Research On Petroleum Migration And Accumulation Model In The Different Types Of Slope Of The Baiyun Depression In The Northern South China Sea

The Baiyun Depression is located in the deep water area of the Northern South China Sea.The discovered petroleum reservoirs are mainly distributed on the slopes around the depression.This thesis studies the geological characteristics,the transport systems,and petroleum migration mechanism in the slopes around the Baiyun Depression.Then the petroleum migration and accumulation models in the different types of slope are established and the controlling factors of petroleum accumulation in the Baiyun Depression are clarified.Considering the structure and construction of the slopes,this study identifies two types of slope,including the differential subsidence slope and the fault-controlling slope,in the Baiyun Depression.The differential subsidence slope has the simple structure and does not develop large boundary faults.It is caused by the differential subsidence between the depression and the low uplift,including the northern slope,the northeastern slope,and the northwestern slope.The fault-controlling slope is controlled by the large faults and the structure is complex,including the southeastern slope and southwestern slope.In the differential subsidence slope,the transport systems are controlled by the sedimentary characteristics and late tectonic activity.They are the“sandstone-fault-sandstone” double-layers superposed transport systems.The lower layer is the sandstones of the Enping Formation.The faults can transport the petroleum to the lower member of the Zhujiang Formation.The sandstones in the upper layer have good physical properties.The petroleum migration and accumulation of the differential subsidence slope in the Baiyun Depression can be divided into multiple processes and called the “multi-steps and relaying migration,convergenced accumulation” model.Firstly,the petroleum in the mudstones gradually migrates into the sandstones along the pores and micro-fractures.Then,the episodic activities of the faults open the fractures in the fault zones.The overpressures in the deep drive the petroleum to migrate along the fault zones.The activities and fault zone control the vertical migration.Finally,the petroleum enters the lower member of the Zhujiang Formation,which have good physical properties.The structural morphology controls the migration pathways.The petroleum ultimately converges into the traps on the structural ridge.In the fault-controlling slope,the faults and diapirs are the efficient vertical transporting systems.The petroleum can migrate to the reservoirs along these vertical transporting pathways.Therefore,they are the “vertical pathways-dominated” transport systems.The petroleum migration and accumulation of the fault-controlling slope is called “vertical migration,parallel accumulation” model.The petroleum migration is mainly along the faults and diapirs.The petroleum accumulation is mainly controlled by the vertical migration and is parallel accumulation.Overall,the petroleum accumulation in the Baiyun Depression is controlled by the characteristics of the source rocks and the efficiency of petroleum migration.The property and phase of petroleum are controlled by the lithofacies and maturity of Enping source rocks.The migration efficiency of faults is controlled by the tectonics activities and the migration efficiency in the sandstone transport layers is related to their connectivity,physical properties and geological occurrence.