南堡凹陷天然气赋存状态与碳同位素组成特征

Occurrences and Carbon isotopic compositions of the natural gas in Nanpu sag

为了明确南堡凹陷天然气成因和分布规律,通过天然气赋存状态和碳同位素组成特征研究,综合地质条件,认为南堡凹陷主要发育有3种成因类型天然气,且天然气赋存状态和碳同位素组成主要受两套烃源岩性质的差异性和断裂的垂向疏导控制.结果表明:南堡凹陷天然气深层以游离气为主,中浅层以油溶气为主,浅层含有少量游离气;沙三段和奥陶系天然气乙烷δ13 C值一般大于-27.5‰,丙烷δ13 C值大于-25.5‰,为煤型气,来源于沙三段烃源岩,且碳同位素部分倒转主要是同源不同期天然气混合造成的;沙一段天然气乙烷δ13 C值一般小于-27.5‰,丙烷δ13C值小于-25.5‰,为油型气,来源于沙一段烃源岩,且碳同位素部分倒转主要是同源不同期天然气混合造成的;东营组和新近系天然气乙烷δ13 C值为-31.5‰^-25.5‰,丙烷δ13 C值为-28.1‰^-22.4‰,为沙三段和沙一段烃源岩的混源气,且碳同位素部分倒转主要是不同源不同类型天然气混合、同源不同期天然气混合以及细菌氧化作用共同造成的,断裂为垂向运移通道.

To determine the genetic types and distribution characteristics of gases in Nanpu sag,the occurrences and carbon isotopic compositions of the natural gases were analyzed.Integrating with geological characteristics,it is considered that three genetic types of gas occur in Nanpu sag,with their occurrences and carbon isotopic compositions controlled by the varied characteristics of two source rocks and the vertical conduct of faults.The results show that free gas is predominant in deep layers,and the occurrence in medium-shallow layer is dominated by oil-dissolved gas while in shallow layers mainly a little free gas.Gases with highδ13C2（〉-27.5‰）andδ13C3（〉-25.5‰）from Es3（the third member of Shahejie formation,Eogene）and Ordovician reservoirs are coal-type gases generated from Es3 source rocks;partly reversed order of carbon isotope can be attributed to the mixing of gases from one source rock interval ofvarying maturity.Gases with lowδ13C2（〈-27.5‰）andδ13C3（〈-25.5‰）from Es1（the first member of Shahejie formation,Eogene）reservoirs are oil-type gases generated from Es1 source rocks,and partly reversed order of carbon isotope can also be attributed to the mixing of gases from one source rock interval of varying maturity.Gases with wide range ofδ13 C2（-31.5‰—-25.5‰）andδ13 C3（-28.1‰—-22.4‰）from Ed（Dongying formation,Eogene）and Neogene are derived from Es1 and Es3source rocks,partly reversed order of carbon isotope is mostly due to a combination of reasons including mixing of gases from one source rock interval of varying maturity,mixing of different types of gases from different source rocks as well as bacterial oxidation.Faults are the vertical migration pathways.