Formation, Distribution And Prediction Of Lower Cambrian Black Rock Series In South China

The early Cambrian is an important period of biological evolution in geological history.The transgression has led to the widespread development of black shale rich in organic matter in the world,making it one of the most important exploration strata in the world.Although the lower Cambrian black shale is widely developed in South China.However,due to factors such as paleomorphology,sedimentary facies,and structural changes,the spatial distribution of source rocks is very different and the heterogeneity is strong,which brings great risks to oil and gas exploration.Based on this situation,detailed geochemical analysis of organic carbon,major elements,and trace elements was conducted on three typical source rock profiles in South China to study the control of paleoclimate,paleoproductivity,and paleo-environmental changes on the formation of source rocks,and analyze the development model,formation mechanism and coupling relationship between biological activities and changes in marine environment and the formation of source rocks,In addition,comparing the quality of source rocks in different facies and the seismic response characteristics of source rocks in platform rift areas,reveal the spatial distribution of source rocks under various factors.The analysis results show that the lower Cambrian black shale has a high organic carbon content（1²5%）,a large sediment thickness（30^>200m）,and a distribution area of more than 100,000 km².It is highly enriched in redox sensitive trace elements（Mo,U,V）and formed in anoxic environment.Geochemical analysis of typical profiles shows that the lower Cambrian source rocks are vertically characterized by euxinic to reduction to dysoxic,TOC gradually decreasing,and the quality of the source rock gradually deteriorating from bottom to top.Reveals the control of the marine environment on the formation of excellent source rocks.The quality of the source rocks from west to east on the plane is getting better,the TOC is gradually increasing,and the thickness is increasing,indicating that the sedimentary facies controls the distribution of source rocks,revealing that the source rocks in the rift troughs of the slope and platform facies are of the best quality.Comparison of the spatial distribution of source rocks shows that platform source rocks are mainly developed in rift troughs,with an average TOC of 2.7%,and the thickest source rock in the center of rift（>100m）.The slope facies source rocks are mostly related to the H₂S-rich water environment,with an average TOC of 6%and a thickness greater than 50m;Basin facies source rock deposition environment is relatively stable,the main water body is dominated by anoxic environment,the average TOC is 8%,and the thickness is more than70m.Comprehensive studies suggest that there are two thickness centers in the lower Cambrian source rocks in South China,the source rocks in the platform rifts and slope facies source rocks,the two source rocks in the center of thickness are formed in the anoxic water body,while the other areas outside the thickness center are controlled by the paleomorphology.the source rock quality and thickness are quite different.