The objective of this paper is to evaluate the possible migration of coal-bed methane and carbon dioxide to the earth's surface in a selected areas from the southern part of the Upper Silesian Coal Basin (USCB) and resulted hazards produced by near-surface gas migration, referred to the geological setting and the origin of these gases. The surface geochemical survey was run over the areas of both the operating and the abandoned bituminous coal mines, including the remediated shafts and the regional tectonic dislocations: the Bzie-Czechowice and the Jawiszowice fault zones. The survey included a total of 453 soil gas samples collected at a depth of about 1.2 m and along 7 horizontal profiles as well as in the vicinities of remediated shafts. All collected gas samples were chromatographically analysed for methane and carbon dioxide concentrations. Selected samples showing increased concentrations of methane and/or carbon dioxide were analysed also for stable carbon isotopes of these gases. Additionally, gas emission was measured at 5 sites selected due to the increased concentrations of the gases. The maximum detected concentrations of methane and carbon dioxide in soil gas samples were 52.8 and 17.4 vol%, respectively. The closure of high methane-emitting bituminous coal mines in the USCB, including the termination of mine ventilation, caused the increase of average methane concentrations in the near-surface zone. Vast anomalies of both gases can be the effects of migration from secondary accumulations of "free" methane present in the elevated parts of Carboniferous formation capped by Miocene claystones and mudstones. Methane originated from primary accumulations located beneath the degassing zone and migrated along fault zones. However, carbon dioxide migrated from both the primary and the secondary accumulations, and, sometimes, it originated also from secondary oxidation of methane. The methane recovery captured with degassing wells run in the area of the abandoned "Morcinek" Mine disturbed the naturaly, vertically migrating methane towards the earth's surface and caused the local decrease of average methane concentrations in soil gas collected along the DD' sampling line. It was additionally confirmed by relatively high methane concentrations (over 50 vol%) detected in samples collected from the wells degassing the shafts I, II and III of the closed "Morcinek" Mine. The methane is of microbial origin and migrated from secondary accumulation located within the Carboniferous (Pennsylvanian) coal-bearing strata. In the regional tectonic zone of the Bzie-Czechowice Fault, over 4 times higher average carbon dioxide concentrations were detected, in comparison with those analysed in the Jawiszowice Fault Zone. Carbon dioxide was of microbial origin, migrating from a secondary accumulation. Relatively low values of both the methane and carbon dioxide emissions were noticed at sampling sites, where elevated concentrations of both gases were encountered previously in the near-surface zone. This suggests that gases may accumulate in the near-surface zone as a result of long-term, and relatively slow migration from the depth. Hence, and in this case the potential gas hazard caused by uncontrolled and fast inflow of methane and carbon dioxide to a basements of buildings is insignificant.
