Influence of movable fluid occurrence in tight sandstone from a multiscale fractal perspective: a case study of the Chang-6 reservoir in the Huaqing area, Ordos Basin, China

Tight sandstone reservoirs commonly develop multiscale pore-throat systems, and differences in pore-throat structure influence movable fluid occurrence and seepage capacity. In this study, the Chang-6 3 tight sandstone reservoir within the Chang-6 interval in the Huaqing area, Ordos Basin, was investigated using casting thin sections, scanning electron microscopy, X-ray diffraction, high-pressure mercury intrusion porosimetry, nuclear magnetic resonance, and multiscale fractal analysis to clarify pore-throat structure and its control on movable fluid occurrence. The results show that reservoir space is dominated by residual intergranular pores, dissolution pores, and intercrystalline pores, with pore-throat sizes mainly concentrated between 0.04 and 0.18 μ m. Movable fluid saturation ranges from 20.98% to 55.90%, averaging 35.23%, and decreases from type II to type IV reservoirs. NMR pore partitioning based on the T₂ cutoff shows that movable fluids are mainly hosted in mesopores, contributing 34.75%-76.28%; micropores mainly control bound fluid occurrence, whereas macropores contribute limited movable fluids where connectivity is poor. NMR and HPMI fractal results indicate that mesopore- and macropore-related fractal dimensions generally fall within the effective fractal range, whereas micropore-related dimensions deviate from the conventional range. SEM box-counting results show that poorer-quality reservoirs have more complex pore boundaries. Sensitivity analyses of SEM thresholds, NMR pore partitioning based on the T 2cutoff , and HPMI segmentation boundaries confirm that the main mesopore- and macropore-related interpretations remain stable. These findings identify mesopores as the dominant pore-throat scale controlling movable fluid storage and seepage, providing multiscale fractal evidence for evaluating movable fluid occurrence and reservoir quality in tight sandstones.