北部湾盆地涠西南凹陷东南斜坡原油成因类型及成藏特征

北部湾盆地涠西南凹陷东南斜坡是重要的油气聚集区。为研究其原油成因类型及成藏特征，开展了原油生物标志物分析和同位素等地球化学分析。结果表明：①涠西南凹陷东南斜坡原油可划分为2类，Ⅰ类为具有较低的4-甲基甾烷丰度和较轻的碳同位素特征的原油，主要分布在流一段；Ⅱ类原油具有较高的4-甲基甾烷丰度和较重的碳同位素特征，主要见于流二段、流三段和涠洲组。②东南斜坡钻遇流沙港组烃源岩，地球化学特征揭示其存在上、下2个供烃组合，即包括流一段泥岩-流二段中部泥岩的上组合和流二段底部油页岩-流三段上部泥岩的下组合。③应用生物标志物和同位素组成等参数进行油源精细对比可以得出，第Ⅰ类原油来源于上组合，第Ⅱ类原油来源于下组合。烃源岩岩相控制了不同类型原油的分布，东南斜坡的流二段、流三段、角尾组、下洋组及基底风化壳均是寻找第Ⅱ类原油的有利区，东南斜坡流一段岩性圈闭是寻找第Ⅰ类原油的有利区。该研究成果对北部湾盆地涠西南凹陷油气勘探具有重要指导意义。

Genesis types and accumulation characteristics of crude oil in southeast slope of Weixinan Depression,Beibuwan Basin

The southeast slope in Weixinan Depression of Beibuwan Basin is an important hydrocarbon accumulation area. In order to study the genesis types and accumulation characteristics of its crude oil,the biomarkers and isotopic geochemical characteristics of the crude oil in the southeast slope were analyzed. The results show that:(1)The crude oil in the southeast slope can be divided into two types. The first type of crude oil has low 4-methyl sterane abundance and low carbon isotope, and mainly distributed in the first member of Liushagang Formation,while the second type of crude oil has high 4-methyl sterane abundance and higher carbon isotope,and mainly distributed in the second and third member of Liushagang Formation and Weizhou Formation.(2)Hydrocarbon source rocks in the southeast slope can be divided into two hydrocarbon combination through geochemistry characteristics in combination with sedimentary environment. The upper hydrocarbon combination mainly includes the mudstone of the first member of Liushagang Formation and the middle mudstone of the second member of Liushagang Formation,while the lower hydrocarbon combination mainly includes the bottom oil shale of the second member of Liushagang Formation and the upper mudstone of the third member of Liushagang Formation.(3)Through analyzing the biomarkers and isotopic characteristics of oil and source rocks, it is concluded that the first type of crude oil is from the upper hydrocarbon combination,and the second type of crude oil is from the lower hydrocarbon combination. The petrofacies of source rocks controlled the distribution of different types of crude oil. The second and third member of Liushagang Formation,Jiaowei Formation,Xiayang Formation and basement weathering crust in the southeast slope are all favorable areas for searching for the second type of crude oil,while lithologic traps in the first member of Liushagang Formation are favorable areas for searching for the first type of crude oil.