Characteristics and accumulation model of bauxite gas reservoirs in the southwestern Ordos Basin

A significant breakthrough has been made in natural gas exploration in the aluminum (Al)-bearing rock series of the Permo-Carboniferous System in the Longdong (LD) region of the Ordos Basin, and this has sparked great interest in studying bauxite reservoirs within the oil and gas industry. Although recent research has focused on the depositional environment and reservoir well logging identification, there is still a lack of understanding regarding the characteristics, formation mechanism, and accumulation model of bauxite gas reservoirs. This knowledge gap hinders the rapid exploration of this new type of gas reservoir. Based on core observations, major and trace element analyses, whole-rock X-ray diffraction, thin section examination, and scanning electron microscopy (SEM), the pore types, physical properties, reservoir formation mechanisms and accumulation models of the bauxite reservoirs were determined. The results are as follows: (1) The main mineral components of the Al-bearing rock series are diaspore and clay minerals. The bauxite that formed by the weathering, leaching, sedimentation and diagenesis of the parent rock has a high diaspore content and secondary dissolution pores. (2) Bauxite with a high aluminum content has a high porosity and permeability and can be used as a potential natural gas reservoir, and the reservoir space is mainly composed of dissolution pores. There is a strong correlation between the aluminum content and porosity, indicating that high Al2O3 or diaspore contents can serve as reliable indicators for an effective reservoir. (3) The overlying coal-bearing source rocks and the underlying karst are key to bauxite reservoir formation. The concurrent presence longitudinally of coal, bauxite, fractures, and karst collapse offers valuable guidance for the exploration of natural gas in bauxite within the Ordos Basin. These findings have significant implications for natural gas exploration in Al-bearing rocks in North China and other regions.