鄂尔多斯盆地大牛地下古生界天然气地球化学特征及其来源综合判识

近年来，鄂尔多斯盆地大牛地古生界天然气勘探取得重要进展，尤其是下古生界奥陶系风化壳连续高产工业气流的发现，展示了良好的勘探前景，但大牛地下古生界天然气成因及气源研究相对薄弱，直接制约了下一步勘探部署，为此系统开展了大牛地古生界天然气组分、碳氢同位素、稀有气体组分和同位素等地球化学分析。大牛地古生界天然气中甲烷占绝对优势，含有一定量的重烃，非烃气CO₂和N₂含量相对较高，既有典型的成熟阶段生成的湿气，也有高过成熟阶段生成的干气。大牛地上古生界天然气为典型煤型气，而下古生界天然气既有煤型气，也有油型气。古生界天然气中CO₂主要为有机成因，少数为无机成因，无机成因CO₂赋存于下古生界储层，可能主要由下古生界烃源岩中碳酸盐岩热解产生。综合运用烃源岩岩石脱附气碳同位素—天然气碳同位素—干酪根碳同位素、天然气甲烷氢同位素、稀有气体Ar同位素定年及混源模拟实验等综合地球化学手段，明确了大牛地上、下古生界煤型气均来源于上古生界煤系烃源岩，而下古生界油型气中混有0~45%的煤型气，主要来源于下古生界马家沟组烃源岩。

Evaluation of geochemical characteristics and source of natural gas in Lower Paleozoic,Daniudi area,Ordos Basin

In recent years,great progress has been made in the exploration of natural gas in the Paleozoic in the Daniudi Gas Field in the northern Ordos Basin,especially the discovery of high yield industrial gas flow in the Ordovician weathering crust.However,the study of the origin and source of natural gas in the Lower Paleozoic in the Daniudi area is relatively limited,which directly restricts the next exploration project in the Lower Paleozoic in the northern Ordos Basin.The systematic analyses of gas components and carbon,hydrogen and noble gas isotopes showed that natural gas of the Paleozoic in the Daniudi area is dominated by methane and contains a certain amount of heavy hydrocarbons,and the carbon dioxide and nitrogen content is relatively high.It is not only dry gas generated at high to over maturity,but also wet gas originated from the mature stage.The natural gas of the Upper Paleozoic in the Daniudi area is a typical coal-type gas,while the natural gas of the Lower Paleozoic is both coal-type gas and oil-type gas.The carbon dioxide in the natural gas of the Paleozoic is mainly organic and with a small amount in the Lower Paleozoic being inorganic,which may be produced by the pyrolysis of carbonate minerals in the Lower Paleozoic source rocks.Geochemical analyses such as the carbon isotopes of source rock desorption gas,natural gas and kerogen,the hydrogen isotope of methane,the argon isotope dating,and the end-member gas mixed simulation experiments demonstrated that the coal-type gas in Upper and Lower Paleozoic in the Daniudi area in the northern Ordos Basin originated from the Upper Paleozoic coal-bearing source rocks,while the oil-type gas in the Lower Paleozoic was mixed with 0-45%coal-type gas,and the oil-type gas was predominant in the natural gas,which mainly originated from the source rocks of the Majiagou Formation in the Lower Paleozoic.