Effect of heterogeneous diagenetic alterations on pore structure of tight sandstones: a case study from Yanchang Formation sandstones in Ordos Basin

Tight reservoirs require integrated characterization methods to capture their full-scale pore size distribution， yet current approaches often rely on subjective determination of merging points without solid theoretical basis. This study introduces seepage theory and proposes a novel HPMI-CT integration method for comprehensive pore size characterization. In order to elucidate the relationship between differential diagenetic processes and complex pore structures， this study focuses on tight sandstones of the Yanchang Formation， deposited within a delta-lacustrine system. Three types of tight sandstone with distinctly different diagenetic facies were selected for analysis. Results indicate that pore structure is predominantly controlled by differential diagenesis， while reservoir permeability is mainly governed by the crucial radius. Type A sandstone has larger grains and well-developed chlorite rims， preserving large intergranular pores. Type B， with smaller grains and intense dissolution， has many small intragranular pores. Despite differing pore sizes， both have similar crucial radius and high permeability， making them high-quality reservoirs. Type C， however， underwent strong compaction and cementation， leaving few pores and the lowest crucial radius， resulting in low permeability and poor reservoir quality.