鄂尔多斯盆地上古生界凝析油成因研究

鄂尔多斯盆地古生界凝析油具有低密度、低凝固点、低初馏点的特征；Pr/Ph值均大于2.0，具有姥鲛烷优势；轻烃中甲基环己烷相对含量高于60％；碳同位素值主要分布在-21‰～-26‰之间，正构烷烃单体分子碳同位素值主要分布在-20‰～-29‰之间，且碳同位素值随碳数的增大逐渐变轻，呈分段式分布。将凝析油地球化学特征与上古生界暗色泥岩、煤及下古生界碳酸盐岩进行比较，结果表明鄂尔多斯盆地古生界凝析油与煤的地球化学特征一致，是煤在高成熟阶段的产物。对古生界凝析油成因的探讨为认识鄂尔多斯盆地古生界天然气成因提供一定的依据，一般有凝析油伴生的天然气烃类碳同位素组成相对较重，主要为煤的产物；而烷烃碳同位素组成相对较轻的天然气与泥岩厚度分布特征有一定的关系，可能以上古生界暗色泥岩生成物为主。

Genesis of Upper Paleozoic Condensates in Ordos Basin

The Paleozoic condensates in the Ordos basin are characterized as low density,low freezing point,and low dropping point,as well as pristine predominance and high Pr/Ph ratios(Pr/Ph>2.0).The relative content of methyl cyclohexane in the light hydrocarbon is more than 60%.Carbon isotope values(δ13C) of n-alkane monomers are generally ranged from-20‰ to-29‰ and δ13C value decreases with carbon number increase,which shows the sectionalized distribution.Comparing geochemical characteristic of condensates with that of dark-colored mudstones and coal in upper Paleozoic and carbonates in lower Paleozoic,the results show that the geochemical characteristic of Paleozoic condensates in the Ordos basin is similar to that of high mature coal.The determination of Paleozoic condensate supports the Paleozoic natural gas sourced from coal.The carbon isotope value(δ13C) of hydrocarbon gases in the condensates is heavier,indicating they are sourced from coal,in contrast that natural gases with lighter carbon stable isotopes are related to mudstone thickness and distribution in upper Paleozoic,inferring that they would stem from the upper Paleozoic mudstones.