become the focus of future exploration.