Prototype Restoration Of Paleogene Fault Depression And Tectonic Evolution In Weixinan Sag,Beibuwan Basin

Weixinan sag is a third-order structural unit belonging to the northern depression of Beibuwan Basin.Weixinan sag has two periods of fault systems,the Paleogene continental rifting system and the Neogene South China Sea spreading system,which have complicated relationship of accommodation,utilization,cutting and reconstruction.The continental rifting structural layers(Changliu Formation,Liushagang Formation and Weizhou Formation)have undergone different degrees of transformation during the formation of the spreading structural layers in the South China Sea,and the Paleogene extensional rifting prototype structures have been severely damaged in some areas.The prototype of the tectonic framework of the Paleogene continental faulted depression in Weixinan sag directly determines the original distribution of source rocks and reservoirs.However,the residual pattern is often used to replace the prototype pattern in the current oil and gas evaluation work,which often leads to misunderstandings.This paper clarifies the formation process of the Paleogene extensional fault system in the study area by classifying and staging the faults in the study area,and analyzes the transformation and superposition of Paleogene fault system in Neogene tectonic movement,determines the evolution process of the study area,and then correctly understands the mechanism of fault control depression formation in the study area,it is of great significance to objectively understand the original distribution characteristics of the oil generation layer of the Liushagang Formation and the reservoir layer of the Weizhou Formation in the study area.In this paper,from the perspective of the Weixinan sag mainly studies of fracture geometry,through 3D seismic data for various faults in the study area,the analysis of regional structure and the restoration of the extension pattern of the Paleogene fault prototype,finally formed several understandings:1.The synsedimentary faults in Weixinan sag can be divided into three grades according to the scale of the controlling fault strata.The No.1 and No.3 faults control the subsidence pattern of the Paleogene fault depression and are the first grade faults.No.2 fault controls the local subsidence of the fault depression and is a second-order fault.The tertiary fault is a series of forward and reverse faults which control the local fault depression and are smaller in size than the No.2 fault.It is determined that the faults formed in the sedimentary period of Liushagang Formation,Weizhou Formation and the early Neogene.2.The No.2 fault in the study area is a secondary fault of the regional No.1 fault.The eastern section of the No.2 fault was formed in the deposition period of Liushagang Formation in the Eocene and the western section was formed in the deposition period of Weizhou Formation in the Oligocene.The NE segment of the No.1 and No.3 faults in the study area is a synsedimentary low-angle extensional controlled depression fault in Paleogene,while the NE segment is a high-angle differential depression fault in Neogene.The existing trace of No.1and No.3 faults is the product of the combination of two stages of faults.3.The subsidence axis in the study area is NEE in Paleogene and NE in Neogene;The Paleogene prototype subsidence framework in the study area controlled the filling pattern of the Paleogene,but the prototype structure of the sag suffered severe damage during the spreading tectonic movement in the South China Sea.4.The western section of the existing No.1 fault in Weixinan sag is not the original controlled depression fault,but the reformed fault at the end of Paleogene,so the northwestern boundary of Weixinan sag is not the original extensional boundary,it's the residual boundary of the later reconstruction.