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 3 0 seismic studies and integrated geophysical andgeological interpretations have led to a number of major gas discoveries. The reef complexes in the Koscian-Wielichowo region are thought to have recoverable gas reserves of more than 32B cu m.The basin-centre reefs have been identified on the basis ofpalaeogeographic analysis and from the seismo-acoustic properties of the Zechstein Limestone of which they are composed. 3 0 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 lithologicat units which are capped by the Werra Anhydrite. Four of these units rejlect 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 10-2096. The permeability varies significantly (0-400 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 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 otherparts of the Variscan externides may host similar reefal build-ups which could be of exploration potential.
The Middle Eocene Rus Formation in Qatar and adjacent area consists ofevaporite-carbonate sequences. The Rus Formation extends over a large area in the Middle East and can be correlated with confidence throughout the eastern Arabian Peninsula. In this study, the RusFormationinQatarisredescribedandsubdividedintotwonewmembersfrombottomto top:thegypsiferous dolomiticlimestone TrainaMember and the chalky limestoneAI-KhorMember. These units were depositedin two different depositionalsettingsranging from supratidalto subtidal environments. Sevenlithofaciesarerecognizedfromstudiedsurfaceand
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