Joan Southeast Basin Structure Of The Dynamical Evolution And Hydrocarbon Accumulation In The Study

Qiongdongnan basin is a Tertiary basin developed on the NW continental margin of the South China Sea with an area of 30,000 km2. In regional tectonics, it located at the transition area of the Pacific realm and Tethys realm. The Qiongongnan basin has experienced complex deformation history in Mesozoic and Cenozoic. From Eocene to early Oligocene, the rifting of the Qiongdongnan basin was strongly affected by Indochina Peninsula clockwise extrusion; from Late Oligocene to Middle Miocene, the Qiongdongnan basin shifted from rifting to depressing stage controlled by South China Sea opening. Since Late Miocene, the Qiongdongnan basin has changed into active rifting stage associated with mantle activity. Controlled by above geological events, the basin shows a double-layer vertical structure with faulting structures in the lower and depressing structure in the upper sector. The rifting structures in Qiongdongnan basin include both grabens and half-grabens. From plan view, the basin includes three zones from north to south and three blocks from west to east. The change of accommodate space in Paleogene was controlled by block activity, regional background subsidence and global sea level change, while the supply of sediments is mainly affected by pre-rifting water depth, boundary highs near the basin and tectonic morphology. Controlled by balance degree between accommodate space and sediment supply, the Qiongdongnan basin deposited lacustrine sediments in Eocene, an alternate continental-marine sequence in Yacheng group and separated neritic facies in Lingshui group. Mainly controlled by thermal subsidence and sea level change, the basin has experienced open neritic filling period from Sanya to Meishan group and open neritic to half abysmal filling period from Ying-huang group to Quaternary.The thermal status of the lithosphere during rifting stage directly controlled the deformation pattern of the upper crust. The northern depression stretched with relative colder lithosphere and was characterized by half grabens with all the faults dipping toward south, while the central depression stretched with relative hotter lithosphere and was characterized by complex rifting composed of half grabens and grabens. The present PT field formed mainly during active rifting period, but was controlled by early rifting architecture. The complex rifting zone was characterized by high PT, while the rifting zone with faults dipping toward the same direction was characterize by normal PT. Yacheng- Songtao uplift developed between above two rifting zones belongs to transition area of PT field, the pressure curvature of single well here is featured by four segments: the normal compacted area + upper over-pressured zone +relative low or normal pressure zone + lower strongly over-pressured zone.Confined by source rock and basin sedimentary architectures, the Qiongdongnan basin developed two series of hydrocarbon bearing systems in vertical. They crosscut and link through early Miocene sedimentary sequences. Oil and gas mainly came from lower hydrocarbon bearing system. Controlled by Paleogene structures and sedimentary architectures, the sags in the northern-3-depression and Ledong, Lingshui, Baodao sags in the central depression developed main source rocks, which has oil generation potential; other sags and low uplifts mainly beared gas generation source rock. The maturation degree of the organic materials determined that the northern depressions mainly generate oil and the sags in the central depression mainly produce gas. Resource evaluation shows that hydrocarbon-rich sags include Ya'nan, Lingshui, Songnan and Baodao sags.Regional cover and PT condition controlled the formation and .evolution of hydrodynamic system in Qiongdongnan basin. Initial migration of oil and gas is mainly through micro-fractures. The northern depression mainly developed low-energy compact flow, the secondary migration of oil and gas are driven by buoyancy and hydrodynamics toward prevailing slope. The central depression mainly developed high-energy PT flow,...