Geochemical Characteristics Of Source Rocks And Origin Of Crude Oil In The Baiyun Sag,Pearl River Mouth Basin

Baiyun Sag is located in ZhuⅡdepression of Pearl River Mouth Basin in the northern part of South China Sea.It has been confirmed that there are three sets of source rocks in the study area,namely Wenchang Formation（WC E₂w）,Enping Formation（EP E₂e）and Zhuhai Formation（ZH E₃zh）.Due to the lack of offshore drilling source rock samples in the early stage,the study on the distribution and geochemical characteristics of source rock and the origin of crude oil in this area are not well studied.Based on the hydrocarbon generation potential evaluation of source rocks and gas chromatography-mass spectrometry（GC-MS）analysis of saturated hydrocarbons and aromatic hydrocarbons in source rocks and crude oils,the study of hydrocarbon geochemistry in the Baiyun Sag has been improved.According to sequence and sedimentary facies,the source rocks in the Baiyun Sag can be divided into four types:WC,EPSQ1-2 shore-shallow lacustrine mudstone,EPSQ1-2 fan-delta mudstone,EPSQ3 delta coal-measure mudstone and EPSQ3 littoral-neritic mudstone.Hydrocarbon generation potential evaluation and molecular organic geochemical analysis of each type of source rocks were carried out respectively:on the whole,the source rocks in the Baiyun Sag are all of gas-inclined source rocks with high abundance（the average value of TOC between 0.83%and 1.93%）,which have high hydrocarbon generation potential.The WC and EPSQ1-2 shore-shallow lacustrine mudstone have the characteristics of low oleanane and low bicadinanes abundance.The EPSQ1-2 fan-delta mudstone have the characteristics of high oleanane and low to medium bicadinanes abundance.The EPSQ3 delta coal-measure mudstone is characterized by medium oleanane and medium abundance of bicadinanes.The EPSQ3littoral-neritic mudstone is characterized by medium to high oleanane and high abundance of bicadinanes.The novel C₁₅ sesquiterpene,tricyclic and tetracyclic terpanes（X,Y,Z and X1,Y1,Z1）with special structures,bicadinanes and oleanane are typical biomarkers for organic matter input from terrigenous higher plants.In this thesis,these compounds were detected and identified,and their geochemical significance was discussed.According to the correlation of molecular structure and abundance,the novel C₁₅ sesquiterpene,Y1,Z1and bicadinanes may have different biogenetic derivation or evolution process.Combined with the abundance differences of typical biomarkers and conventional geochemical indicators,the crude oils in the Baiyun Sag can be divided into two groups:The groupⅠcrude oils are characterized by high novel C₁₅ sesquiterpene,high bicadinanes and low Y1 and Z1,mainly distributed in the North slope.The groupⅡcrude oils are characterized by low distribution of novel C₁₅ sesquiterpene,low distribution of bicadinanes and slightly higher distribution of Y1 and Z1,mainly distributed in the Eastern region.In order to explore the origin of crude oil,this thesis conducted two-dimensional simulation of thermal evolution history of source rocks in typical sections passing through the center of the main depression in the Baiyun Sag.Based on the comparison between thermal evolution degree of source rocks in key accumulation periods and molecular organic geochemistry oil source,it is concluded that:the groupⅠoil reservoir may be mainly derived from the EPSQ3 delta coal-measure mudstone,and the groupⅡoil reservoir may be mainly derived from EPSQ3 littoral-neritic mudstone and EPSQ1-2 fan-delta mudstone.