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

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.