Study On Microscopic Pore Throat Structure And Grading Evaluation Of Chang 7 Tight Sandstone Reservoirs In Jiyuan Area,Ordos Basin

The tight sandstone reservoir contains tight oil with great resource reserves and development potential,which is the most realistic key field of increasing crude oil storage and production in China,and the research and evaluation of reservoir characteristics are the key to realize the effective exploration and development of tight oil.Tight sandstone reservoirs have the characteristics of strong diagenetic transformation,tight rock,complex pore throat structure and strong oil-bearing heterogeneity,and there are many problems,such as difficult quantitative characterization of microscopic pore throat structure,unclear control factors of fluid mobility,weak research on reservoir formation mechanism,and ambiguous meaning of gradational boundary of reservoir evaluation,which seriously restrict the quality evaluation of tight sandstone reservoirs and the prediction of high quality reservoir.Therefore,taking Chang 7 tight sandstone reservoirs of Triassic Yanchang Formation in Jiyuan area of Ordos Basin as the research object,this manuscript carried out cast thin sections,scanning electron microscope,X-ray diffraction,constant rate mercury injection,micron CT,high pressure mercury injection（HPMI）,low-temperature nitrogen adsorption（LTNA）,nuclear magnetic resonance（NMR）,starting pressure gradient and oil-water relative permeability experiments to quantitatively characterize the microscopic pore throat structure characteristics of tight sandstone reservoirs,and to determine the mobility degree,occurrence characteristics,movable condition and seepage characteristics of fluid in pore throat space.On this basis,the diagenesis-pore evolution process and densification mechanism of the reservoir were clarified,and the reservoir forming boundary of tight sandstone was accurately determined through various methods.The tight sandstone reservoirs were quantitatively graded and evaluated by combined with the type of pore throat structure and reservoir forming boundary,and the comprehensive evaluation standard of tight sandstone reservoirs was finally established,which provides scientific basis for the next effective exploration and development of tight oil in this area.The main insights gained are as follows:（1）According to the shape of capillary pressure curve,displacement pressure and average pore throat radius,the pore throat structure of tight sandstone reservoirs in the study area can be divided into four types.From type I to type IV pore throat structure,the average pore throat radius gradually decreases,the heterogeneity of pore throat structure gradually increases,and the corresponding reservoir fluid mobility and seepage capacity become worse in turn.（2）The T₂spectrum of movable fluid was constructed based on the T₂spectrum obtained before and after NMR centrifugation,and the pore throat size distribution curves obtained by HPMI and LTNA experiment were combined to transform the NMR T₂spectrum of movable fluid,combined with the piecewise fitting method can accurately characterize the full pore throat size distribution characteristics in the range from 0.001 to 10μm,and the conversion results can quantitatively characterize the occurrence characteristics of movable fluid in different pore throat structures and pore throat types.（3）The full pore throat size distribution has multi-fractal features.According to the turning point of the fractal characteristic curve,the pore throat space of tight sandstone reservoir can be divided into four types:large pore-throats（>1μm）,medium pore-throats（0.1～1μm）,small pore-throats（0.01～0.1μm）and microscopic pore-throats（<0.01μm）,and the fractal dimension of corresponding pore throat types has a relationship D₁>D₂>D₃>D₄.Medium pore-throats and small pore-throats are the main contributors to the physical properties of tight sandstone reservoirs,and they are also the main occurrence place of reservoir movable fluid.（4）The different combinations and development of different pore throat sizes and types in tight sandstone reservoirs result in different microscopic pore throat structures,and its heterogeneity is the main factor affecting reservoir physical properties and fluid mobility.In addition,reservoir fluid mobility is comprehensively affected by reservoir physical properties and various clay minerals.Lower clay mineral content and higher reservoir quality are conducive to the high flowability of tight sandstone reservoir fluids.（5）Compaction and early authigenic mineral cementation are the main factors leading to the densification of the reservoir in the study area.Late carbonate mineral cementation ultimately determines the densification degree of the present reservoir,while dissolution has limited ability to improve the reservoir physical properties,and the reservoir has been basically densified before large-scale charging of oil and gas.（6）Tight sandstone reservoir has three types of reservoir forming boundaries:theoretical lower limit of reservoir forming,physical property lower limit of oil accumulation and effective seepage lower limit.Among them,the theoretical lower limit of reservoir forming is the boundary between tight sandstone reservoir and non-reservoir,and the corresponding porosity and permeability are 3.5%and 0.02×10^-3μm²,respectively.The physical property lower limit of oil accumulation is the boundary between oil-bearing tight reservoir and ineffective tight reservoir,and the corresponding porosity and permeability are 5.5%and0.05×10^-3μm²,respectively.The effective seepage lower limit is the physical property lower limit of effective tight reservoir that can be produced in oil-bearing tight reservoir,and the corresponding porosity and permeability are 8%and 0.2×10^-3μm²,respectively.（7）There is a good correspondence between various types of reservoir forming boundaries and the classification boundaries of pore throat structure.Combining the pore throat structure types with the reservoir forming boundaries,the Chang 7 tight sandstone reservoirs in the study area can be divided into four classes:I,II,III and IV,representing easily movable tight reservoir,movable tight reservoir,oil-bearing tight reservoir and ineffective tight reservoir,respectively.Among them,the class I and class II tight sandstone reservoirs are the favorable target areas for increasing tight oil storage and production in the study area.