东营凹陷原油单体烃碳同位素特征及其在油源识别中的应用

Characteristics and Application of Compound Specific Isotope in Oil-source Identification for Oils in Dongying Depression,Bohai Bay Basin

为揭示东营凹陷部分复杂成因油气的来源,对该区原油、烃源岩单体烃碳同位素进行精细特征分析与油源追踪。东营凹陷不同成因原油具有不同正构烷烃单体烃碳同位素特征。沙三段淡水成因原油碳同位素曲线分别以nC15、nC32为主峰,碳同位素总体偏重、前峰极其富集13C;沙四段咸水成因原油一般以nC23为分界线,近似呈对称的双峰分布,1δ3C值较沙三段成因原油的轻。沙三段、沙四段成因原油碳同位素分别与沙三段、沙四段烃源岩的聚类相关。牛庄洼陷沙三中段透镜体岩性油气藏原油单体烃碳同位素值介于沙三段、沙四段烃源岩之间,与沙四段烃源岩及相关原油具有更好的相关性,验证以往关于牛庄洼陷沙三中段隐蔽油气藏原油为沙三段、沙四段成因的混源油观点。东营凹陷南斜坡孔店组原油碳同位素总体相对较轻、近直线型分布,与孔店组烃源岩、黄骅坳陷孔店组成因原油的碳同位素曲线聚类相关,指示其与孔店组烃源岩具有成因联系。在正常油窗范围内,成熟度对东营凹陷烃源岩及相关原油碳同位素的影响较小,后者主要受控于母源与古沉积环境。单体烃碳同位素可以很好地应用于东营凹陷油气成因与油源研究,该方法是对以往生物标志物地球化学研究的补充与深化。

Compound specific isotope analysis was performed to help to investigate the origin and source rocks for some oils with source rock still confusing.The Es3 genetic affinity oil from lacustrine with fresh-water is characterized by bimodal distribution with the main-peak to be around nC15 and nC32,respectively and the fore-peak particularly enriched in 13C.This kind of oil generally has a little heavier δ13C values relative to the other oils.The brackish Es4 genetic affinity oil also shows a bimodal distribution,displaying a symmetrical ＂double peak＂ shape with the boundary consistently to be around nC23.It was observed that the Es3 and Es4 source rocks are plotted cluster with the Es3 and Es4 genetic affinity oils（based on biomarkers in previous studies） respectively,suggesting their good genetic relationship.The oils from subtle pools of EsM3 in the Niuzhuang Sag have an intermediate δ13C values between the Es3 and Es4 genetic affinity oils and much more approaching the latter,suggesting oil mixing of the two types oil,which is consistent with the previous opinion about mixed-source oils of the EsM3 from the Es3 and Es4 intervals for the Niuzhuang oils based on biomarkers.It was observed that high-waxy oils from Kongdian Formation（Ek） in the Dingjiawozhi tectonic belt of the Niuzhuang South Slope have flat isotope distributions,which are plotted with the Ek source rock in the Niuzhuang Sag and Ek genetic affinity oils in the Huanghua Sub-basin.It suggests a close genetic relationship between the Ek oil and the Ek source rock in the Dongying Depression.We conclude that maturity only has a minor effect on the isotope for the oil and source rocks in the Dongying Depression,and it is the organic input as well as paleoenvironment of the source rock which have an Absolute controlling on the isotope after detailed oil-oil and oil-source correlation.Therefore,compound specific isotope approach is an excellent tool for the oil-source rock correlation in the area studied,which can be a complementation for the previous studies based on biomarkers.