Enrichment and accumulation patterns and favorable area evaluation of deep coalbed methane in the Fugu area, Ordos Basin

The Shenfu block on the eastern margin of the Ordos Basin is a typical deep coalbed methane (CBM) field with abundant resources and considerable potential for exploration and exploitation. In October 2023, the large-scale Shenfu deep CBM field successfully reported proven geological reserves exceeding 110 billion m³. The Fugu area, located in the central part of the Shenfu block, is the earliest experimental area for deep CBM production. However, there is a lack of a clear understanding of CBM enrichment and accumulation patterns, as well as the potential for exploration and development, of this area. By comprehensively utilizing the data from seismic surveys, logging, drilling, and coal tests, this study identified the geological characteristics, enrichment and accumulation patterns, and favorable area distribution of deep CBM in the Fugu area. The results show that the dominant coal seams 4+5 and 8+9 exhibit stable distribution and large thicknesses (coal seams 4+5: 3.2‒5.8 m; coal seams 8+9: 8.7‒13.5 m), with favorable intervals primarily occurring in the middle-upper parts. Macroscopically, the coals are dominated by bright to semi-bright coals, and their structures predominantly include primary and cataclastic structures. Due to the influence of plutonic metamorphism, the coals mainly comprise gas coal, fat coal, and coking coal. Regarding the metamorphic degree, the coals are at the peak of pyrolysis and gas generation, with coal reservoirs exhibiting medium to high gas contents (coal seams 4+5: 3.0‒12.0 m³/t; coal seams 8+9: 7.5‒18.5 m³/t) and medium to high gas saturation (35.0%‒115.0%). The dominant coal seams are low-permeability reservoirs (permeability: (0.01‒0.09) ×10⁻³ μm²), with pores primarily consisting of micropores and small pores. This study proposed a CBM enrichment and accumulation mode involving flexural folds, faults, and hydrodynamic force, determining that the CBM enrichment areas include the gentle tectonic zone and the slope zone. Furthermore, this study established an evaluation system for geological and engineering double sweet spots of deep CBM, identifying that the Class I geological and engineering sweet spots are located in the eastern and southwestern parts, which serve as the preferred areas for deep CBM exploration and exploitation in the study area. This study further enriches the theory on deep CBM enrichment and accumulation, serving as an important guide for the exploration and exploitation of deep CBM on the eastern margin of the Ordos Basin.