Abstract: Tight sandstone features small pore throats,a complex pore structure and severe heterogeneity.To accurately characterize the pore throat structure characteristics of tight sandstone reservoirs,taking the tight sandstone of the third member of Yingcheng Formation in Changling Fault Depression,Songliao Basin as an example,the pore structure of tight sandstone was investigated by means of field emission scanning,casting thin section,constant-rate mercury injection and nuclear magnetic resonance method.Through the combination of NMR with CMP experiment,the distribution of pore and throat radius was effectively identified,and the lower limit of pore throat radius of movable fluid was defined.The experimental results indicate that the average throat radius of the target reservoir ranges from 0.331 to 0.824 μm,the average pore radius ranges from 17.600 to 24.290 μm,and the average pore-throat connectivity ratio ranges from 27.62 to 53.17.The lower limit of the pore throat radius of movable fluid is within a range.The lower limit r₁ and the upper limit r₂ can divide the fluid into the bound zone,the transition zone and the movable zone.As the permeability decreases,r₁ gradually increases,r₂ gradually decreases,the range of the bound zone becomes larger,and the range of the transition zone and the movable zone becomes smaller.The increase of r₁ will lead to the increase of adsorption pore porosity,the decrease of seepage pore porosity and the deterioration of reservoir quality.r₁ can be used as an index to evaluate the boundary of reservoir flow capacity.The research results can provide a basis for optimizing high-quality tight sandstone reservoirs.

Key words: tight sandstone, pore structure, throat, constant-rate mercury injection, nuclear magnetic resonance