Fault Structural Characteristics of Wuerxun-Beier Depression in the Hailar Basin and Their Reservoir-Controlling Mechanism

Based on the structural analysis principle that different boundary conditions in the same stress field build different nature faults,it is commonly considered that fault subsidence of the Hailar Basin is controlled by the SSE-NNW-trending extensional stress field,forming NNE-and NEE-trending faults,with NNE-trending faults displaying distinct strike-slip deformation.During rifting-subsidence transformation phase of the Hailar Basin,the extensional stress field orientation of faulting and subsidence was adjusted to near EW direction,forming near NS-trending faults,and twisted deformation of NNE-,NEE-and SN-trending faults.The basin returned again at the end of Yimin Formation deposition and the near EW-trending compressional stress field resulted in the sinistrial deformation of the basin and intensive inversion of NNE-and NEE-trending faults.Based on the superimposion relatioinship between faulting and deformation,the Hailar Basis can be divided into four faulting systems:early extensional faulting,interim tensile shearing faulting,early extensional and interim tensile shearing faulting,and early extensional and interim tensile shearing and late reversal faulting.The early extension and interim tensile reverse faults of rift and subsidence layers resulted in fault sheltered traps.Early extensional faulting formed a complicated anticline traps by cutting through the uplifts within depression and slopes,and complicated fault block traps by assembling with interim tensile shearing faults.The early extension,interim tensile shearing and late reversal faults appear as "comb-like" composite,forming typical fault block traps in the rift-depression structure layer.Based on the coupling relation between fault activity period and accumulation period,and analysis results of typical oil and gas reservoirs,it is concluded that the early extension faults and early extension plus interim tensile shearing faults constituted sheltering faults at a critical moment of accumulation,with the sealing hydrocarbon column height generally less than the traps amplitude,and early extension and interim tensile shearing and late reversal faults were adjusted to faults.On the basis of trap styles,and roles of faults in reservoir-forming and transporting system,the modes of fault controlling reservoir in the Hailar Basin can be divided into 2 groups and 4 styles.Two groups include primary and secondary reservoirs.The primary reservoirs consists of three styles:the first is the model in which oil and gas migrate laterally in hydrocarbon kitchen margin and is sheltered by reverse faults,with the early extension and interim tensile shearing fault system as leading reservoir-controlling faults;the second is "in the box" reservoir-forming model in which oil and gas primarily migrate within hydrocarbon kitchen and is sheltered by faults;and the third is the dispersive-type reservoir model in which oil and gas in hydrocarbon kitchen of the depression migrates laterally.Reservoir-controlling faults of both models are early extensional faults.Secondary reservoirs are an "umbrella-shaped" reservoir-forming model,in which oil and gas migrates vertically along faults,with the early extension and interim tensile shearing and late reversal fault system as reservoir-controlling faults.