Fluid characteristics, gas accumulation controlling factors and gas enrichment modes in coal reservoirs: A case study of the Upper Paleozoic in the central-eastern Ordos Basin, NW China

Based on the test and experimental data from exploration well cores of the Upper Paleozoic in the central-eastern Ordos Basin, combined with structural, burial depth and fluid geochemistry analyses, this study reveals the fluid characteristics, gas accumulation control factors and accumulation modes in the Upper Paleozoic coal reservoirs. The study indicates findings in two aspects. First, the 1 500–1 800 m interval represents the critical transition zone between open fluid system in shallow–medium depths and closed fluid system in deep depths. The reservoirs above 1 500 m reflect intense water invasion, with discrete pressure gradient distribution, and the presence of methane mixed with varying degrees of secondary biogenic gas, and they generally exhibit high water saturation and adsorbed gas undersaturation. The reservoirs deeper than 1 800 m, with extremely low permeability, are self-sealed, and contains closed fluid systems formed jointly by the hydrodynamic lateral blocking and tight caprock confinement. Within these systems, surface runoff infiltration is weak, the degree of secondary fluid transformation is minimal, and the pressure gradient is relatively uniform. The adsorbed gas saturation exceeds 100% in most seams, and the free gas content primarily ranges from 1 m³/t to 8 m³/t (greater than 10 m³/t in some seams). Second, the gas accumulation in deep coals is primarily controlled by coal quality, reservoir-caprock assemblage, and structural position governed storage, wettability and sealing properties, under the constraints of the underground temperature and pressure conditions. High-rank, low-ash yield coals with limestone and mudstone caprocks show superior gas accumulation potential. Positive structural highs and wide and gentle negative structural lows are favorable sites for gas enrichment, while slope belts of fold limbs exhibit relatively lower gas content. This research enhances understanding of gas accumulation mechanisms in coal reservoirs and provides effective parameter reference for precise zone evaluation and innovation of adaptive stimulation technologies for deep resources.