The Evolution Of Carbonate Platform In The Xisha Sea Area And Tectonic Contrals

As a kind of very important reservoir, Carbonate is investigated in petroleum exploration because of its high porosity and good permeability. In later Cenozoic, there abundant carbonates developed in northern margin of the South China Sea, especially in the Xisha Sea area. Its promises huge oil-gas resource, which gradually become a hot research point. Therefore, it is important for the theory and reality to study carbonate in the Xisha sea area.Based on the latest drilling and 2D/3D seismic data and regional geological document, this study was carried out mainly on the carbonate in the Xisha sea area. Innovative results had been achieved as follows. 1)Concluded on well YC35-1-1, we firstly adusted former sequence boundary T30 to T31 and reinterpreted T30. 2) established three margin symbols of carbonate platforms, tidal channels, fault escarpments and lithologic shifts. Tidel channels mainly distribute on the gradual slope of the Xisha Uplift and Guangle Uplift, fault escarpments centralize on the two uplift margins, while, the shifts of carbonate rock and clastic rock local on the relative steep slopes around uplifts. 3) straightened out the space-time of the Xisha carbonate platform and identified seven periods of carbonate platform evolution which were divided into four stages, preliminary stage, flourishing stage, recessed stage and surviving stage. Drew six sedimentary faces of carbonate platforms and established an evolutionary model of isolated carbonate platform. 4) firstly restored paleogeomorphographies of carbonate platforms. This resesrch suggested that the gradient around the south of Guangle Uplift was steeper than on the north and the gradient around Xisha and Guanugle Uplifts increased in late Miocene. The different subsidences in every tectonic unit controlled those topography features. Special topographies, such as tidal channels and channels, had a relationship with carbonate platform. Those early volcanoes were benefit for isolated carbonate platforms. Reefs main centralized on west of the Xisha Uplift. 5) Quantitatively calculated factors of extension and subsidence by balanced cross-section and backstripping techniques, detailed analyzed tectonic evolutional histories. The results demonstrated that the carbonate platforms initialized on the tectonic high in early Miocene and flourished during the middle Miocene dependent on the stable tectonic conditions. However, later speed subsidence led to the demise of carbonate platforms. Nowadays, only small isolated carbonate platforms, such as Yongle Atoll and Xuande Atoll, survived on the Xisha Uplift. 6) The change of relative sea level had a very important influence on the evolution of carbonate platforms. The second order of relative sea level controlled evolutionary trend of carbonate platforms and the third order of relative sea level affected the evolutionary periods in each stages.