| Baiyun Sag is located in the deep water area of Zhuer Depression in the Pearl River Mouth Basin in the north of the South China Sea.Although predecessors have carried out a lot of research work on Baiyun Depression,there are still some problems to be solved in terms of hydrocarbon source rocks.This paper comprehensively utilizes geochemical,biomarkers,and logging data,combined with the regional tectonic and sedimentary background of Baiyun Depression,to conduct a detailed evaluation of potential hydrocarbon source rocks in the study area,establish a prediction model for organic carbon content,and clarify the planar distribution characteristics of hydrocarbon source rocks.First,three sets of Source rock developed in Wenchang Formation,Enping Formation and Zhuhai Formation of Baiyun Sag are divided into six lithofacies types based on sedimentary background and logging data.Based on the analysis of basic geochemical characteristics of Source rock in the study area,it is considered that the Source rock of Wenchang Formation and Enping Formation are high-quality Source rock in the area,and the hydrocarbon generation potential is evaluated as"medium-good".Among them,the average TOC value of lacustrine mudstone in the Wenchang Formation and Enping Formation is 1.40%,the types of kerogen are I-II1type and II2-III type,respectively;The average TOC value of the coal bearing mudstone in the Enping Formation is 2.91%,and the parent material type is Type III;The overall hydrocarbon generation ability of the Zhuhai Formation is relatively weak,with a"medium"level.The average TOC value of marine delta mudstone is 0.74%,the kerogen type is II2 type,the average TOC value of shallow marine shelf mudstone is 1.30%,the parent material type is II2 type,the average TOC value of semi deep-sea continental slope mudstone is 0.68%,and the kerogen type is III.The Ro and Tmax data indicate that the overall maturity of the mudstone in the Wenchang Formation is the highest,with an average Ro of 1.27%.The maturity of the mudstone in the Enping Formation is second,and it is in the mature to high maturity stage,with an average Ro of 1.17%.The organic matter evolution of the mudstone in the Zhuhai Formation is relatively weak,and it is in the immature to mature stage,with an average Ro of 0.60%.The profile of Ro and depth shows that Source rock began to generate a large amount of hydrocarbon during the sedimentation period of Zhuhai Formation,and Source rock was at the peak of hydrocarbon generation during the sedimentation period of Enping Formation and Wenchang Formation.The content of organic matter group components（saturated hydrocarbon,Aromatic hydrocarbon,non hydrocarbon and asphaltene）in the three sets of Source rock increases with the increase of thermal evolution degree.Secondly,select typical biomarker compound parameters to conduct molecular geochemical analysis of six lithofacies types of hydrocarbon source rocks.The results show that the source rock composition and formation environment in Baiyun Depression are mainly characterized by two major types:one is the lacustrine mudstone of Wenchang Formation and Enping Formation,and the other is the mudstone of shallow marine shelf and semi deep continental slope of Zhuhai Formation.The first type of source rock was formed in a reduction-weak oxidation sedimentary environment,with a lower vegetation ratio（Pr/Ph）,and a lower content of biomarker compounds such as OL,W/T-diduropinane,which indicate the input of terrestrial parent materials,The hydrocarbon generating parent material is mainly low level aquatic organisms such as algae;The other type is the delta/coal measure mudstone of the Enping Formation and the marine delta mudstone of the Zhuhai Formation.This type of hydrocarbon source rock is formed in an environment that is slightly oxidized,with a higher vegetation ratio（Pr/Ph）,and a greater contribution of terrestrial organic matter.Finally,in the study area theΔlogR method is used to predict the organic carbon content,and the measured and predicted TOC values are used to draw a TOC contour plan.The TOC values of the three sets of hydrocarbon source rocks are the largest in the center of the depression,and gradually decrease towards the edge of the depression.Based on the principle of hydrocarbon generation and expulsion,three sets of effective source rock lower limit values are determined,and then effective mudstone thickness is predicted.The results show that the effective mudstone thickness of Wenchang Formation is the largest,with the effective mudstone thickness in the main depression of about 2000 m,followed by the effective mudstone thickness of Enping Formation,with the thickness in the center of the main depression of about 1000 m,and the effective mudstone thickness of Zhuhai Formation is the thinnest,widely distributed in the plane,with significant thickness changes,distributed between 200 m and 1000 m. |
