Accumulation mechanisms and classification of CBM reservoir types: A case study from the eastern margin of the Ordos Basin

With the continuous deepening of coalbed methane (CBM) exploration and development in China, it is becoming more and more important to define and summarize CBM reservoirs with diverse occurrence states and accordingly to guide the optimization of favorable CBM exploration targets. In this paper, domestic and foreign CBM enrichment and accumulation theories were systematically analyzed. Then, based on geological laws of CBM exploration in the easter n margin of the Ordos Basin, the concept of CBM traps and the classification scheme of CMB reservoirs were put forward in reference to the classification method of conventional oil and gas reservoirs based upon the genesis of traps. And the research results are as follows. First, the gas sealing pattern of "mudstone-surrounding reservoir, sandstone-connecting coal and water-enclosing gas" is depicted based on the vertical combination relationship of coal beds, sandstones, mudstones and limestones. The pattern of "mudstone-surrounding reservoir" refers to the hydrocarbon accumulation with tight rock beds (e.g. mudstones) to seal coal reservoirs. In the pattern of "sandstone-connecting coal", a gas reservoir is formed jointly by the coalbeds and its overlying sandstones when they are connected. In the pattern of "water-enclosing gas", gas is enriched and accumulated in coal beds under the effect of hydraulic sealing and plugging. Second, the effects of physical sealing, gas pressure sealing, water pressure sealing and adsorption sealing during the formation of CBM reservoirs are illustrated. And it is pointed out that the upper and lower limits of gas reservoir development are the efflorescent oxygenized zone and the in-situ stress transition zone, respectively. And third, the gas reservoir classification scheme with structural, lithologic, hydrodynamic and compound traps as the leading factors is developed, and various CBM reservoirs are dissected, including anticline, fault, syncline, stratigraphy-lithology, hydraulic sealing and hydraulic assistance CBM gas reservoirs. © 2017, Natural Gas Industry Journal Agency. All right reserved.