Geological characteristics and exploration potential of deep coalbed methane in the Permian Longtan Formation, Sichuan Basin: A case study of well NT1H

To investigate the exploration potential of deep coalbed methane (CBM) in the Longtan Formation, Sichuan Basin, we conduct coring of coal seams in risk exploratory well NT1H. Through experiments and tests on the coal quality and the physical properties, adsorption, and gas-bearing properties of coal reservoirs in the Longtan Formation, we examine the geological characteristics and exploration potential of deep CBM in the formation. The results indicate that the deep coal seams in the Longtan Formation exhibit coals with intact structure and well-developed cleats, vitrinite reflectance (R_o) ranging from 2.70 % to 3.13 %, average vitrinite, inertinite, and inorganic component contents of 60.6 %, 23.4 %, and 16.0 %, respectively, and minerals dominated by clay. These coal seams feature high total porosity, with a porosity varying from 4.83 % to 10.13 % and effective porosity from 2.82 % to 9.66 % (average: 6.53 %). Their pores, with strong structural heterogeneity, are dominated by micropores with sizes less than 2 nm, followed by macropores or fractures with sizes greater than 10 μm, while pores with sizes ranging from 2 to 50 nm are relatively underdeveloped. The coals in the Longtan Formation display Langmuir volume generally exceeding 25 m³/t and average Langmuir pressure of 3.24 MPa at 120 ℃ . Pressure coring reveals that the coal seams manifest gas content varying from 26.52 to 31.24 m³/t, gas saturation from 123 % to 146 %, free gas content from approximately 5 to 10 m³/t, and low in-situ water saturation from 18.2 % to 48.6 %. Their total gas content increases with burial depth. In all, the comprehensive investigation suggests that the deep coal reservoirs in the Longtan Formation exhibit conditions favorable for gas-bearing properties. Therefore, the deep reservoirs with supersaturated CBM in the Upper Permian Longtan Formation in the Sichuan Basin have great exploration potential. For these reservoirs, it is recommended to target closely spaced coal seams for integrated volume fracturing and three-dimentional reconstruction for CBM recovery.