Fractal characteristics of micropore structure of Taiyuan Formation bauxite reservoir in Longdong area of Ordos Basin

Abstract

As an unconventional gas reservoir, bauxite gas reservoirs have often been studied as weathered crust cover layers rather than reservoirs in the past. The Taiyuan Formation bauxite in the Longdong area of the Ordos Basin has achieved breakthroughs, and some wells have obtained industrial airflow. Therefore, it is necessary to conduct basic geological research on the bauxite reservoir. The application of casting thin sections, scanning electron microscopy (SEM), X-ray diffraction (XRD), high-pressure mercury injection (HPMI), and low-temperature nitrogen adsorption (L-TNA) techniques provides a comprehensive framework for studying reservoir characteristics as well as the micropore fractal structure. The results indicate that (1) the bauxite reservoir has logging characteristics of high natural gamma (500API) and low acoustic wave time difference (60–90 us/m), with natural gamma being the most critical parameter in identifying the bauxite reservoir in the logging curve. (2) The bauxite rocks in the Longdong region belong to sedimentary formation, and three types of micro ancient landforms (buried pits, terraces, and grooves) have different lithological combinations. Among them, the combination of the bauxite rocks in the buried pits and carbonaceous mudstones has been subjected to freshwater leaching for a long time, and the thickness of the bauxite rocks is the largest. (3) The pores of bauxite reservoir are mainly dissolution pores within particles, intergranular dissolution pores, intergranular pores, and microcracks. The average porosity of the reservoir is 8.49%, and the average permeability is 0.261 × 10⁻³ μm², with a low permeability standard greater than 0.4 accounting for 17.19%. (4) The fractal dimension D2 of micropore D1 and nanopore D2 were calculated by high-pressure mercury injection and nitrogen adsorption experiments, respectively. Through correlation analysis, we found that the fractal dimension D2 (calculated from nitrogen adsorption data) more effectively characterizes the comprehensive quality of nanopores within the bauxite reservoir compared to the fractal dimension D1 obtained from high-pressure mercury injection (HPMI). We have established classification evaluation standards: Type I D1 is 2.2∼2.6, Type II D1 is 2.6∼2.8, and Type III D1 is 2.8∼3. Among them, Class I bauxite reservoir is a high-quality reservoir and can be the preferred target for exploration in the study area.