Abstract
The Upper Cretaceous stratigraphic sequences are widespread over the whole of Sirt Basin and consist of marine shale and limestone, ranging in age from the Turonian up to the Maastrichtian. The sequences, in the study area (Zaltan platform), are divided into five clearly recognizable formations and their distribution is related mainly to the paleogeography of the pre-existing topography. In ascending order they are; Pre-upper Cretaceous sandstone, evaporitic Turonian Etel Formation (shallow water restricted platform), Coniacian/Santonian Rachmat Formation (shallow open marine, slightly protected), Campanian Sirte Shale (shallow restricted platform) and Maastrichtian Waha Limestone (shallow carbonate platform). Upper Cretaceous sea transgressed towards the south either over the basement and granite wash or over Pre-upper Cretaceous sandstone. It encroached gradually on the Al Wahah and Ad Deffah areas at the close of Santonian and the beginning of Campanian time. During Maastrichtian time, sea continued to transgress southwards and covered most of paleohighs in the Zaltan platform. Two separated islands were left uncovered, forming the paleohigh (Al Wahah and Ad Deffah field area). The carbonate belts around them grew narrower as the sea got too deep for the carbonate deposition. Carbonate facies, known as Waha reservoir facies, were developed on the highs and changed laterally basinward into shaly sequence deposits of the Sirte Shale.