Vertical coalbed gas content distribution in the Walloon Subgroup of Daandine gas field and its geological controls

The Middle Jurassic Walloon Subgroup of Daandine gas field contains numerous coal seams with abundant coalbed methane (CBM) resources. However, the vertical gas content distribution of these coal seams is unclear currently, as well as its influencing factors, affecting the formulation of CBM exploration and exploitation strategy. For this purpose, using diverse geologic, experimental, and engineering data from recent exploration and development, this article ascertains the vertical coalbed gas content distribution in the Daandine gas field and discusses how the geological factors affect it. The results show that the coalbed gas contents have two basic trends as the depth increases: (1) increase, and then decrease; (2) increase. The former trend is dominant, and the inflection point occurs in the coal groups within or close to the Tangalooma Sandstone Formation. The correlation analysis shows that the macerals have little influence on the vertical coalbed gas content distribution. The moisture contents affect the adsorption capacity of coalbeds and show a negative correlation with the gas contents, and coal groups with higher moisture contents generally have lower gas contents. Three sets of independent gas-bearing systems exist in the Walloon Subgroup, and the coalbed gas content-depth relationship is relatively independent among different gas-bearing systems. The better the caprock condition, the higher the gas content, which is particularly evident in the coals of Condamine group. The permeabilities show a significant effect on the coalbed gas contents of Daandine coals. The coal groups within or close to the Tangalooma Sandstone Formation have higher permeabilities, as well as higher gas contents, and they are positively correlated. The reason is that the CBM in the study area is dominantly microbial in origin, and the higher permeability coals are conducive to the introduction of more microbial consortia into the coal seams, enhancing the generation of secondary biogenesis methane.