D004 ...... NEW CONCEPTS ON MESOZOIC AND COENOZOIC EVOLUTION OF OIL BASINS; IN

PIERRE F. BUROLLET' and FATHIA MEJRI 2 CIFEG - GEA, 1 place Viei//e, 04300 Forcalquier, France 2 Enterprise Tunisienne d'Activités Pétrolières

An extensive seismic coverage, including 3-D surveys and new geological field works in the North- West, allow new interpfetations on basin fi ll and subsidence in Tunisia .

At the crossing between Western and Central Mediterranean and between the Saharan craton and the Alpine ranges, Tunisia gathers several types of basins (see fig .). • In the South, on the Saharan platform, Ghadames and Chotts areas are palaeozoic sag basins divided by the Caledonian and Hercynian Dahar arch. Hydrocarbons are produced from Palaeozoic and Triassic sandstones. • North-East of the Arch, a set of NW-SE and E-W faults formed steps along the border of the Late Carboniferous and Permian Palaeo-Tethys . Marine Permian shales may reach thickness over 3000 m. and reefs developped on the hinge line. • The South Gabes Gulf evolved also as a distensive margin during Triassic and Jurassic, mainly due to E-W faults. Thick Triassic evaporites form salt domes and walls in the Eastem part . Oil is produced from Jurassic and Mid-Cretaceous reservoirs (Ezzaouia field and El Biban and, farther West, Limaguess suspended discoveries) .

The Atlasic Tunisia is underlain by thick Triassic evaporites . As a consequence the Mesozoic and Coenozoic series are disconnected from the basement and form "couverture" folds . • The trough is a Jurassic and Cretaceous intracratonic basin with very thick restricted and neritic sediments, which are folded as "plis en échelons" by strike-slip movements along deep seated E-W faults . No hydrocarbon has been discovered there at the present time ; however some Jurassic and Triassic source beds have been described East of the area . • Between the basins, Island is a regional high devoid of Late Cretaceous and Coenozoic marine sediments .

A set of N-S trending deep basement faults, named N-S Axis, separate Western and Eastern Basins . Very often, along this axis the series are reduced with a lot of gaps and unconformities . • The North-western basins have been shaped by E-W basement faults in Triassic and Jurassic times (Early Tethys). An active subsidence caused large thicknesses and open marine facies of the Cretaceous series. In the Douleb basin Aptian carbonates received oil from Vraconian source beds . The Cenomanian Bahloul Horizon is also a rich source rock . • The Medjerda basin belongs also to the Tunisian trough . It has been strongly tectonized during the Miocene and acted as a front basin with the deposition of thick Miocene and Pliocene molasse . The Northern part is covered by nappes . The youngest nappe, named Numidian flysch, comes from a deep mediterranean furrow . A lot of oil seeps are known along the Triassic extrusions . • The basin, with the oriental plains on land and the Northern Gulf of Gabes off-shore, has been formed by Cretaceous distension followed by a general thermal subsidence in Coenozoic times . Folding is less sharp than in Western Tunisia . Due to thick Coenozoic covers and overburden the area is oil and gas rich, Turonian, Senonian and Lower Eocene carbonates being the main reservoirs .

Both in Eastem and Western Tunisia transversal grabens (NW-SE or E-W) correspond to various distension phases . North-West they may be epidennic features involving only the post-Triassic series . East of Sfax Basin; in the Pelagian Sea, they are basement grabens . North of the Sfax Basin, near for example, their deep significance is still unknown. • The Gulf of Hammamet and the Cap-Bon peninsula are a composite basin cut by many Neogene grabens. Tilted blocks form a mosaic where hydrocarbons have been found in Cretaceous and Miocene reservoirs. Deep synclines and grabens are convenient kitchens for oil and gas maturation.

EAGE 59th Conference and Technical Exhibition - Geneva, Switzerland, 26 - 30 May 1997 - Petroleum Division i