Differential control of structures over reservoirs and its significance in Qiongdongnan Basin

Using the method of structural analysis, the differential structural evolution characteristics and their control on difference in reservoirs between the east and west blocks in the deep-water areas of the Qiongdongnan Basin are studied in depth through fine interpretation of seismic data and analysis of fault activities. The boundary between Songnan sag and Baodao sag is delineated by detailed structural analysis, and the structural transitional zone is considered as the boundary of differential tectonic evolution between the east and west blocks of the Qiongdongnan Basin. Taking this as the boundary, the western basin is NE trended and characterized by the development of large grabens and half graben depressions controlled by large simple listric faults; the eastern basin, however, is EW in trend and characterized by the development of large grabens and composite grabens jointly controlled by Y-shaped faults, stepped spade and plate-shaped faults. Analysis shows that there are three controlling factors for the differential tectonic evolution of the east and west blocks of the Qiongdongnan Basin, i.e the restrictive boundary conditions, differences in crustal structure and heat flow, and differential structural geological influence factors. The differences in restrictive boundary conditions and the change in regional tectonic stress field are the main factors controlling the temporal and spatial differences of faults in the basin, while the last two factors eventually aggravate the differential evolution of the eastern and western regions of the basin. On this basis, a comprehensive study on the differences in petroleum systems in the deep-water area of the Qiongdongnan basin is carried out, which includes the differences in source rocks, sedimentary systems and petroleum accumulation systems. It is considered that the western deep-water area is dominated by the source rocks of transitional facies and terrigenous marine facies under large burial depth, high thermal evolution degree, and late hydrocarbon generation and expulsion time, controlled by the structure of half graben depression structure. Three accumulation systems are well developed: vertical migration-accumulation system through depression controlling fault in steep slope zone, vertical migration-accumulation system through high pressure diapirs-fractures in the central zone and lateral migration-accumulation system in the gentle slope zone. Borehole drillings have confirmed that the former two systems are the best. In contrast, the eastern deep-water area is dominated by marine source rocks under shallow burial depth, moderate thermal evolution, early generation and expulsion of hydrocarbon. Controlled by the graben structure, four zones of hydrocarbon accumulation systems are developed symmetrically: the lateral migration-accumulation systems through faults and sand ridge migrating framework in uplift zones and vertical migration-accumulation systems on both sides of the stepped faults in near depression zones. The accumulation conditions of the two areas are favorable for hydrocarbon accumulation and should be the next target for exploration.