Optimum selection of favorable areas for multi-layer CBM mining and reservoir characterstics in North Yunnan and Guizhou exploration area

Due to the abundance of coal-bearing synclines, multiple seams and high gas content, the exploration area in North Yunnan and Guizhou Provinces is an important expansion block in the periphery of Junlian County. Based on a large number of coalfield holes and test wells, the geological conditions of CBM are analyzed, and the optimization and evaluation method of multi-layer favorable area is established, i.e. multi-level fuzzy mathematics plus key indicators. Firstly, the evaluation model is constructed based on hierarchical optimization, and the key indicators are determined as co-mining coefficient, coal structure and gas content. The co-mining coefficient is composed of the optimal co-mining span and the cumulative coal thickness, and the quantitative evaluation table of the key indicators is given. Then, the fuzzy mathematics calculation formula is used to obtain the reservoir score results, guiding the optimization of multi-layer co-mining favorable areas. The results show that the coal seam of Longtan Formation/Xuanwei Formation in Late Permian in the study area can reach up to more than 20 layers, and the recoverable coal seam is generally about 3 layers with the total thickness of more than 6 m. From southeast to northwest, coal seams pinch out and the number of layers gradually decreases. In most areas, the main coal seam is C5 (M11), with a thickness of more than 2 m and an average ash content of 27.73%, belonging to medium ash coal, and the coal rank is mainly lean coal–anthracite. The gas content of main coal seams in each syncline varies significantly, and the maximum gas content can reach 30.53 m³/t. The coal structure in the study area is dominated by primary structure coal and fractured structure coal. The co-mining coefficient is calculated by the upward or downward co-mining of seam C5 (M11), thus forming two co-mining layers. The multi-layer co-mining type I favorable area is mainly located in the central and western Kele syncline, the south of Niuchang-Yigu syncline, the south of Zhenxiong syncline, the southeast of Miaoba syncline, the central and western Luowang syncline, and the central and western Shikan syncline.