CHARACTERISTICS OF UNCONFORMITY STRUCTURE AND ITS CONTROL ON HYDROCARBON ACCUMULATION :A CASE STUDY FROM THE WUSHI SAG, BEIBUWAN BASIN, SOUTH CHINA SEA

As the Paleogene-Neogene boundary, the T60 unconformity in the Beibuwan Basin witnessed the transition of the faulted basin into the depression basin. Unconformity controls the distribution of stratigraphic hydrocarbon reservoirs. However, the unclear characteristics and vertical structures of the T60 unconformity in Wushi sag limit the exploration. Eight types of unconformities are identified to occur during the deposition of the T60 unconformity in the Wushi Sag based on changes of seismic reflection characteristics at the unconformity. Given the three layered structures of these unconformities, this study develops an efficient method to recognize the vertical structures of unconformities on logging curves. Vertical structures of the T60 unconformity are accurately divided based on this method. Through the denudation restoration in the wells drilled with the T60 unconformity, the main controlling factors of the thickness of the weathered clay layer are revealed. Combined with the stratigraphic position of source rocks and the spatial distribution of migration system, The hydrocarbon accumulation models are established in the eastern Wushi Sag. The research has four significant outcomes. (1) Unconformity types include parallel-truncation unconformities, overlap-truncation unconformities, parallel-folding unconformities, and so on. Among them, truncation unconformities have the widest distribution range in the plan view. The plane distribution of each type of unconformity is controlled by structural location and deformation strength. (2) Step-like changes occur in several logging curves at the contact surfaces between the middle weathered clay layer and the upper and lower layers of the unconformity structure. Moreover, the weathered clay layer corresponds to the low-value intervals in multiple changing amplitude difference curves. (3) The planar distribution of the weathered clay layer is found to be mainly controlled by unconformity types and denudation thicknesses. (4) Under the combination of fault, sand body and unconformity, linear reservoir formation mode, "Z-shaped" reservoir formation mode and "step-shaped" reservoir formation mode are developed in the eastern Wushi Sag.