Detailed oil-source correlation within the sequence and sedimentary framework in the Fushan Depression, Beibuwan Basin, South China Sea

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The Fushan Depression is one of the petroliferous depressions in the Beibuwan Basin, South China Sea. Previous studies have preliminarily explored the origin and source of crude oils in some areas of this depression. Nevertheless, no systematic investigations on the classification and origin of oils and hydrocarbon migration processes have been made for the entire petroleum system in this depression, which has significantly hindered the hydrocarbon exploration in the region. A total of 32 mudstone and 58 oil samples from the Fushan Depression were analyzed to definite the detailed oil-source correlation within the sequence and sedimentary framework. The organic matter of third member of Paleogene Liushagang Formation (Els3) source rocks, both deltaic and lacustrine mudstone, are algal-dominated with high abundance of C23 tricyclic terpane and C30 4-methylsteranes. The deltaic source rocks occurring in the first member (Els1) and second member (Els2) of the Paleogene Liushagang Formation are characterized by high abundance of C19+20 tricyclic terpane and oleanane, reflecting a more terrestrial plants contribution. While lacustrine source rocks of Els1 and Els2 display the reduced input of terrigenous organic matter with relatively low abundance of C19+20 tricyclic terpane and oleanane. Three types of oils were identified by their biomarker compositions in this study. Most of the oils discovered in the Huachang and Bailian Els1 reservoir belong to group A and were derived from lacustrine source rocks of Els1 and Els2. Group B oils are found within the Els1 and Els2 reservoirs, showing a close relation to the deltaic source rocks of Els1 and Els2, respectively. Group C oils, occurring in the Els3 reservoirs, have a good affinity with the Els3 source rocks. The spatial distribution and accumulation of different groups of oils are mainly controlled by the sedimentary facies and specific structural conditions. The Els2 reservoir in the Yong’an area belonging to Group B oil, are adjacent to the source kitchen and could be considered as the favorable exploration area in the future.