DEVELOPMENT AND FORMING MECHANISM OF MAJOR FAULTS IN LEIDONG SAG, BEIBUWAN BASIN

As one of the earliest offshore basins for oil and gas exploration in China, the Beibuwan Basin has experienced more than 50 years of oil and gas exploration. The current oil and gas exploration blocks are mainly located in some local regions, such as the Weixinan Sag, Fushan Sag and so on. The exploration density of the whole basin remains low. And no breakthrough has been made recently. Therefore, it is necessary to carry out more researches on geology and to make further resource evaluation in backup blocks, such as the Leidong Sag. In this paper, the characteristics as well as the evolution and formation mechanism of some major faults in the Leidong Sag are analyzed and their control over the sediment infilling and oil and gas accumulation discussed. There are three major faults in the region which control the distribution pattern of sedimentary facies and two sub-sags with individual depositional centers are formed. The North Fault in the Eastern Sub-sag and the South Fault in the Western Sub-sag are extensional in NE direction, influenced by NW-SE stretching stress, while the North Fault in the Western Sub-sag is a transform one characterized by transformation and shearing. In Early Eocene, the activity of the South Fault in the Western Sub-sag was intensified. The North Fault in the Eastern Sub-sag and the North Fault in the Western Sub-sag started to develop. In Late Eocene, the activity of the South Fault in the Western Sub-sag slowed down and eventually stopped. The North Fault in the Eastern Sub-sag and the North Fault in the Western Sub-sag kept active until late Oligocene. The whole sag turned into a depression stage in Miocene with little fault activity. The oil and gas exploration in the Leidong Sag should restart from the evaluation of advantageous migration system. Breakthroughs may be achieved on the uplift, southern slope, and northern steep slope.