K. Dyjaczynski*, M. Górski**, S. Mamczur* and T. M. Peryt+
*PGNiG S.A., O/Zielonogórski Zaklad Górnictwa Nafty i Gazu, ul. Bohaterów Westerplatte 15, 65-034 Zielona Góra, Poland.
**Geofizyka Torun, ul. Chrobrego 50, 87-100 Torun, Poland.
+ Panstwowy Instytut Geologiczny, Rakowiecka 4, 00-975 Warszawa, Poland.
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Zechstein (Upper Permian) reefs have long been known to occur along the margins 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 studies and integrated geophysical and geological interpretations have led to a number of major gas discoveries. The reef complexes in the KoscianWielichowo 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 palaeogeographic analysis and from the seismo-acoustic properties of the Zechstein Limestones of which they are composed. 3D seismic profiles show that the reefs have irregular shapes in map view. Drilling results indicate that their thickness 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 five lithological units which are capped by the Werra Anhydrite. Four of these units reflect deposition in subaqueous environments whereas the uppermost unit originated in a very shallow-water or temporarily subaerial setting. Typical "ecological" bryozoan-stromatolitic reefs occur in the middle and upper (but not the uppermost) parts of the Zechstein Limestone, while the lower parts are dominated by bioclastic packstones and grainstones. Average reservoir porosity is 1020%. The permeability varies significantly (0-400 mD) but values of few tens 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 tectonic origin. Blocks of contemporaneously lithified Zechstein Limestone as well as steeply-dipping laminar sheets are common in the reefs of the Wolsztyn Ridge and have also previously been recorded in the English Zechstein. Palaeoslopes elsewhere in the Wolsztyn Ridge and possibly also in other parts of the Variscan externides may host similar reefal build-ups which could be of exploration potential.