Geological mechanisms of the accumulation of coalbed methane induced by hydrothermal fluids in the western Guizhou and eastern Yunnan regions

• The characteristics of hydrothermal fluid activities are clarified.
• Two stages of hydrothermal fluid activity occurred in the western Guizhou and eastern Yunnan regions.
• Propose three geological mechanisms by which hydrothermal fluids affected the accumulation of CBM.
• Clarify the process and influencing factors of CBM accumulation in this complex geological setting.

Based on the geological characteristics of hydrothermal fluid in the western Guizhou and eastern Yunnan regions, this paper discusses the geological mechanisms of hydrothermal fluid on the coalbed methane (CBM) system. From the perspective of hydrothermal fluid activity in the regional strata, we analyze the geological processes by which coalbed methane is enriched and accumulated in this region. Our results demonstrate that two stages of regional thermal fluid activity occurred in the western Guizhou and eastern Yunnan regions. In the earlier stage, calcium fluid acted as the carrier, but in the later stage, it was replaced by siliceous fluid with a small amount of calcium fluid. The former resulted from thermal cycling of fluid through the strata, and the latter was caused by injection of deeply sourced high temperature fluids. We propose three geological mechanisms by which hydrothermal fluid affected coalbed methane accumulation: stimulation for hydrocarbon generation, transformation for physical properties and fluid supercharging. The hydrothermal fluid impels coalbed methane to generate and migrate at high intensity; it also produces dense swarms of tubular pores that coincide with tectonic stress. As a result, the coal body is broken, and there are changes to the porosity and permeability of the coal. Pressure development in the regional gas reservoir depends on the intensity of the fluid effect and the differential leakage of fluid pressure in the coal reservoir. The relatively closed and stranded hydrothermal fluid environment of coal measure strata tends to lead to a high pressure coalbed methane system. The platform uplift in northwestern Guizhou is one such environment, in which the geological configuration of the high temperature fluid provided favorable conditions for the enrichment and accumulation of coalbed methane.