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Sedimentary and Reservoir Structuring of Aptian Deposits in North- Central Tunisia

Abstract

A detailed facies analysis of the Serdj Formation has been carried out to give insight into the depositional environments with a view to work out the Aptian paleogeography of the Central North Tunisia. In onshore and offshore North-Central Tunisia, Aptian deposits represent dolomite and sandstone reservoir series that correspond to proven oil fields (Douleb, Semmama). The main problems of hydrocarbon exploration levels are their basin tectonic configuration and their sequence distribution. The Central Atlas of Tunisia is characterized by deep seated faults directed to Northeast-Southwest, Northwest-Southeast and North-South. These faults limit inherited tectonic blocks and show intruded Triassic salt domes. It is essentially characterized also by synsedimentary extensional tectonics for whom the main stress is oriented Northeast-Southwest. This fact allows the subsurface identification of subsiding zones and thus sequence deposit distribution. The basin was extensive, shallow, with a moderate deeper to the North. A facies analysis of the Aptian (Serdj Formation) has been carried out to give insight into the depositional environments with a view to work out the Aptian paleogeography of the North-Central Tunisia. Our surface and subsurface data provide information about the palaeoenvironmental evolution and sea-level change of the Tunisian shelf in Aptian. The unusually expanded deposits are composed of mudstone, wackestone and packstones which reflect mid- and outer-ramp facies deposits. Outcrop and subsurface cuttings descriptions have been carried out to give insight into the depositional environments deposits of shallow marine carbonate-platform drowning and indicates that the Aptian reservoir is characterized by bioclastic dolomite with silty marl intercalations. The Aptian reservoir is characterized by lateral extension with three different facies. The first zone which is identified by dolomite facies is the place where the petroleum system is proven. The second zone is the same as carbonate reef facies and sub-reef. The third zone is North-South axis with detritic deposit. These siliciclastic were accumulated especially in the lower part of the Aptian reservoir and confirmed by the palaeogeography of central Tunisia distinct by the emergence of Kairouan Island characterizing the paleogeographic evolution of North-Central Tunisia in the Cretaceous. The facies analysis of these zones make easy to appreciate the hydrocarbon reservoir and their lateral distribution around these domains.