Study On Microscopic Pore Throat Structure And Porous Flow Mechanism Of The Chang 7 Shale Oil Reservoir In The Ordos Basin

As one of unconventional petroleum resources,shale oil is important for increasing crude oil reserves and stabilizing production in China.The seventh oil layer group of third member of the Yanchang formation of the upper Triassic（Chang 7）in the Ordos basin is characterized by low sand ratio and small thickness of single sand layer,which meets the standard of shale oil reservoir.The Chang 7 shale oil is mainly distributed in the tight sandstone.Reservoirs are characterized by complex microscopic pore throat structure and strong heterogeneity.It is necessary to adopt multiple technologies to characterize the microscopic pore throat structure of the reservoir at multiple scales.The Chang 7 sand bodies of delta front in the Ansai area and turbidite fan in the Heshui area were selected as the research objects in the thesis.The characteristics of deposition,petrology,physical properties,diagenesis and pore evolution of tight sandstone reservoirs were studied by cast thin section,high pressure mercury injection,constant speed mercury injection,CT scanning,nuclear magnetic resonance,oil-water relative permeability and other technologies.The differences of porous flow characteristics of the two types of reservoirs and their causes were revealed by analyzing the microscopic pore throat structure,fractal and movable fluid distribution characteristics.The classification and evaluation parameter system suitable for shale oil reservoirs were established in the study area.The Chang 7 sandstone reservoir is mainly low porosity,ultra-low porosity and ultra-low permeability fine sandstone in the study area.The pore throat structure is controlled by sedimentation and diagenesis and has the characteristics of micron-nanometer pore throat development,poor connectivity and strong heterogeneity.The Chang 7experienced depth increase and temperature rise before the early Cretaceous and depth decrease and temperature drop after the late Cretaceous.The diagenetic environment evolved from alkaline to acidic,and then from acidic to alkaline.After compaction,early cementation,dissolution and medium-late cementation,the unconsolidated sediments finally evolved into the reservoir dominated by submicron pore and nanoscale throat.Strong compaction and cementation were the main reasons for the tight reservoir and the complexity of pore throat structure.Reservoir porosity in the Ansai area decreased from34.48%in the penecontemporaneous stage to 6.94%in the early diagenetic stage,then increased to 13.51%in the medium diagenetic A₁ stage,and finally decreased to 8.30%in the medium diagenetic A₂ stage（now）.Reservoir porosity in the Heshui area decreased from 33.44%in the penecontemporaneous stage to 9.10%in the early diagenetic stage,then increased to 16.05%in the medium diagenetic A₁ stage,and finally decreased to9.19%in the medium diagenetic A₂ stage（now）.Influenced by sedimentation and diagenesis,the size and distribution of pore throats,connectivity and heterogeneity of reservoir in the two areas are obviously different.The pore throat structure models of four types of sand bodies:（1）For the branch channel sand bodies,Intergranular and feldspar dissolved pores are relatively developed,the maximum mercury saturation mainly 70.00%-95.00%,the submicron pore throat content 20.00%-30.00%,and the fractal dimension 2.40-2.60;（2）for the underwater distributary channel sand bodies,intergranular pores are relatively developed,the maximum mercury saturation mainly 55.00%-80.00%,the submicron pore throat content 15.00%-25.00%,and the fractal dimension 2.50-2.70;（3）for the lateral edge sand bodies of branch channel,inter-crystalline and feldspar intragranular dissolved pores are relatively developed,the maximum mercury saturation mainly 40.00%-65.00%,the submicron pore throat content 15.00%-25.00%,and the fractal dimension 2.60-2.80;（4）for the underwater distributary bay sand bodies,inter-crystalline pores are relatively developed,the maximum mercury saturation mainly 30.00%-50.00%,the submicron pore throat content 5.00%-15.00%,and the fractal dimension 2.70-2.90.The movable fluid in the Chang 7 reservoir is mainly distributed in nanoscale and submicron pores and controlled by medium and coarse throats.The oil-water porous flow capacity is weak.The average relative permeability at the iso-permeability point is 0.0832,and the average co-permeability interval is 26.91%.With the increase of water saturation,the decreasing rate of oil phase relative permeability tends to slow down.Oil displacement mainly occurs in submicron pores,and crude oil in nanoscale pores is not easy to be displaced.Physical property,mineral composition,microscopic pore throat structure and heterogeneity are the key factors for reservoir porous flow differentiation.The movable fluid distribution of the turbidite fan sand bodies in the Heshui area was characterized by more movable fluid,stronger fluid mobility and higher movable fluid content in submicron pores.The movable fluid distribution in the delta front sand bodies in the Ansai area is characterized by less movable fluid,poor fluid mobility and low movable fluid content in submicron pores.Based on the analysis of the basic characteristics of the reservoir,the pore throat structure and porous flow characteristics were mainly considered.A classification and evaluation parameter system applicable to the Chang 7 shale oil reservoir was established.The Chang 7 reservoirs were divided into typesⅠ,Ⅱ,ⅢandⅣ,and four types of reservoir microscopic characteristics models were constructed.The microscopic characteristics models of typeⅠandⅡreservoirs are characterized by more submicron pore throats,strong heterogeneity,movable fluid saturation greater than 40%,movable fluid mainly distributed in nanoscale and submicron pores,movable fluid mostly controlled by medium and coarse throats,and strong oil-water porous flow ability.The microscopic characteristics models of typeⅢandⅣreservoirs are characterized by less submicron pore throats,strong heterogeneity,movable fluid saturation less than 40%,movable fluid mainly distributed in nanoscale pores,movable fluid mostly controlled by medium and fine throats,and weak oil-water porous flow ability.