Research On Development Characteristics Of Faults In Bonna Sub-sag

In this dissertation,the geometrical characteristics of faults in Bonan sub-sag were studied,on the basis of seismic,core,reservoir,regional geology,and rock mechanics data.Combining geometrical characteristics and fault activity,the faults in Bonan sub-sag were divided into four levels.We made statistics of fault throw and calculated the fault activity rate,which was used to do research on fault activity.Meanwhile,balanced cross-section technique was introduced into structural evolution study.Then,five stages and nine periods of tectonic evolution were divided,including Indosinian-to-early Yanshanian stage(T₃-J₁₊₂)characterized by thrust fault development,middle-later Yanshanian stage?J₃-K?characterized by tectonic reverse,Yanshanian-to-Himalayan transitional stage?Ek-Es₄?characterized by the development of level-I and level-II faults and the formation of Bonan sub-sag structure,early-middle Himalayan stag?Es₃-Ed?characterized by secondary faults development in quantity,and late Himalayan stag?Ng-Q?characterized by faults dying out totally.The Indosinian-to-early Yanshanian stage includes two periods of late Triassic for intensive thrusting and early-middle Jurassic for weak thrusting.The early-middle Himalayan stag makes up of three periods,consisting of intensive activity in Es₃ period,relatively quiet period of Es₂,and resurrection period of Es₁-Ed.Late Himalayan stag can be divided into Ng period of weak activity and Nm-Q period of no activity.On the whole,Bonan sub-sag undergone an evolution of intensive thrusting-to-weak thrusting-to-intense normal faulting-to-weak faulting-to-resurrecting-to-dying out.Based on regional tectonic stress field and the ancient structure pattern,the tectonic stress field of Bonan sub-sag can be be divided into four phases similar to tectonic evolution.Bonan sub-sag suffered from compression by direction of NE35°-SW215°in Indosinian-to-early Yanshanian,and extension in the same direction in middle-later Yanshanian.In Yanshanian-to-Himalayan transitional phases,Bonan sub-sag undergone intense extension in the direction of NW340°-SE160°and weak extension in the direction of NE70°-SE250°.However,there was strong extension in one direction of NW348°-SE168°in early-middle Himalayan.Then,stress field numerical simulations of the four phases were done by ANSYS15.0 software.The simulation results show that the orientation of thrust faults were mainly controlled by the maximum principal stress.The orientation of normal faults was dominately controlled by the planar shear stress and the minimum principal stress.The shear fractures were formed in high shear stress area,with the same rotation fractures developing preferentially.In low shear stress area,the orientation of normal faults was perpendicular to the direction of minimum principal stress.Furthermore,the differential stress decided the possible fault development area.In conclusion,fault development mainly depends on local and regional tectonic stress field.The distance from main faults,the shape and patterns of faults,and structural configuration can influence the stress distribution locally,controlling the formation of low level faults consequently.Hydrocarbon accumulation can be influenced by faults in many aspects.The high level faults controlled the type and spatial distribution of sedimentary facies obviously.And the fault activities influenced the variation gradient of formation thickness.In addition,tectonic stress field can indicate the direction of oil and gas migration,from low stress value to high stress value.Moreover,the permeability of fracture in reservoir forming period indicated that oil and gas in Bonan sub-sag mainly migrated through fracture in EW and vertical directions.The fracture permeability in SN direction is poor,and therefore oil and gas mainly migrated along faults.