Geochemical Characteristics Of Source Rock And Oil-source Correlation In Baiyun Sag Of The Pearl River Mouth Basin

Baiyun sag is located in southeast of Pearl River Mouth Basin,which is one of the hydrocarbon prolific basin in South China Sea.Due to the relative shortage of hydrocarbon source rock samples in deep water sag,less work has been done on the geochemical characteristics of rock in Baiyun sag.The source rocks of Paleogene Wenchang,Enping and Zhuhai Formation are proven to be effective source rocks in this area.A total of four types of source rocks i.e.lacustrine mudstone,coaly mudstone,shelf mudstone and deltaic mudstone,can be correspondingly distinguished in terms of the characteristics of lithology and deposition.In this study,the geochemical characteristics of source rocks were investigated on the basis of systematic gas Chromatography-Mass Spectrometry and Rock-Eval analysis of 27 source rock samples.The results show that lacustrine mudstone of Wenchang and Enping Formation show with similar geochemical characteristics,such as medium TOC content,ranging from 0.4%to 2.4%?with an average value of 1.4%?,with dominance of type?₂ kerogen,medium Pr/Ph ratios ranging from 0.9 to 2.8,and relatively lower bicadinanesratios.The coaly mudstones of Enping Formation are characterized by relatively higher TOC contents in the range of 0.4%-5.9%?with an average value of 2.5%?,mainly type?kerogen,relatively higher Pr/Ph?>2.0%?ratios,higher bicadinanesand highervalues.The shelf mudstone of Zhuhai Formation with the kerogen of?₂type,show with relatively medium TOC ratios?average value 1.2%?,relatively lower Pr/Ph?<2.0%?ratios,bicadinanesandvalues.In contrast,deltaic mudstones of Zhuhai Formation are characterized by medium TOC with an average value of 1.1%,dominanted type?kerogen,relatively lower Pr/Ph ratios?<2.0%?,and being rich in bicadinanes W,T and oleannane.In addition,a two-dimensional basin-modeling study was applied to analyze the source rock thermal evolution history.And the oil source of eastern area,northern slope and northeast area of the Baiyun sag were analyzed,combining with the hydrocarbon generation experiments result.Simulation results indicate that the timeing of hycarbon generation for source rock of Wenchang Formation is earlier than that of source rocks of Enping and Zhuhai Formation.lacustrine mudstone of Wenchang and Enping Formation has the strongest strength of oil generation,followed by coaly mudstone of Enping Formation and shelf mudstone of Zhuhai Formation,deltaic mudstone of Zhuhai Formation in succession.The crude oils from oil reservoirs of the northern slope mainly derived from lacustrine mudstone and coaly mudstone of the Enping Formation in main subsag of the Baiyun sag.The crude oils of northeast area oil reservoirs may mainly come from lacustrine mudstone of the Enping Formation in eastern subsag of the Baiyun sag.And the crude oil of eastern area oil reservoirs mainly generated from source rock of the Enping Formation in eastern subsag and main subsag of the Baiyun sag,with some contribution of oil generated from shelf mudstone of Zhuhai Formation.