PRESENCE OF NATURAL GAS IN BASINAL FACIES OF THE ZECHSTEIN LIMESTONE IN A COPPER MINE IN SW POLAND

Zechstein (Upper Permian) dolomitic grainstones saturated with high pressure natural gas (including N-2, CH4 and CO2) are present in the roof of workings in the Rudna copper mine in SW Poland. Natural gas commonly occurs in Zechstein reefal carbonates but this is the first occurrence of gas in basinal facies of the Zechstein Limestone in Poland. The reservoir dolomites are thin (ca. 1m), with porosity of 2-15% and permeability of less than a few mD. Pore size and pore distribution was analysed using microscope observations, X-ray computed microtomography, mercury porosimetry and argon adsorption to investigate the dolomitic reservoir interval. The dolomites have variable reservoir properties, and stratigraphic traps with dimensions of several metres are present where high poro-perm dolo-grainstones are sealed by tighter dolomites. Geochemical data indicate that the methane and carbon dioxide present are derived from source rock intervals in the Kupferschiefer (Late Permian) and the Zechstein Limestone. The high content of nitrogen in the gas may be related to the thermal decomposition of Carboniferous organic matter. However, a biogenic origin for the nitrogen is also possible.