THE MECHANISM AND CHARACTERISTICS OF OIL AND GAS MIGRATION AND ACCUMULATION IN SHALLOW AND DEEP WATER OF ZHUJIANGKOU BASIN, NORTHEASTERN SOUTH CHINA SEA

The geological conditions for oil and gas accumulation are of significant difference in shallow and deep waters in the ZhuJiangKou Basin. The difference is resulted mainly from the different mechanism of migration and accumulation of oil and gas and their distribution pattern. ①oil occurs in the north and gas in the south. In the northern part of the basin, including the rift zone, the DongSha uplift zone and the area further to the north, there are low geothermal gradient and low heat flow, and the thermal evolution of source rocks is within the oil window. A large amount of oil with a small amount of oil type gas was accumulated in the sallow water of north continental shelf, which mainly contains the Wenchang, Yun Ping, Xijiang, Huizhou, Lufeng and Liuhua oil fields. To the contrary, The geothermal gradient and heat flow are high in the deep water of the south rift zone and south uplift zone, where the thermal evolution of source rocks had dropped into a mature or over mature stage. Thus there mainly produce natural gas and a small amount of condensate oil, such as the cases in the gas reservoirs of LW3-1,LH34-2 and LH29-1 and the oil reservoir of LH16-2 in the eastern Baiyun depression as well as the group of small gas reservoirs in the Panyu uplift of Baiyun depression. There also oil and gas hydrate occur sometimes. ② The area of shallow water is dominated by the allochthonous hydrocarbon accumulated in the Oligocene and Miocene deltaic sandstone and Miocene reef limestone, whereas the hydrocarbon of mixed source dominates the area of deep water. So far, some large oil fields of these types have been discovered in the basin. The mechanism of hydrocarbon migration and accumulation in the half-grabens has been verified by exploration but no breakthrough has been achieved so far. In the area of deep water, The hydrocarbon of mixed sources has been verified in the Oligocene deltaic sandstone and deep-water fan systems, characterized by the coexistence of gas hydrates in seafloor and shallow gas and conventional oil and gas in the deep.