Structural Characteristics And Formative Mechanism Of Southern Sag, Qiongdongnan Basin

South China Sea (SCS), which is one of the largest marginal sea of west Pacific Ocean, is a hot research point in the world for its special location of structure and abundant hydrocarbon resources. Qiongdongnan Basin (QDNB) locates in the northwest part of north SCS, with Yinggehai Basin on the west separated by Fault No.1, and Zhujiangkou Basin on the northeast. Because of its special structural location, structural evolution of Qiongdongnan Basin is both affected by Pacific Structure Domain and Tethys Structure Domain, and formative mechanism of the basin is very complex. The South Depression of QDNB is a weak study and exploration area because of the lack of data in the past. The study of structural characteristics and formative mechanism of the area has a significant meaning for analyzing the formative mechanism of northern SCS, and also can give us some suggestions in petroleum exploration of SCS.On the basis of predecessors' research, this dissertation interpreted 2D seismic data acquired by CNPC in recent years, and divided the main structure sequences of South Depression based on the sequence stratigraphy. The author realized that T60 is the breakup unconformity and it separates the strata into the upper structural sequence and the lower structural sequence. On the basis of studying the fault characteristics systematically based on the fine sequence interpretation, the author found that the structural direction of the depression was controlled by the NNE trend faults, and the depression could be divided into west uplift area and mid break-step area. By subdividing of fault period the author found that the faults in the area developed in three different periods: Eocene, early Oligocene and mid-Miocene. Then the plan characteristics of faults in Eocene and early Oligocene were summarized ,and realized that faults developed in Eocene have a main trend of NNE and NE, faults developed in early Oligocene have a main trend of EW. By analyzing the main structural styles, concluded that the depression was under the regional extensional environment when major faults developed. Through a sequential restoration of two structural sections (D—D' and E—E'), combined with the regional structural movements, the structural evolution history of this depression were analyzed. By comparing with the main Cenozoic rift basins, the author analyzed the structural Characteristics and formative mechanism of QDNB and realized that the structural Characteristics of South Depression and northern SCS were both affected by the Red River Strike Fault.