Reservoir Characteristics and Evolution Model of Carboniferous Bauxite Rock Series of Benxi Formation in the Ordos Basin, China

The Benxi Formation bauxite series in the Ordos Basin holds substantial natural gas exploration potential, and reservoir characterization is pivotal to delineating favorable exploration blocks. This study focuses on the bauxite series in the Baode–Xingxian area, eastern Ordos Basin, integrating mineralogical analysis, scanning electron microscopy, nuclear magnetic resonance, low-temperature CO 2 /N 2 adsorption, and fractal analysis to systematically investigate its mineral composition, pore types, porosity-permeability properties, pore distribution, and controlling factors. The sequence mainly comprises ferroaluminous rocks, bauxites, and claystones, dominated by diaspore, hematite, chlorite, gibbsite, and kaolinite, with aluminum-rich minerals exhibiting cryptocrystalline/clastic textures and directional alignment. Reservoir spaces are dominated by dissolution pores (primary), intercrystalline pores, and microfractures. Porosity ranges from 0.81 to 18.19% (average: 10.75%) and permeability from 0.001 to 10.883 mD (average: 2.025 mD), showing an exponential positive correlation. High-quality reservoirs primarily develop in bauxite and ferruginous aluminous rocks. Mesopores dominate the pore structure, contributing to most of the pore volume and specific surface area. Fractal analysis reveals higher fractal dimensions for mesopores and macropores than micropores, indicating stronger heterogeneity. Reservoir evolution underwent three stages: penecostemporaneous dissolution-induced porosity enhancement, burial intercrystalline pore formation, and organic acid modification. Pore development is jointly controlled by the sedimentary environment and mineral composition.