Reservoir Characteristics and Formation Model of the Bauxite Gas Reservoir: The Benxi Formation in the LX Block, Northeast of Ordos Basin, China

Recently, horizontal well L47–1CH in the Longdong area of the southwestern Ordos Basin made a significant breakthrough in bauxite natural gas exploration, changing the traditional geological understanding that bauxite could not form effective reservoirs. To further explore the exploration and development potential of bauxite natural gas reservoirs in the northeast of the Ordos Basin, it is urgent to carry out basic geological studies. This paper discusses the sedimentary environment, reservoir characteristics, and formation patterns of the bauxite gas reservoirs in the LX Block using trace elements, thin sections, X-ray diffraction, scanning electron microscopy, conventional physical properties, constant pressure mercury, etc. Then, the distribution pattern of bauxite was studied according to the restoration of the karst paleogeomorphology, and the formation model of bauxite was ultimately established. The results show that bauxite developed a clear triple-segment structure vertically, characterized by rich iron at the bottom, high aluminum at the middle, and low iron at the top. The mineral composition of the bauxite section mainly includes diaspore, iron minerals, titanium minerals, and birnessite. The types of pores mainly include intra- and intergranular dissolved pores, matrix-dissolved pores, intercrystalline pores, and microfractures. The porosity ranges from 1.51 to 9.90%, with a relatively good sorting and connectivity of the pore and throat. The bauxite was formed in a hot and humid climate, deposited in a shallow-water tidal flat and lagoon sedimentary environment with oxygen depleted, and experienced oscillatory regression during the deposition process. The thickness of bauxite is significantly controlled by karst paleogeomorphology, and it is mainly distributed at the negative terrain positions of karat pits and karst terraces. The above results can provide a geological basis for the exploration and development of bauxite in the Ordos Basin and similar basins worldwide.