Deepwater area——the new prospecting targets of northern continental margin of South China Sea

The deepwater area with over 300 meters in water depth in the northern continental margin of South China Sea experienced pre-rift phase,rift phase,post-rift phase and Neotectonic phase.In this area,a bump-sag pattern was formed.The Paleogene system with 3~6 kilometers in thickness was deposited in the sags during the rift phase.The thickness of Lower-Middle Miocene deposited during postrift phase was less than 2000 meters.The late Miocene-Quaternary system with a tremendous thickness was formed and also reformed in east part during Neotectonic phase.The potential hydrocarbon sags in deepwater area are the faulted depressions formed in the rift phase.The proven source rocks are the early Oligocene mid-deep lacustrine mudstone,transitional mudstone,marine mudstone and coal system.The probable source rock is the Eocene mid-deep lacustrine mudstone,and the possible one is the upper Oligocene and Miocene marine mudstone.There are at least three reservoir cases in deepwater,including Oligocene transitional sandstones,Neogene-Quaternary marine deepwater sandstone and reef,and Eocene terrestrial sandstone.The seal rocks are developed widely,and the Neocene marine mudstone is the regional seal.There are a lot of traps with the main styles of draping anticline,fault trap and deepwater fan.The oil-gas migration style varies with time and space and is controlled by the Neotectonic fault activities.The parallel migration dominates in the deepwater of Qiongdongnan Basin and in the early phase of Baiyun Depression,and the vertical migration dominates in the late phase of Baiyun Depression.The deepwater play and traps contain bulge,secondary bulge in sag,fault zone and deepwater fan.Exploration activity should first aim at the structural traps in the sag and in the forward direction of structure around the sag and then gradually develop toward the non-structural trap.