Abstract: [Objective] Significant progress has been made in the exploration of Cretaceous volcanic rocks in the Honghaoershute Depression of the Erlian Basin. However, the study of reservoir genesis mechanisms, particularly the primary pore filling and subsequent modification, remains relatively weak. [Methods] In this study, core samples were collected from three oil-bearing structures (Baer, Hailute, and Nugeda) within the study area. Comprehensive analyses were conducted using core observations, thin-section identification, scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and energy-dispersive spectrometry (EDS) point-to-area scanning. These analyses were used to investigate the pore-filling series and mechanisms of Cretaceous volcanic rocks in different structures. [Results] The results show that the pore-filling materials in the volcanic rocks of the study area primarily consist of calcite, dolomite, chlorite, quartz, and kaolinite, which are similar to the materials filling fractures. These materials are predominantly controlled by the compositional characteristics of the volcanic rock matrix.In the Hailute structure, the andesites exhibit high Fe-Mg content and abundant calcic-albite phenocrysts. The pore-filling materials are dominated by Fe-bearing dolomite, with a high degree of filling.In the Baer structure, the andesites also exhibit high Fe-Mg content, but the calcite content in the albite phenocrysts is low. Calcite fills only the edges of the pores, while the interior is primarily filled with chlorite. In the Nugeda structure, the andesites show low Mg, low Fe, and high K characteristics. The pore-filling materials are dominated by quartz and kaolinite, with a low degree of filling.The pore-filling patterns depend on the connectivity between pores and the matrix. Three filling modes are identified and established: (1) fracture-connected type, (2) compaction-damaged type, and (3) uneven filling type. [Conclusion] The differences in physical properties of the andesite reservoirs in the study area are mainly controlled by the combined effects of fracture connectivity and the degree of dissolution, while vesicle filling intensity and mineral types to some extent influence the development of dissolution and primary reservoir space.