Tectonic Influence on Oil and Gaz Field Distribution in the Sahel Region ( Sfax Basin, Tunisia) and Related Petroleum Systems

Derbel Damak, Fayrouz¹, Ayedi Ben Gacha¹ (1) Entreprise Tunisienne D'Activités Pétrolières, Montplaisir Tunis, Tunisia

The Central Mediterranean Sfax basin is located East of the North South Axis; It lies on the Sahel region and the Pelagian Platform.

The hydocarbon potential assesment of the Sfax basin led to the following results:

Active synsedimentary tectonic and post carbonate reservoir deposits has the main control on oil and gas field distribution in Sfax basin.This latter was intensively subsident during Triassic, Jurassic and Neocomian times. At Middle Cretaceous, extension is synchronous with halokinesis and the early prospects are induced.During Upper Cretaceous, distention climaxed and NW-SE grabens are formed.

Nevertheless, Cretaceous (Albo-Cenomanian, Maestrichtian) and Tertiary (Intra Paleocene, Oligocene and Miocene)unconformities are recorded (in several wells)expressing compressive phases which may result in an angular unconformity.

Distension continues during Ypresian times. Relaxation phase succeeds and EW grabens are formed.

In the middle part of Sfax basin, the El Jem Fault is a major structural straight NW-SE oriented that separates two petroleum provinces:

The northern one comprises three Cretaceous plays confirmed by:

-The Sidi El Kilani oil field producing from fractured chalky Upper Cretaceous Abiod limestones. Its offshore equivalent is the Upper Cretaceous Miskar gas field. -the Cenomanian reefal Isis oil field also considered as related to an obvious extention in the offshore of the northern province. -and the good oil shows recorded in the Serj reefal limestones of MAH-1 and MAHE-1 offshore wells.

A younging of these plays seems to be noted from the NE towards the El Jem fault.This may be explained by a more important overburdening of the series towards the major fault.

The Southern province yields Eocene and Cretaceous plays confirmed by the Upper Lutetian, the Ypresian, the Coniacian and the Lower to Middle Turonian carbonates oil fields.