Study On The Cenozoic Transition Structure Of Huimin Sag

Huimin sag belongs to a secondary structural unit in the southwest of Jiyang depression of Bohai bay basin.The research on the Cenozoic extensional transition structure in this area not only has important practical significance for oil and gas exploration,but also has important theoretical significance for understanding the structural characteristics of the extension basin in this area.Based on the comprehensive analysis of geology,geophysics and oil-gas exploration data and previous research results,this paper further identifies the distribution of the Cenozoic extensional transition structure zone and divides the types of transition structure by systematically analyzing the geometry,kinematics and chronology of the Cenozoic fault structure.Then we disscuss the controlling factors,tectonic evolution and their relationship with oil and gas.Cenozoic faults in this area can be divided into four levels.These faults can be divided into five major types according to plane distribution and combination types:zigzag-elbow bend,fault branch,en echelon-broom fault combination,relay-parallel fault combination and restricted-cut intersecting fault combination.According to the symbiotic combination in the section,it can be divided into five combinations of structural style:shovel-slope low angle normal fault and associated structural combination,oblique-normal slip fault,shovel-normal fault fan,step or domino-type structure and graben-horst structure.The Ningnan-Wunan fault zone was active in the sedimentary stage of the E_k.The Linshang fault zone is most active in the sedimentary stage of the E_s3-E_s2,while there are still some strong activities in the sedimentary stage of the E_s1-E_d.Xiakou fault zone was active in the sedimentary stage of E_k.The depositional activity of the E_s4 is weak.The activity was the most intense in the sedimentary stage of the E_s3-E_s2.There are still strong continuous activities in the E_s1-E_d.This area can be divided into three first-order transition structure zones,eight second-order transition structure zones and ten main third-order transition structure zones.The main factors controlling its development are the preexisting basement fault of Mesozoic,segmenting activity and growth connection of large normal fault system,and uneven extension.In the sedimentary stage of E_k,the embryonic form of the first-order transition structure zone and the outline of the second-order transition structure zone developed along the Ningnan-Wunan fault.In the sedimentary stage of the E_s4,the embryonic form of the second-order transition structure zone developed along the Xiakou fault.In the sedimentary stage of the E_s3-E_s2,the outline of the second-order transition structure zone developed along Linshang fault.A number of three-order transition structure zones were formed in the sedimentary stage of the E_s1-E_d.The transition structure zones control the formation of major hydrocarbon-generation sags,and affect the development of reservoirs,and provide related hydrocarbon traps.The next oil and gas exploration targets in this area should be mainly concentrated in Linnan sub-sag and the secondary and tertiary extensional transition structure zone.