Reservoir Characteristics Of Shahejie Formation In The Western Slope Of Baxian Depression

Baxian depression is an important oil-bearing block.With the development of exploration and exploitation these years,further research about the western steep zone of Baxian depression is becoming more necessary.Guided by principles of sedimentary and theories of reservoir geology,mineral petrology,diagenesis,pore space and physical property are studied using core and thin section observation combined with other analytical data.Based on these experimental tests,the contolling factor of the favorable reservoir is discussed,then the distribution of favorable reservoir is predicted.There are two kinds reservoir in the western steep zone of Baxian depression which are fan delta and nearshore subaqueous fan.Controlling by sedimentary facies,sand type of the western steep zone of Baxian depression reservoir is mainly lithic arkose sandstone,which is poor or moderate sorted and sub-angula rounded.There are five types of diagenesis which are crucial to the quality of reservoir in the western steep zone of Baxian depression,including compaction/pressure solution,cementation,metasomatism,dissolution and recrystallization.Under the background of continuous bury,the diagenetic environment of the western steep zone of Baxian depression underwent an evolution of alkalescence-acid-alkalescence process,and the corresponding diagenetic evolution sequence is compaction/early carbonate cementation-feldspardissolution/quartzovergrowth/authigenickaolinite-lime-mudmatrix recrystallization/late carbonate cementation/anhydrite cementation/quartz dissolution-pyrite cementation.The diagenetic stage of Es₁ formation is from B sub-period of eogenetic to A sub-period of mesogenetic,while the diagenetic stage of Es₂ and Es₃ formation is mainly A sub-period of mesogenetic.According to diagenesis intensity,nine types of diagenetic facies can be divided.From the edge of the delta to the interior,carbonate cementation transits to mixed cementation.Deeper the reservoir burries,the compaction effect is more intense and at the depth from 3500m to 4200m,a secondary pore belt is developed because of acid dissolution.Physical property data and thin section observation indicate that reservoir of the western steep zone of Baxian depression are mainly low porosity and super-low permeability and the pore types are mainly residual primary pore and secondary pore.Deeper the reservoir burries,less the primary pore is.Through mercury injection capillary pressure analysis,three types of pore are reconiged,which include medium pore with thin throat,thin pore with thin throat and fine pore with fine throat.Based on comprehensive analysis of influences on reservoir property caused by sedimentation,diagenesis and abnormal fluid pressure,the reservoir evaluation standard is established and the distribution of favorable reservoir is predicted.The results shows that underwater distributary channel of the fan delta front sandbody and axial gravity flow channel sandbody in Es₃ and axial gravity flow channel sandbody in Es₁ are all favorable reservoir.