The occurrence of coalbed methane in the depocentre of the Upper Silesian Coal Basin in the light of the research from the Orzesze-1 deep exploratory well

The Orzesze-1 exploratory well with a depth of 3708 m (TVD) was drilled in 2019–2020 in the depocentre of the Upper Silesian Coal Basin (USCB). The methane content in the coal seams has been tested to a depth of 2840 m and the sorption capacity of the coal to a depth of 2576 m. These are the deepest measurements in the USCB so far. The vertical distribution of methane content in the borehole shows two depth zones of interest, the first at a depth 883 m to about 1300 m (maximum methane content about 12 m³/t coal^daf) and another in the range of 1500–2840 m, that is, to the maximum measurement depth, so the actual lower boundary depth of this zone is unknown. The maximum methane content here exceeds 18 m³/t coal^daf at a depth of >2800 m. Both zones are separated by an interval of reduced methane content of about 5 m³/t coal^daf at a depth of approximately 1400 m. The gas composition is dominated by methane (∼90%), and the content of carbon dioxide increases to approximately 15% at a depth of >2300 m. The methane-bearing zone at ∼900–1300 m corresponds to the zone of high- and medium-volatile bituminous coal (second coalification jump), while the highest methane content at a depth of >2800 m was determined in anthracite. The methane sorption capacity of the coal seams oscillates between 16 and 40 m³/t coal^daf with a maximum in anthracite at a depth of >2800 m, where the temperature of the rock approaches 100 °C and the deposit pressure exceeds 28 MPa. The highest sorption capacity in anthracite results from its inner structure characterised by the predominance of ordered aromatic lamellas and the dominance of vitrinite macerals (>70%), which contain coal micropores accumulating adsorbed methane. The comparison of the sorption capacity of the tested coal and the measured methane content displays undersaturation of 11–59%, however, due to significant gas content in the deep zone (depth > 1500 m), the drilling area can be considered as a prospect for further exploration and development of coalbed methane (CBM).