The Genesis and Influence of Pore-Throat Structural Multifractionation in Dense Sandstone Reservoirs as an Example of Chang 8 in Zhenjing Area, Ordos Basin, North China

The heterogeneity of the pore-throat structure in tight sandstone is a key factor influencing its reservoir quality and fluid seepage capacity. This study, based on 20 tight sandstone samples from the Chang 8 section in the Zhenjing area of the Ordos Basin, employs a range of experimental techniques, including cast thin section analysis, X-ray diffraction, high-pressure mercury injection, nitrogen adsorption, and nuclear magnetic resonance. These methods, combined with mineralogical and petrological analyses, are used to investigate the fractal characteristics of the pore-throat structure, its genesis, and its impact on fluid movability. The results reveal that the combination of different mineral compositions and pore-throat types leads to three distinct pore-throat structures: Type I, characterized by large pore throats, high connectivity, and weak heterogeneity; Type II, exhibiting a bimodal distribution with well-organized pore throats; and Type III, dominated by intercrystalline pores, which show poor connectivity and physical properties. Overall, the pore-throat structure is highly heterogeneous, exhibiting multiple fractal characteristics with triple fractals as the dominant form and double fractals as secondary. The large pore-throat fractal (<i>D</i> ₃) is primarily controlled by the number of large pore throats and is also influenced by pore morphology and the development of interparticle pores. The middle-pore-throat fractal (<i>D</i> ₂) is mainly determined by the number of medium-sized pore throats and is constrained by the development of interparticle dissolution pores and microfractures. The small pore-throat fractal (<i>D</i> ₁) depends on the content of intercrystalline pores and the variation in the microporosity. The overall heterogeneity of the system reflects the coupling of pore throats across different scales in terms of spatial distribution, geometry, and development. Additionally, mineral composition has a certain effect on the fractal characteristics of the pore-throat structure. Rigid particles promote pore distribution homogenization, while carbonate minerals increase the fractal complexity of pore throats. An increase in clay mineral content suppresses the fractal dimension of small pore throats but enhances the complexity of medium-sized pore throats. The fractal dimension of pore throats is positively correlated with fluid mobility, with <i>D</i> ₃ having the greatest influence, followed by <i>D</i> ₂, while <i>D</i> ₁ has a minor effect. The development of large pore throats enhances connectivity and reduces network complexity, thus improving the fluid permeability and movable saturation. Conversely, the accumulation of clay minerals significantly inhibits the fluid mobility. These findings demonstrate that the nonhomogeneity and multiple fractal characteristics of the microscopic pore-throat structure primarily control the fluid seepage performance of tight sandstone reservoirs, providing critical theoretical support for the evaluation and efficient development of such reservoirs.