Distribution of methane and carbon dioxide concentrations in the near-surface zone over regional fault zones and their genetic characterization in the Pszczyna-Oświęcim area (SE part of the Upper Silesian Coal Basin, Poland)

Abstract

The objective of this paper was to characterize the variability of methane (CH4) and carbon dioxide (CO2) concentrations in the near-surface zone, measured along 4 selected sampling lines positioned perpendicularly to regional fault zones, in the SE part of the Upper Silesian Coal Basin (USCB), Poland. This way it was possible to check the migration activity of these gases to the surface through low-permeable Miocene strata. It was also possible to evaluate the applicability of surface geochemical survey methodology to the identification of potential accumulations of free natural gas in the topmost parts of the Carboniferous formations in the USCB. The results of 319 soil-gas samples were supported by stable isotope analyses and related to the geological setting and the distribution of methane potential at various depths. Moreover, the sampling included the area of the closed and remediated Andrzej VII mining shaft, where 5 soil-gas samples were taken. Additionally, gas flux was evaluated at 3 measurement points at which increased gas concentrations were detected. The maximum concentrations of CH4 and CO2 in soil gases were 15.4 and 9.39 vol%, respectively. Soil gases contain both thermogenic and microbial CH4 and CO2 that migrated from secondary and primary gas zones within the Pennsylvanian coal-bearing strata. In some sites, CO2 was also generated by the oxidation of soil organic matter and/or CH4 in a near-surface zone. Anomalous CH4 concentrations were generally found over elevated fragments of erosional structures of Carboniferous coal-bearing formations. This indicates the presence of free CH4 accumulations beneath the Miocene overburden. However, the surface anomalies did not always correlate with the zones of CH4 secondary accumulations. Anomalous concentrations of CH4 and CO2 correlated with the regional fault zones that cut through the Carboniferous formations. This indicates the potential presence of fault traps saturated with free CH4 in the upper parts of Carboniferous formations. Relatively low values were observed for both CH4 and CO2 emissions (maximum: 25 and 4320 mg m−2 d−1, respectively). This suggests relatively low dynamics of gas migration from the depth to the soil zone.