Key geological factors and evaluation methods for huge low-rank coalbed methane reservoirs：Taking Bayanhua depression in Erlian basin as an example

In order to evaluate the potential of low rank CBM resources with large thickness, Bayanhua depression in Erlian basin is studied. By studying the genesis of CBM, buried depth of coal seam, thickness of coal seam, physical properties of coal reservoir, gas content, hydrogeology and cap rock, the key geological elements of thick and low coal rank coal reservoirs are summarized. Furthermore, a new low rank CBM resource potential evaluation method is established, and the favorable and target zones for CBM exploration and development in Bayanhua depression are delineated. The results show that the main coal-bearing strata in Bayanhua depression is the Lower Cretaceous Tenggeer Formation(K₁t), which is dominated by lignite and long-flame coal, and the maceral is mainly vitrinite. CBM is mainly biogenetic and mixed genetic CBM is auxiliary. A total of three coal groups are developed in the study area. The coal reservoir has large thickness, wide development area, moderate buried depth, good preservation conditions, high gas content, and gas content of air drying base is up to 4.45 m³/t. No.1 and No.2 coal groups are conducive to the formation of CBM reservoirs, and No.3 coal group is conducive to the formation of coal-bearing sandstone gas reservoirs. According to the established resource potential evaluation method, the optimal target area for CBM exploration and development is located in the northern part of Bayanhua depression.