The Difference In Space-Time Of Cenozoic Fault System In Northern Depression Of Beibu Gulf Basin

The northern Depression of Beibu Gulf Basin is a Cenozoic faulted sedimentary basin developed in the background of Mesozoic regional uplift,and the complex fault system is developed in the basin.This paper makes a detailed description of the present fault system based on the current seismic data and drilling logs.The spatial difference of the Cenozoic fault system is analyzed from three aspects: The differences of the characteristics of different faults,The difference of the fault system of each structure layer and the difference of the fault combination.The evolution process of fault system and the time difference of Cenozoic fault system is clarified based on the analysis of fault activity and equilibrium profile.Tectonic stress field analysis and physical simulation experiment are carried out based on regional dynamics background investigation,and the mechanism of the difference of time-space of the Cenozoic faults in the northern depression is analyzed.Based on the above analysis,this paper discusses the control effect of fault system difference on basin structure and petroleum geology condition.The faults in the northern depression can be divided into four main directions: NE,NEE,EW and NW.Among them,the plane spreading and development process of large composite faults are segmented such as wei-southwest fault and no.1 fault.The fault properties are diverse,tension,slippage,extrusion and other fault properties coexist.There is stretch fault such as WZ12-8 south fault,stretch-slip fault sunch as no.2 fault,sea 2 fault has compressive.The fault of north west trend has some concealment.The Cenozoic fault system in the northern depression has the difference of the upper and lower faults,bounded by the subface of Liu-1 member of Liushagang formation(T83).There are obvious differences between the different trend and the activity of each period.The depositional stage between Changliu formation to Liu-2 member of Liushagang formation is the first stage of rifting.The main control fault activity of NE is strong,which shows the strong NW-SE extension.The depositional stage between Liu-1 member of Liushagang formation to Wei-3 member of Weizhou formation is the second stage of rifting.The main fault activity of NEE is strong and shows a certain slip property,this period is dominated by the extension of near NS.The depositional stage between Wei-2 member of Weizhou formation to Wei-1 member of Weizhou formation is the transition period of rifting and depression,there is an increase in the number of near EW direction secondary faults with a sliding property.The depositional stage after Wei-1 member of Weizhou formation and before Xiayang formation is the period of uplift and denudation.The depositional stage between Xiayang formation to Wanglougang formation is the period of sedimentation,at this period,the fault activity basically stops.The regional tensile stress field since the late Eocene has shifted from NW-SE to near s-n.The oblique extension caused by this stress orientation controls the difference of different strata in the Cenozoic fault system in the study area.The separation of two preexisting basement faults of EW direction has affected the change of the basin structure from west to east.The difference of the fault system controls the development of hydrocarbon source rocks,reservoir and oil and gas transmission in the study area,and further controls the two kinds of accumulation modes of "Upper hydrocarbon generation and lower reservoir" and "Lower hydrocarbon generation and upper reservoir".