Accumulation characteristics and exploration potential of deep coalbed methane in Changning area of Sichuan Basin

The Sichuan Basin is rich in coalbed methane resources, and shallow coalbed methane in the Junlian area has been commercially developed. In the adjacent Changning area, multiple drilling wells have tested and obtained gas in the Permian coal seam, revealing significant potential for coalbed methane in the Sichuan Basin. With significant breakthrough in the exploration and development of deep coalbed methane in China, and drawing from experiences in the Qinshui Basin and the Ordos Basin, a comprehensive study was conducted to assess the resource potential of deep coalbed methane in the Changning area. This study utilized data from logging, coal seam coring, and experimental testing to analyze the geology of the coal seams, evaluate gas-bearing properties, and investigate the main factors influencing reservoir formation and favorable zones. The study found that the 7th and 8th coal seams in the study area are thick, regionally stable, and high-quality, mainly composed of primary structural coals with high rank and high fixed carbon content. These seams are at the peak of pyrolysis gas generation, suggesting significant hydrocarbon generation potential. Coal seams have characteristics of high porosity, high permeability, and high cleat density, providing ample storage space, while the coal seam roof and floor—predominantly mudstone—offer excellent preservation conditions. Compared to the shallow coalbed methane in Junlian, the deep coalbed methane in the Changning area features a more stable structure, a higher proportion of free gas, and a more intact coal matrix. Coalbed methane reservoirs are formed in the target area far from erosion boundary and Class Ⅰ fault zones. The abundant free gas is more conducive to subsequent development. Based on geo-engineering conditions, a dual geo-engineering “sweet spot” index system for evaluating coalbed methane in the Changning area has been established. A favorable deep coalbed methane development area of 1 300 km² has been identified, with a calculated resource volume of 1 700×108 m3, primarily located in the Luochang and Jianwu synclines. The research results have effectively guided the deployment of coalbed methane wells in the region, contributing to the high-quality development of unconventional natural gas in the Sichuan Basin.