Research progress on accumulation mechanism and formation geological conditions of coalbed methane

The study of coalbed methane accumulation mechanism and formation geological conditions is of great significance to rationally evaluate coalbed metha ne resources and guide coalbed methane development. CBM accumulation is a dynamic geological evolution process of CBM generation,occurrence,migration and pres ervation in coal-bearing basins,and each stage is controlled by various geological factors. Based on the research achievements in related fields at home and abroad in「 ecent years,from the perspective of coalbed methane accumulation process and fluid characteristics,the CBM formation mechanism,CBM storage and migration mechan ism and the formation geological conditions of CBM reservoir were summarized in sequence. The results show that rich organic carbon is the material basis for the form ation of coalbed methane ,the formation of biogenic coalbed methane mainly depends on the characteristics of microbial communitys in the coalbed,and the formation of thermally derived coalbed methane depends on the degree of coal metamorphism. The mechanism of adsorption and desorption is to study the storage of coalbed meth ane. Based on the basic theories,solid-liquid adsorption/desorption theory can better explain the adsorption and desorption of water-bearing coalbed gas. The diffusion and seepage mechanisms are the microscopic theories of CBM migration. The diffusion model based on time change and non-Darcy seepage model are more suitable t 0 explain the CBM migration in low permeability reservoirs in China. The geological conditions for the CBM reservoir formation can be summarized as depositional cond itions,structural conditions,and hydrogeological conditions. The depositional conditions control coal composition,coal seam thickness,roof and floor characteristics and 0 ther influences to affect the accumulation of CBM reservoir. The influence of structural conditions on the formation of CBM reservoir can be reflected by the tectonic mo vement,the burial depth,magmatic activity,and structural type. The hydrogeological conditions have a dual effect on the formation of CBM reservoir. In short,the geologic al structure of coal-bearing basins in China is complex,and the formation of CBM reservoir is controlled by a variety of geological factors. The analysis of the main contr olling factors of CBM reservoir should be combined with the history of coalbed deposition and burial,thermal evolution,structural evolution,and hydrodynamic characteris