A tight-gas reservoir in the basinal facies of the Upper Permian Ca1 in the southwestern Zechstein Basin, Poland

Abstract

The Ca1 basinal deposits of the Werra cycle in the southwestern Zechstein Basin, Poland, are similar to those in other parts of the basin, although a high porosity (15 %) dolomite reservoir unit in the middle part near the Rudna Copper Mine (KGHM Polska Miedź S.A.) has a gas accumulation comprised mainly of nitrogen. The reservoir horizon is developed within dolomitic grainstone and occurs in the upper part of the second regressive cycle of the Zechstein Limestone as a result of redeposition of unconsolidated sediment from the carbonate platform margin to the basin floor. Early dissolution of the aragonite components of the grainstone and early dolomitization created moldic and minor intercrystalline porosity. The most important process that formed the porosity was deep burial release of carbon dioxide from the thermal decomposition of organic matter within over- and underlying deposits. The acidic porewater resulted in the dissolution of the rock fabric, which increased the porosity, and on the other hand caused the formation of a Mn-rich dolomite cement that precipitated in the intercrystalline space surrounding the newly formed pores. The created isolated pores were partially filled by euhedral dolomite crystals with Mn-rims. Later dissolution and cementation had a significant influence on the isotopic results. The δ18O values show a clear decrease in the grainstone section from −2 to −5 ‰. The δ13C values (from −3.84 to −5.40 ‰) are typical for the Polish Zechstein Basin; however, there is no significant increase in the δ13C values up-section as is the case for the coeval platform facies. This may indicate that the basinal facies was deposited in more uniform conditions than the platform facies.