Abstract: Due to the significant variation in fluid distribution,complex pore structure,and relatively low percolation capacity of the Chang 8 reservoir in the Fuxian Area of the Ordos Basin,five representative rock samples from the study area were selected for testing and analysis.The primary controlling factors affecting fluid mobility were analyzed using thin section casting, scanning electron microscopy (SEM),X-ray diffraction clay mineral analysis (XRD),and constant velocity mercury injection experiments.The results indicate that the movable fluid saturation in tight sandstone reservoirs ranges from 26.31% to 50.61%,with an average of 35.15%.The pore throat radius for movable fluid is between 0.10 μm and 0.50 μm;the T₂ cutoff value falls within the range of 6.69 ms to 49.90 ms,and the minimum pore throat radius for movable fluid is between 0.16 μm and 0.36 μm.The permeability of a reservoir exerts greater control over the mobility of fluids than porosity.The median radius,maximum pore throat radius,and average pore throat radius exhibit a positive correlation with movable fluid,with the median radius exerting a greater influence.Higher feldspar content facilitates the formation of feldspar dissolution pores,resulting in greater movable fluid saturation.Type Ⅰ and Type Ⅱ reservoirs exhibit high kaolinite content,with pores filled with dispersed particles.Type Ⅲ reservoirs exhibit high illite and mixed-layer illite content,with pores bridged and segmented,leading to pore throat plugging.These research findings provide valuable insights for enhancing the recovery of tight oil reservoirs.

Key words: microscopic pore throat, movable fluid, nuclear magnetic resonance, tight sandstone, main controlling factors, Ordos Basin