Exploration and development process of coalbed methane in eastern margin of Ordos Basin and its enlightenment

The buried depth of coal seam in the eastern margin of Ordos Basin changes greatly, and the characteristics of coalbed methane reservoir formation and reservoir reconstruction methods vary greatly with different buried depths. At present, coalbed methane exploration gradually extends from a shallow area of 1 000 m to a deeper area of 2 000 m. To study the theory and technology of efficient exploration and development of coalbed methane resources under different burial depths, this paper reviews the exploration and development practices in the eastern margin of Ordos Basin in the past 30 years. According to the changes of geological knowledge, technological development, exploration workload, exploration achievements and gas production,the process of coalbed methane exploration and development in the eastern margin of Ordos Basin is divided into three stages In the first stage of exploring shallow coalbed methane(CBM), under the guidance of the theory of “enrichment of tectonic highs in shallow coal-rich areas”, the gas field in Hancheng WL1 well area is discovered,with coal seam burial depth less than 800 m, large coal seam thickness and high gas content. In the second stage of. shallow-scale of deep coal bed methane exploration, Under the guidance of “hydrodynamic gas control-tectonic adjustment-gently sloping monocline” formation theory, we selected the sweet spots with “burial depth less than 1 500 m, good hydrodynamic closure conditions, large coal seam thickness, high gas content, gently sloping monocline and positive tectonics”, and discovered the coal-bed methane fields in Baode and Linfen, changing the concept of reservior transformation, and realized the effective production enhancement and transformation of Hancheng tectonic coal. In the breakthrough stage of deep CBM exploration, the reservoir formation model of “temperature -pressure controlled gas and high saturation” is put forward to guide the exploration breakthrough of CBM in depth of more than 2 000 m. On this basis, the occurrence, enrichment characteristics and target direction of coalbed methane in typical blocks are analyzed, and three enlightenments from coalbed methane exploration and development in the eastern margin of Ordos Basin are summarized: Preservation conditions are important factors for enrichment and accumulation of shallow coalbed methane in Baode block; Three types of indirect roof fracturing and perforating modes support the coal seam stimulation in Hancheng structure; The “geology-engineering” sweet spot evaluation boosts the exploration breakthrough of deep coalbed methane in Daning-Jixian Block.