Seismic and well log post-Cretaceous reservoir correlations in the Sahel, east Tunisia
Résumé
The eastern part of Tunisia is made up of the Atlasic chain foreland, a domain connected to a continental crust which thins northeastward. It is limited to the west and northwest by the corridor of the NS axis faults and by the Zaghouan fault which separates it from the Atlasic domain and trends toward the Pelagian Sea eastwards. It is regarded as a stable zone. The signs of the main events which generated the Atlasic structures are hidden under a thick Plio-Quaternary cover. From surface observations, microtectonic measurements, and lithostratigraphic sections and logs, including those of oil well log data and seismic profiles onshore and offshore, one notes (i) an in-depth and lateral variation of the lithostratigraphic series and (ii) a considerably faulted area at subsurface characterized by a complex structural evolution and shears associated with basins. Such variations are controlled by tectonics and eustatism. The eastern platform is affected only by N45° oriented folds, often accompanied by reverse faults and associated with N90–110° dextral and N160–180° sinistral shears. It corresponds to a tectonic foreland domain representing a folded and sheared deformation zone. The tectonic deformations recognized by seismic data only affect narrow corridors, lengthened and oriented according to three major directions: N45°, N100–120°, and N160–180°. These zones, mobile at several geological periods and tectonically complex, delimit large sectors which have undergone minimal deformation or none at all. Well developed in the Sahel, the reservoirs are carbonated and fractured types such as the limestones of Abiod, Metlaoui, Souar and Chérahil, Aïn Grab, and the Oum Dhouil group. Structural traps for the accumulation of hydrocarbons were formed following these Atlasic compression and folding phases. The deposit of thick argillaceous series during the Eocene (Souar and Chérahil) and during the Miocene (Oum Dhouil) constitutes good continuous covers which sealed the folded and faulted reservoir structures. The seismic profiles show traps which are associated with folded and closure structures by faults and stratigraphic traps through facies changes. The structural inversions and the thrust tectonics in eastern Tunisia play a significant role in the structuring of the cover and also in the evolution of the oil system emplacement. The reservoirs are supplied by bed rocks of the Cretaceous (Bahloul Formation) and the Eocene (Metlaoui Formation).