Fractal Characterization of Pore-Throat Types and Fluid Occurrence Mechanisms in Tight Sandstones: A Case Study of the He 1 Member, Ordos Basin

The He 1 Member of the Ordos Basin is a key interval for tight sandstone gas exploration and development. The complex interplay between mineral fabric and pore-throat structure makes movable fluid evaluation challenging, which is a critical factor restricting the efficient development of gas reservoirs. Current research primarily analyzes fluid occurrence characteristics from the perspectives of pore-throat structure parameters and clay mineral types, yet the mechanisms by which clay minerals influence pore-throat structure and subsequently control fluid mobility under different lithological settings remain unclear. This study focuses on the tight sandstones of the He 1 Member in the Xinzhao area, Ordos Basin. Integrating thin section identification, scanning electron microscopy, X-ray diffraction, high-pressure mercury injection, and variable-speed nuclear magnetic resonance (NMR) techniques, this work classifies rock types and employs segmented fractal theory based on T₂ cutoff values to elucidate the control mechanism of mineral fabric on pore-throat structure and fluid mobility under lithological constraints. The results show that the tight sandstones in the study area can be divided into four lithologies: Type Ⅰ (gravel-bearing coarse-grained lithic quartz sandstone) is dominated by intergranular/intragranular dissolution pores and kaolinite intercrystalline pores; Type Ⅱ (medium- to coarse-grained lithic quartz sandstone) is characterized by intragranular dissolution pores and clay mineral intercrystalline pores; Type Ⅲ (gravel-bearing coarse-grained lithic sandstone) mainly develops intergranular matrix dissolution pores and intragranular dissolution pores; and Type Ⅳ (coarse- to medium-grained lithic sandstone) primarily develops intragranular dissolution pores and illite intercrystalline pores. Segmented fractal analysis reveals that the pore-throat space can be divided into large and small pore systems. The fractal dimension of the large-pore system shows a significant negative correlation with movable fluid saturation, indicating that the structural complexity and connectivity of the large-pore system are key factors controlling fluid mobility. The occurrence of movable fluid is jointly controlled by mineral composition and diagenesis. Types Ⅰ and Ⅱ are characterized by high quartz content, where a rigid framework resists compaction, and dissolution pores, kaolinite intercrystalline pores, and micro-fractures constitute a well-connected large-pore system with a low fractal dimension, resulting in high movable fluid saturation. In contrast, Types Ⅲ and Ⅳ are characterized by illite and chlorite occurring in filamentous, bridging, or flaky forms, which fill the pores, leading to complex pore-throat structures, a high fractal dimension, and low movable fluid saturation. This study reveals the control mechanism of mineral fabric on pore-throat structure and movable fluid under different lithological settings, providing a theoretical basis for the efficient exploration and development of tight sandstone gas reservoirs.