A study on the mechanism and geological model of CBM reservoir-forming dynamic system in Huainan Coalfield

The Permian coalbed methane(CBM) reservoir with mixed secondary biogenic and thermogenic gas in the Huainan Coalfield, Anhui Province, is affected by complex geological and hydrogeological factors. The purpose of this paper is to provide a basic understanding of CBM reservoir-forming dynamic system mechanism, which involved the thermal flux and geotemperature fields, palaeo- and modern tectonic stress fields, and hydrodynamic field. Although the CBM reservoir in the Huainan Coalfield is usually considered as a conceptual model of synformal or/and basin-centered gas accumulation, but this model emphases that, after Permian coals reaching their maturity, it had suffered uplift in large scale, unload and reburied. Due to lose of thermogenic adsorbed gas, it resulted in that the coal reservoirs were in the unsaturated status. When meteoric water seeps through coal beds, in them secondary biogenic methane and CO₂ are generated from wet gas and other organic matter by the metabolism of bacteria activity. In the process of groundwater centripetal drainage, the secondary biogenic gas carried in the front (or "washing" soluble gas) was pushed to the impermeable barrier or/and the structure hinge line, and was stayed and resorbed by coal matrix, or/and was full in coal cleats, which supplements the coal reservoirs and leads to the increase of gas content and saturation, and then, it was enriched and formed CBM reservoir. Therefore, the CBM accumulation in the Huainan Coalfield is mainly controlled by the evolution of coal reservoir-forming dynamic system, and the coupling relationship between the tectonic style and the energy fields.