Accumulation characteristics and exploration prospects of deep coalbed methane in the Longtan Formation of the Nanchuan block on the southeastern margin of the Sichuan Basin

Vertical well Y2 in the Nanchuan bloc on the southeastern margin of the Sichuan Basin has yielded daily production of natural flowing gas of 13000 m³, signifying a strategic breakthrough in deep coalbed methane (CBM) exploration on the margin. This study aims to systematically reveal the enrichment and high-yield patterns of CBM in the Nanchuan block. Using data on the regional geology, tests, assays, and production, this study investigated the accumulation characteristic and conducted exploration potential evaluation for deep CBM in the Nanchuan block from the perspective of the quality, pore and fissure characteristics, gas content, and fracability of coals. The results are as follows: (1) The deep coal reservoirs in the study area exhibit stable distribution, with thicknesses ranging from 1.5 m to 4.2 m and vitrinite reflectance (R_ran) from 1.6% to 2.2%, establishing them coal seams with high R_ran values and low ash content. These coal seams provide a material basis for CBM accumulation. (2) The coal reservoirs are of micro pore-micro and -fissure type, with micro-pores accounting for 78.4% and contributing 95% to the total specific surface area. These are conducive to CBM adsorption and preservation. Furthermore, these reservoirs exhibit well-developed micro-fractures, which are beneficial for fracturing and gas seepage. (3) The coal seams feature high gas content and rich free gas, with a total gas content ranging between 12 m³/t and 37 m³/t and free gas generally accounting for 24% to 36% of the deep CBM. The critical conversion depth of absorption is about 1500 m. (4) The deep coals exhibit small horizontal stress difference coefficients, favorable coal structures, strong stress barriers as roof, and small angles between natural cracks and the maximum principal stress, which contribute to high fracability. (5) Wide and gentle anticlines in the Dingshan and Lintanchang areas on basin margins emerge as favorable zones for deep CBM exploration. The results of this study will serve as a guide for deep CBM exploration on the southeastern margin of the Sichuan Basin and provide a reference for deep CBM exploration in similar basins.