Characteristics of coalbed methane accumulation in Bide-Santang syncline, western Guizhou and favorable sector

The coalbed methane(CBM) resource in the Bide-Santang synclineis hosted in three coal groups of the Longtan Formation, of which the coal seam No. 6 and 7 in group Ⅰ, the coal seam No. 16 and 17 in group Ⅱ, and the coal seam No. 20, 23, 27 and 30 in group Ⅲ are stably distributed. The seam depth is obviously controlled by syncline structure, reaching 1500m near the axis of Bide sub syncline. The coal seams are mainly high metamorphic lean coal, lean coal and anthracite, and the maximum vitrinite reflectance increases gradually from west to east and north to south. The methane adsorption capacity of coal is generally high(Langmuir volume, 18.32~39.32 m³/t), and is significantly affected by the coal metamorphism degree. Under the in-situ condition, the seams belong to typically low permeability reservoir, but with high gas content(average of 10~15.78 m³/t) and high gas saturation(generally >50%). Combined with the effect of multi-layer co-production in this area, it is found that the gas production effect of the gas wells with the inclined depth of more than 800m in Bide is poor, and the geological conditions of coal gas development in Zhizang and Santangxi syncline are relatively good. Among them, four sets of main coal seams(seam spacing < 60 m) of Ⅲ coal group in Zhizang syncline are potential high-yield and high-quality reservoirs.