The Tectonic Characteristics And Forming Mechanism Of Wushi Sag

Wushi sag which is secondary structural unit of Beibu Bay basin and rich in hydrocarbon, which is viewed as a potential zone of exploration. In this paper, based on the 3D seismic interpretation, well logging and drilling data, the tectonic geometry characteristics of Wushi sag were analyzed and studied. According to the technique of fault activity and balanced section, the evolution stages and the structure characteristics of those stages were made clearly. Based on all the research work above, with the application of numerical simulation of tectonic stress field and structural physical modeling, the tectonic mechanism of Wushi sag was discussed.The main trends of the fault system in Wushi sag were NNE and EW, and the NW and NE were rest. Because the structural styles developed here were mainly stretching structural style, strike-slip structural style and strike-slip-stretching tectonic style, this means that Wushi sag was mainly influenced by tensile stress and strike-slip shear stress. Wushi sag is a typical double-fault depressed lacustrine basin, which can be divided into east zone, west zone and middle accommodation zone.The basin evolution process of Wushi sag can be divided into four stages. First, the third stage to second stage of Liushagang Period, the original faults were formed by the NW-SE extension and in Paleocene the direction of extensional stress field swing clockwise for 20, so NE and NEE faults began to develop. Second, the first stage of Liushagang Period to the third stage of Weizhou Period. In this stage, with the influence of strong extension by mantle upwelling and Ailaoshan-Red River Fault sinistrorotation, Wushi sag formed reverse NEE right-lateral strike-slip fault such as Fault 6 and Fault 7 and NW left-lateral strike-slip fault, composing Conjugate strike-slip system. Third, the second to first stage of Weizhou period. During this stage, the evolution of this work place was controlled by strong dextrorotation and weak extension, the basin evolution inherited the former stage, whereas the fault activity differed: NNE faults such as Fault 6 and WS21-1 south Fault activated stronger than others, and the NW faults became a series of echelon fractures system. Forth, Xiayanghe period to Jiaowei period. In this period, Wushi sag changed from fault depressed lacustrine basin into depression basin, controlled by the superimposing of depression and strike slipping. The dextrorotation was the primary stress effort and series of near EW secondary faults were developed in en echelon arrangement along NNE, and fault activity was weak. The South China Sea Spreading, the plate motion around Wushi sag, the development and evolution of Ailao-Red River fault, the lithospheric thinning and mantle upwelling were the main geodynamics mechanism of Wushi sag in Cenozoic.