| The Bohai Basin is the offshore part of the Bohai Bay Basin.In recent years,oil and gas exploration achievements have been quite fruitful.In order to explore the structural factors affecting the differential enrichment of oil and gas,three marginal depressions,namely Liaoxi sag,Qinnan sag and Miaoxi sag,were selected in Bohai Basin with different structural locations,structural characteristics and differential enrichment of oil and gas.Based on seismic data,exploration data and previous research results,the fault system,tectonic evolution process,extensional strength and subsidence history of the three sags were compared and studied,and then the structural factors conducive to oil and gas enrichment were analyzed.The main points are as follows:（1）Difference of fault system.Extension-strike-slip shovel-type sag control faults are developed in the Liaoxi sag and Qinnan sag.The fault profile combination style of both sags is dominated by extensional strike slip superposition style,the plane combination style shows the stratification that the lower part is mainly extended style and the upper part is mainly strike-slip style.Miaoxi sag is characterized by vertical-high angle plate strike-slip faults.In the Miaoxi sag,the most important fault profile combination style is flower-like strike-slip structure style,the fault plane combination pattern in each layer interface mainly develops the combination pattern of echelon type,broom type and strike-slip dual type and other strike-slip properties.（2）Structural evolution difference.The Cenozoic in the study area experienced four stages of rifting and depression,and different sags show different activity characteristics in different periods.Liaoxi sag and Qinnan sag have the strongest sag activity during the E2s3 and E3d sedimentary periods,with high extensional strength,high fault activity rate,and fast subsidence rate,after entering the depression stage,the extension and strike-slip of Liaoxi sag and Qinnan sag gradually weaken.Miaoxi sag has strong strike-slip and moderate extensional strength during the rift period.After entering the depression period,the extensional effect is weak,but the strike-slip effect is still strong.（3）Mechanism of structural differences.The clockwise rotation of the stress field in the Bohai Basin controls the Liaoxi sag and Qinnan sag,which shows strong extensional characteristics in the early stage and weak strike-slip characteristics in the late stage during the Cenozoic evolution,while the Miaoxi sag always shows strong strike-slip characteristics.During the rotation of the stress field,the basement faults of each sag will exhibit the characteristics of main extension or main strike-slip based on the angle between them and the direction of the stress field.Therefore,the distribution,mutual contact relationship,and distribution location of the basement faults will affect the Cenozoic evolution process of the depression,causing structural differences between the sags.（4）Tectonic activities affect oil and gas enrichment.Structural activities have different degrees of control over the geological conditions of oil and gas accumulation:strong depression extension,high activity rate of sag control faults,and large subsidence of the depression are conducive to the development of high-quality source rocks;The high activity rate of early faults is conducive to oil and gas migration;Late intense activity will destroy the previously formed oil and gas reservoirs;Fault activity has dual effects on oil and gas migration.According to multiple structural parameters comprehensive comparison sort,the west transition zone and north slope of the east subsag of Qinnan sag,the south slope area of the southeast subsag of Qinnan sag,the horsetail fault area at the end of F5 fault,and the south uplift of Miaoxi in the south of the north subsag of Miaoxi sag are selected as the favorable exploration areas. |
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