Characterization and Development Potential Evaluation of the No. 5 Coal Reservoir in the Shanxi Formation of a Block in Southeastern Ordos Basin

Abstract

The southeastern margin of the Ordos Basin holds immense potential for coalbed methane (CBM) resources, yet its development faces severe challenges due to the extremely low permeability of the reservoirs. To thoroughly investigate the reservoir characteristics and development potential of the No. 5 primary coal seam in the Lower Permian Shanxi Formation within a specific block, this study systematically analyzed the spatial distribution, coal lithology and quality, gas-bearing properties, pore-permeability characteristics, and mechanical properties of the coal seam. The methodology integrated well logging interpretation, core observation, maceral identification, isothermal adsorption experiments, and rock mechanics testing. The results indicate that the No. 5 coal seam is controlled by a gently dipping monocline structure that is high in the east and low in the west, with a thickness ranging from 4.42 to 8.61 m (averaging 6.39 m). Three phases of tectonic fractures have developed: the Indosinian near north-south trending fractures are weakly developed, whereas the Yanshanian northwest-southeast trending and Himalayan northeast-southwest trending fractures are strongly developed. This leads to significant spatial variations in the in-situ stress field and fracture intensity, constituting the macroscopic geological background for reservoir heterogeneity. The coal seam is characterized by high rank, high vitrinite content, and low ash and moisture. The coal texture is predominantly primary and cataclastic. Gas content increases with burial depth, generally exceeding 15 m³/t in deep zones; however, the permeability is overall lower than 0.4 mD, exhibiting an ultra-low permeability feature. The maximum and minimum horizontal principal stresses and the breakdown pressure all increase with depth. The difference coefficient of in-situ stress is generally greater than 0.37, indicating that the reservoir is in a state of strong stress disequilibrium. A comprehensive evaluation suggests that the No. 5 coal seam in the study area has undergone multi-stage tectonic modifications. Despite its high resource abundance, it possesses extremely low permeability and exists in a state of strong stress disequilibrium. It is a typical reservoir type that requires large-scale hydraulic fracturing stimulation to achieve industrial productivity.