Geology, spatial distribution of methane content and origin of coalbed gases in Upper Carboniferous (Upper Mississippian and Pennsylvanian) strata in the south-eastern part of the Upper Silesian Coal Basin, Poland

The subject of the investigation was both vertical and horizontal distribution of methane content in coal series within the south-eastern part of the Upper Silesian Coal Basin. In the study area coal deposits are characterized by various methane structures and varied spatial distribution of methane. “Brzeszcze” coal deposit located in the upthrow of Jawiszowice fault zone represents the so-called “open” methane pattern, in which the upper parts of coal-bearing series, to the depth of 300–500 m (seam 327), are naturally degassed. Beneath, methane content rapidly increases with depth, and often achieves high values (between ten and twenty m3CH4/Mg coal daf). The area located to the south of Jawiszowice fault zone represents the so-called “closed” methane pattern, which means, that the primary methane zone was reworked due to the forming of Carpathian Foredeep in the Miocene period. A thick cover of Miocene claystones overlying the Carboniferous formations favors gas accumulation in the uppermost parts of the Carboniferous coal-bearing complex, especially in the roof elevations. Free methane occurs within the elevated part of porous and permeable Łaziska sandstones covered by Miocene claystones. The highest methane content characterizes the coal seams, which lie within Mudstone or Upper Silesian Sandstone Series. Smaller methane content is typical of the seams lying within Łaziska sandstones. The fault tectonics is another factor influencing the distribution of methane content, therefore, the large-size dislocations i.e. Jawiszowice and Bzie–Czechowice fault zones move the high-methane coal seams according to throw direction – southwards. The Carboniferous lithology, fault tectonics and also Miocene claystones covering the Carboniferous formations are the crucial factors influencing the various methane contents in the area discussed. Methane – the main component of gases accumulated in coal deposits and the minor components such as ethane, propane, butanes and carbon dioxide are of thermogenic origin that resulted from Variscan coalification, whereas the methane of the secondary accumulation in the uppermost part of Carboniferous formations beneath the impermeable Miocene cover contains a large microbial component, which is a result of Paleocene infiltration of meteoric waters with nutrients for methane bacteria into the coal series.

► Methane content in coal decreases within porous Łaziska sandstones.
► Fault tectonics and paleorelief of Carboniferous roof control the methane content.
► The patterns — primary and secondary of methane distribution have been found.
► Methane in a primary (deep) accumulation zone is thermogenic in origin.
► Methane of a secondary (shallow) accumulation can be a microbial type.