渤海辽西凹陷原油地球化学特征及地质意义

Geochemical characteristics and oil source correlation of crude oil in Liaoxi Depression,Bohai Sea,China

探讨辽西凹陷原油地球化学特征的差异性成因。利用原油物性、族组成、生物标志物、碳同位素等方法,研究显示:原油包括轻质油、稠油、特稠油3种类型,从北洼向南洼原油物性依次变差。轻质原油主要分布在北洼,稠油在3个洼陷均有分布,特稠油主要分布在南洼馆陶组。北洼原油整体成熟度中等,但油源和受降解程度差异较大,可分为4类原油,其中有2类原油的碳同位素显示主要为沙四段烃源岩供烃,只是伽马蜡烷含量不同,显示沙一段烃源岩供烃量不同,且它们均未遭受生物降解;另外2类原油则为沙三段烃源岩供烃为主,混入少量沙一段烃源岩供烃,饱和烃色谱特征显示一个遭受严重生物降解,而另一个未遭受生物降解。中洼原油沉积环境偏氧化、高成熟度、遭受严重生物降解,其油源以沙三段烃源岩为主,混入少量沙一段。南洼原油沉积环境偏氧化、低成熟度、遭受强烈生物降解,以沙三段烃源岩供烃为主,混入少量沙一段。油藏层位和埋深控制了原油遭受生物降解程度,烃源岩热演化程度控制了原油成熟度,沙四段烃源岩分布和沉积环境控制了供烃层位。

Study of physical property,group compositions,biomarker characteristics and carbon isotope data of crude oils is carried out in order to discuss the differential origin of geochemical characteristics of crude oils in Liaoxi depression.The result shows that the types of crude oil in the study area include light oil,heavy oil and extra-heavy oil.The physical properties of crude oil deteriorated gradually from northern sag to southern sag.Light oil mainly distributed in northern sag,heavy oil distributed in all three sags and extra-heavy oil mainly distributed in Guantao Formation in southern sag.Crude oils of northern sag are medium maturity oil,which can be divided into four types in terms of difference of oil sources and biodegradation.Analysis of carbon isotopes of the two types of crude oil shows that the crude oils are mainly derived from the source rocks of the Member 4 of Shahejie Formation(E 3 s 4).However,the difference of gamma paraffin content suggests that the hydrocarbon supply for source rocks in the Member 1 of Shahejie Formation is different,and neither of the two types of crude oil has been biodegraded.The other two types of crude oil are mainly supplied by the source rocks of the Member 3 of Shahejie Formation and mixed with a small amount of the Member 1 of Shahejie Formation.The chromatographic characteristics of saturated hydrocarbons show that one of them has suffered serious biodegradation while the other has not suffered biodegradation.The crude oil in the central sag is partially oxidized,severely biodegraded and highly matured.Its oil source is mainly derived from source rocks of the Member 3 of Shahejie Formation and mixed with a small amount of the Member 1 of Shahejie Formation.The crude oils in the southern sag are similar to the crude oil in the central sag,while the crude oils in the southern sag have suffered intense biodegradation.It is considered that the reservoir horizon and burial depth controlled the degree of biodegradation of crude oil,thermal evolution of source rocks controlled the maturity of crude oil,and distribution of source rocks and sedimentary environment controlled the source rocks of the Member 4 of Shahejie Formation.