Structural Characteristics And Control Of Hydrocarbon Accumulation In Liujianfang Area Of Qikou Sag

The Liujianfang area,locate in the central part of the buried hill structural belt in Beidagang of Qikou Sag,Bohai Gulf Basin,is an important oil and gas exploration area controlled by buried hills,yet the study on the structural characteristics of the area is relatively limited.Therefore,the in-depth study on the structural characteristics in the Liujianfang area and its controlling laws of oil and gas guided by the theories and methods such as structural geology,petroleum geology,seismic stratigraphy and sedimentary petrology was carried out to provide reliable geological basis for further exploration and development in this area.The structural characteristics of the Liujianfang area were studied in many aspects and from various angles including fracture combination,fracture evolution and structural style and so on by means of fracture activity analysis,etc.,through which it was considered that faults of 2-4 level existed in the area,with the fault strike dominated by North to East and North East to East.Besides,there were various fracture geometry and fracture combinations.The evolution process of faults could be divided into four parts:the middle stage of the third member of Shahejie Formation(strengthened oscillation),the middle stage of the first member of Shahejie Formation(intense activity-gradual decline)and the late stage of Minghuazhen(renewed activity).The structural style was mainly characterized by extensional structural style,supplemented by the strike slip structural style.The area was further divided into two structural zones:the brush like fault zone which was located in the eastern part of the study area and developed into many broom-like fault combinations and the steep slope zone which was located in the middle part of the study area,and with relatively monotonous structural feature.It was believed that this area had many sets of source bed characterized by great thickness,abundant in organic matter,good conversion ability,high hydrocarbon generating strength and high yield of hydrocarbon,well developed large river--delta sand body of the Neogene period with good reservoir conditions,along with the good regional cap rock,which indicated that the area had good reservoir conditions and belonged to compound oil and gas accumulation zone as a whole where multiple sets of oil-bearing series and multiple types of oil and gas reservoirs coexisted.The conclusion that the formation of traps and oil and gas accumulation were controlled by the structure was drawn through further analysis of the control effect of structural characteristics of oil and gas accumulation in this area.The oil and gas enrichment and distributing layer were closely related to the main fault activity period.The diversity of oil and gas reservoirs was attributed to the three or four grade faults,and the migration process and migration mode were affected by the structure.On this basis,the conclusion that the brush fracture zone located in the eastern part of the study area was a favorable oil and gas exploration zone was made combined with the present exploration and development status in the study area.