Sedimentary environment and enrichment model of bauxite series in Benxi Formation, Linxing area, Ordos Basin

Key words: 
• bauxite rock /
• Reservoir characteristics /
• Sedimentary environment /
• Enrichment pattern /
• Benxi Formation /
• Linxing area

Abstract: The existing body of research on bauxite oil and gas reservoirs is limited in scope, both in terms of its domestic and international dimensions. The discovery of a bauxite reservoir with high-yield industrial gas flow in the Taiyuan Formation in the Longdong area, southwestern Ordos Basin, represents a significant advancement in the exploration of bauxite gas reservoirs. Additionally, the recent identification of bauxite strata in the Benxi Formation in the Linxing area of the northeastern margin of the basin, through the test of actual production, suggests promising exploration potential. [Objective] In order to further explore the potential of natural gas exploration and the development of bauxite series in the Linxing area, it is necessary to conduct a comprehensive analysis of the fundamental geological characteristics of the sedimentary environment, the composition of the rock minerals, the characteristics of the reservoir pores and the structure of the bauxite series. In addition, it is essential to identify the factors that control the development of bauxite in the study area. It would be beneficial to ascertain which areas are most conducive to such activity. [Methods] In this study, a comprehensive analysis was conducted on the sedimentary environment, reservoir characteristics and sedimentary model of the Benxi Formation bauxite rock series in the Linxing area. This analysis was based on a synthesis of logging and seismic data, X-ray diffraction, casting thin sections, scanning electron microscopy, major and trace elements, high-pressure mercury injection, N2, CO2 adsorption and conventional physical properties, and other experimental methods. [Results and Discussions] The bauxite rock series in the study area is characterised by a high concentration of diaspore and clay minerals, exhibiting a distinct three-stage vertical structure. The lower portion is rich in iron, the middle contains a higher aluminium content, and the upper section has a lower iron concentration. The pore types observed in the bauxite reservoir include dissolution pores, intercrystalline pores, organic pores and microfractures. The dissolution pore represents the predominant reservoir type, with pore sizes typically ranging between 2 and 50 nm. The sorting and connectivity of the pores are relatively good. The average porosity is 3.64%, with an average permeability of 7.24 × 10-3 μm2. [Conclusions] A comprehensive analysis indicates that the development thickness of bauxite rock series is contingent upon the sedimentary environment and karst paleogeomorphology. The shallow water sedimentary environment, characterised by high temperatures and humidity, low oxygen levels and alternating marine and terrestrial conditions, provides optimal conditions for the formation and preservation of bauxite rock series. The slope, or buried pit, represents a crucial area for the exploration and development of bauxite rock series. This understanding is anticipated to provide a geological foundation for the exploration and development of natural gas in bauxite rock series with comparable geological backgrounds in this region.