Reservoir forming characteristics and favorable area evaluation of deep coalbed methane in Daning-Jixian Block

The No.8 coal with a buried depth of more than 2 000 m of Lower Permian Taiyuan Formation in Daning-Jixian block, eastern margin of Ordos Basin is the first deep CBM field with an output of 100 billion m³ in China, but the reservoir forming characteristics of deep CBM are still unclear. In this paper, geological information, test and production data are synthetically applied to study the characteristics of deep CBM reservoir formation and evaluation of favorable areas. The results show that the deep coal reservoir in the study area is extensively distributed, with large thickness and high degree of thermal evolution. Two-stage reservoir formation and paleothermal fluid intrusion provide premise for substantial hydrocarbon generation. In addition, fractures and micropores are widely developed in deep coal reservoir, where adsorbed gas and free gas coexistence. The roof and floor are mainly limestone and mudstone, with strong capping ability and free gas preservation conditions. The deep CBM possesses the characteristics of “extensive hydrocarbon generation, high gas content, high saturation, high pressure bound free gas and adsorption gas coexistence”. Originally, twelve geological-engineering double sweet spot identification index systems for deep CBM are established, and three types of geological-engineering sweet spots are identified. Among them, geological engineering class I sweet spot is located in the northwest of the study area, geological engineering class sweet spot is located in the middle of the study area, and geological class Ⅱ engineering class Ⅰ sweet spot is located in the northeast and south of the study area; The flowing well JS-01 implemented in the geological engineering class I sweet spot has a maximum daily gas production of 94 000-97 000 m³, showing a good production potential. The research results effectively guide the optimization of the pilot trial area of deep CBM and the exploration of the first deep CBM field of 100 billion cubic meters in China.