Potential for the production of deep to ultradeep coalbed methane resources in the Upper Permian Longtan Formation, Sichuan Basin

Breakthroughs have been achieved in the high-yield gas flow of coalbed methane (CBM) wells with burial depths ranging from 1000 to 2500 m in the Cainan area of the Junggar Basin and the Daning-Jixian and southern Yanchuan blocks on the eastern margin of the Ordos Basin, signifying the enormous potential for the production of deep CBM. In the Sichuan Basin, the thin to moderately thick coal seam groups (7‒15 layers) in the Upper Permian Longtan Formation primarily exhibit burial depths ranging between 2000 and 4500 m. In the central Sichuan Basin, characterized by north-dipping monoclines and local low-amplitude uplifts, deep coal seams display superior critical production parameters, including gas content, gas saturation, and reservoir pressure, to shallow ones, demonstrating considerable potential for supporting China’s natural gas development strategy of reserve growth and production addition. Key findings are as follows: (1) The Longtan Formation coal measures manifest frequent coal-mudstone interbeds, a large number of coal seams, and huge cumulative resources. Notably, the No. 19 coal seam is well-developed laterally, with an average thickness of 3.2 m and a favorable area (thickness: >4 m) of 700 km². (2) The high thermal maturity (2.53%‒3.18%) of coals suggest a high hydrocarbon generation capacity. The gas logging-derived total hydrocarbon value of coal measures reveals favorable results, with measured gas content, gas saturation, and free gas content at a burial depth of 2500 m varying in ranges of 16.64‒17.61 m³/t, 138%‒151%, and 4.84‒5.60 m³/t, respectively. (3) The reservoirs at a burial depth exceeding 800 m in the southern Sichuan Basin exhibit measured pressure coefficients of above 1.1, and those in the central Sichuan Basin display predicted pressure coefficients of above 1.8, indicating potential abnormally ultra-high pressure reservoirs. Based on the deep to ultradeep, overpressure, and supersaturated resource endowment characteristics of coal seams in the Sichuan Basin and referencing the successful experience in deep CBM production in the Daning area, this study determined the resource potential and gas reservoir characteristics of the No.19 coal seam and established a technology system for the three-dimensional exploration and exploitation of deep to ultradeep thin coal seams. Therefore, this study is significant for large-scale CBM production capacity construction.