Study On The Deformation Mechanism Of Strike-Slip And Oblique Extension Faults And Its Control On Hydrocarbon Accumulation In Penglai 25-31 Area,Bohai Sea

The formation and evolution of Cenozoic rift basins are generally impacted by the reactivation of pre-existing basement structures before basin formation.Strike-slip displacement and dip-slip displacement components simultaneously exist when the strike of the pre-existing basement structure obliquely intersects with the extension direction.This study investigates the Penglai 25-31 area in the Bohai Sea as the research target zone.Firstly,the geometry and kinematics of strike-slip and oblique extensional faults are systematically analyzed to divide the fault system of the strike-slip and oblique extensional composite zone.Then,we disentangle the transfer relationships between strike-slip and oblique extensional faults and determine the deformation mechanisms of strike-slip and oblique extensional faults in different periods by analyzing the displacement field of fault reactivation.Next,the"Dual Angle Relationship Diagram"of fault deformation mechanisms under different angles between the displacement direction and the pre-existing fault strike is established using physical simulation.Finally,we discuss the control effects of strike-slip and oblique extensional faults on source rocks,migration systems,and traps during the deformation process.According to the structural evolution history and the geometric characteristics analysis of the strike-slip and oblique extensional faults and the determination of the main fault activity period.,the basin is divided into four evolutionary phases and six types of fault systems.The four evolutionary phases include the initial rifting phase during the sedimentary period from the Kongdian Formation to the Es₄ Member,the rifting phase of strong strike-slip and oblique extensional during the sedimentary period of the Es₃ Member,the phase of rift compression but strong strike-slip during the period from Es₁ and Es₂ Members to Dongying Formation,and the post rifting evolution phase from the Guantao Formation to Minghuazhen Formation period.The six types of fault systems are long-term active strike-slip faults,long-term active faults during the period from the Kongdian Formation to the Es₄ Member,long-term active faults during the Es₃ Member,active faults during the period from the Kongdian Formation to the Es₄ Member,active faults during the Es₃ Member,and active faults during the period from the Guantao Formation to the Minghuazhen Formation.The transfer relationship of the strike-slip and oblique extensional fault system is studied to identify four types of transfer zones in the Penglai 25-31 area by restoring the segment growth process of the main strike-slip and oblique extensional fault system,including the strike-slip polarity transfer zone,strike-slip overlapping transfer zone,strike-slip and oblique extension cross-transfer zone,and strike-slip and oblique extension adjustable transfer zone.In addition,the discrimination diagrams of fault reactivation under paleo-stress are established using the kinematic vorticity to discuss the deformation mechanisms of strike-slip and oblique extensional faults in the target zone during different geological periods.According to the tectonic analog modeling technology,the fault interaction experiments of the strike-slip and oblique extension composite zone are carried out using oblique extension models with different angles.The distribution characteristics of secondary faults formed during the process of pre-existing fault reactivation are analyzed based on the results of the oblique extension experiments at different angles.Furthermore,a prediction method for determining fault deformation mechanisms under different angles between the displacement direction and the pre-existing fault strike is established by the"Dual Angle Relationship Diagram".The"isolated growth-interaction-connection"evolution process of the extensional faults in the west zone and the secondary faults in the Laizhou East Branch No.1 fault zone in the Penglai 25-31 block under the south-north unified extensional stress field during the depression period is reproduced using fault interaction experiments of the strike-slip and oblique extension composite zone.Based on the segment growth process of strike-slip faults with various scales,we propose a fault development process in which strike-slip faults in the Penglai 25-31 block develop upward and penetrate the paleosurface,and discuss the control effect of strike-slip and oblique extensional faults on hydrocarbon accumulation.The research results indicate that:（1）The scale of sag is relatively large during the combined deformation of strike-slip and oblique extension and the deformation of strong strike-slip,which are prospects for main source rock development.（2）Three types of oil-source faults are primarily developed,including long-term active strike-slip fault system,long-term active oblique extensional fault system,and late active partial strike-slip associated transtensional fault system.From the time and space differences of hydrocarbon migration by oil-source faults,the main oil-source faults for hydrocarbon migration are transtensional faults associated with strike-slip fault zones.In addition,the main strike-slip fault in the strike-slip fault zone carries hydrocarbon relatively later but with higher efficiency.From the perspective of differences in late fault activity,the deformation intensity of the strike-slip fault zone is greater than that of oblique extensional faults,and the fault truncates shallower strata.In addition,the frequency and cumulative activity intensity of the multi-phase seismic activities along the fault are larger in the strike-slip fault zone compared to oblique extensional faults,and the strike-slip fault zone possesses higher hydrocarbon migration efficiency or larger migration volume.（3）The deformation of strike-slip and oblique extensional faults segmentally controls the formation of"two classes and four types"traps,that is,the transpressional faulted anticline traps related to strike-slip deformation,the wrench fault traps,and the faulted anticline and faulted barrier traps related to（oblique）extensional deformation.Combining investigation results of strike-slip faults and oblique extensional faults on the sag evolution,source rock distribution,the distribution of traps related to hydrocarbon accumulation faults,and the difference of hydrocarbon migration ability in oil-source faults,two types of composite reservoir control models based on strike-slip and oblique extensional faults have been constructed.