Reefs in the basinal facies of the Zechstein Limestone (Upper Permian) of Western Poland: A new gas play

Zechstein (Upper Permian) reefs have long been known to occur along the mar gins of the Polish Zechstein Basin where they host important gas reserves. During the last decade, reefal build-ups have also been found in the centre of the basin on pre-Zechstein highs such as the Wolsztyn Ridge in Western Poland. Here, early 2D seismic and subsequent more detailed 3D seismic stud ies and integrated geophysical and geological interpretations have led to a number of major gas discoveries. The reef complexes in the Koscian-Wielich owo region are thought to have recoverable gas reserves of more than 32B cu m. The basin-Centre reefs have been identified on the basis of palaeogeographi c analysis and from the seismo-acoustic properties of the Zechstein Limesto ne of which they are composed. 3D seismic profiles show that the reefs have irregular shapes in map view Drilling results indicate that their thicknes s may exceed 80m, but characteristically their thickness decreases sharply at their margins corresponding to a facies change. In the Wolsztyn Ridge area, the Zechstein Limestone can be divided into fiv e lithological units which are capped by, the Werra Anhydrite. Four of thes e units reflect deposition in subaqueous environments whereas the uppermost unit originated in a very shallow-water or temporarily, subaerial setting. Topical "ecological" bryozoan-stromatolitic reefs occur in the middle and zipper (but not the uppermost) parts of the Zechstein Limestone, while the lower parts are dominated by bioclastic packstones and grainstones. Average reservoir porosity is 10-20%. The permeability, varies significantly (0-40 0 mD) but values of a few tens of mD are common. The occurrence of intraclastic breccia in many boreholes suggests that the reefs may have developed on local palaeoslopes which are generally of tecto nic origin. Blocks of contemporaneously lithified Zechstein Limestone as we ll as steeply-dipping laminar sheets are common in the reefs of the Wolstyn Ridge and have also previously been recorded in the English Zechstein. Pal aeoslopes elsewhere in the Wolsztyn vn Ridge and possibly also in other par ts of the Variscan externides may host similar reefal build-ups which could be of exploration potential.