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Petroleum Development Oman LLC, P.O. Box 81, Muscat, Sultanate of Oman
Thick, shallow-water gypsum banks, formed during Zechstein times at the site of regional palaeohighs along the margins of the Southern Permian Basin, acted as foundations for shallow-water carbonate platforms. The carbonate platform margins, in particular their northern to northeastern windward margins, were characterized by deposition of grainstones dominated by aragonitic ooids, pisoids, intraclasts and bioclasts, while muddy, stromatolitic carbonates and nodular anhydrites accumulated on tidal flats and in lagoons behind the platform margins.
Usually the platforms are surrounded by thick slope deposits consisting of carbonate mudstone and redeposited intervals, but locally the platforms deepened gradually and changed, without intervening slope deposits, into a shallow-marine facies devoid of ooids. The slope aprons grade basinwards into thin successions of finely laminated carbonate.
Porosity creation in Zechstein carbonates was due to meteoric-water leaching and to subsurface leaching by carbon dioxyde-enriched formation waters during burial. Porosity destruction was mainly caused by anhydrite pore plugging and subsurface calcitization. The best reservoir properties are found in the unrestricted platform facies and in parts of the slope facies. Fractures are important in Zechstein carbonates; their presence is a prerequisite for high production rates in gasfield wells.
