Methane contents and coal-rank variability in the Upper Silesian Coal Basin, Poland

The process of coalification results in changes in both the chemical composition and physical properties of the coal. One of the most important stages in the development of bituminous coal is the so called “second coalification jump” corresponding to medium-volatile coals characterized by a significant decrease of volatile matter from 33% to 20% and the release of methane, carbon dioxide and water. In the Upper Silesian Coal Basin (USCB), the highest values of methane content (> 12-14 m3/t coal daf) range from 35-22% (Vdaf) and vitrinite reflectance (Rr) values from 0.84-1.50% range-in approximate accord with values defining the second coalification jump (Vdaf-29%; Rr-1.3%), i.e., coking coals. In the basin, the distribution of the top of high methane zone (4.5 m3/t coal daf) is similar to that of the coking coals. That is why increasing methane contents towards the southern- and western parts of the basin can be explained by the increasing proportion of coking coals there. However, on a basin scale, correlations between present methane contents and individual parameters of coal rank are weak due, most likely, to late gas migration due, in turn, to factors unrelated to earlier coalification including, inter alia, tectonic disturbance and hydrodynamic processes.