Abstract: Petroleum Geology of East Central Tunisia

By

William F. Bishop

The tectonics of Tunisia are extremely complex and include folds, all types of faults, salt diapirs, and the major Saharan flexure which separates a stable Paleozoic province on the south from a subsident zone of Mesozoic and Cenozoic strata represented by the offshore Ashtart and Tripolitania basins. The remainder of the offshore is mostly stable shelf of the Pelagian craton, and the study area is situated on this platform, the onshore part of which is known as the Sahel. This region is poorly understood because of the Neogene-Recent sedimentary cover, but subsurface data indicate it to be structurally complex.

To date, two carbonate zones are proven hydrocarbon reservoirs in the study area: the earliest Eocene (Ypresian) Metlaoui and the Zebbag of Late Cretaceous (Turonian) age. With the exception of a few sandstones in the Oligocene and Miocene, the section above the Metlaoui is limestone and shale. The section below the Zebbag is dolomite, limestone and shale down to the oldest penetration, the probable top of the Jurassic. In addition to complex faulting, the section of interest is complicated by unconformities at the base of the Santonian and at the top of the Cretaceous.

Regionally, the Zebbag is thin or absent on the high side of the Saharan flexure and crops out west of the folds and faults of the N-S axis, which separates the Tunisian Atlas from the Sahel. There is apparent regional truncation on the northeast and local absence, possibly resulting from salt intrusion, on the south. On the north and east are a basinal facies of shale and micrite with planktonic forams and a slope or transitional facies of micrite and wackestone, commonly argillaceous with occasional attributes of shallow water, such as dolomitization, bioclasts, rare oolites, etc. On the shelf are bioclastic wackestones and packstones which tested oil in two wells. Oolite grainstones above the main porosity were productive in one well and tested oil in another. The bioclastic facies appears to be fairly restricted and may not be a continuous shelf-edge deposit. A lagoonal evaporite facies appears to be present on the west, and thin dolomites and anhydrites are common in the cored well in the study area.

Regionally, the well defined facies belts of the Metlaoui trend NW-SE. On the northeast is an open marine, basinal facies of micrite and marl with abundant planktonic forams (Bou Dabbous). Nummulithoclastic packstone crops out west of the N-S axis (as do all other facies) and is thickly developed in the study area; it may fringe the entire nummulite facies on the seaward side. Thick bars of nummulite wackestone/ packstone/grainstone, deposited in shallow water, trend northeastward at an angle to the paleoshelf. This lithology has primary porosity, tested oil in two wells, and is a commercial reservoir at the small Sidi El Itayem field and the very large Ashtart field. A bioclastic wackestone with nummulite debris is well developed in some wells and may be present elsewhere. Lagoonal gastropod coquina is present in one well and common in outcrops. Lagoonal/supratidal mudstone/wackestone and dolomite are widespread between shelf deposits and the thick gypsum and anhydrite which crop out in intermontane basins and probably are present in the subsurface.

Analyses of a considerable number of surface and subsurface samples have identified to date two source rocks, Bahloul (basal Turonian) and Bou Dabbous (Ypresian). The oil in the Metlaoui of one well correlates very well with the

End_Page 4---------------

Bahloul using fluorescence scan data and is a good match with 13c isotopes. The oils of Ashtart and Sidi El Itayem fields are indicated by geological considerations and published data to have been generated from the Bou Dabbous. The Metlaoui oil in another well appears to be a less mature version of these two oils.

Oils from Turonian reservoirs clearly are of the same family as indicated by fluorescence scan , 13c isotopes, pristane/phytane ratio, carbon preference index, and API gravity. Their origin is uncertain, but an intra-Turonian source is suspected because of close correlation among the oils and their occurrence only in reservoirs of that age.

End_of_Record - Last_Page 5---------------

Copyright © 2005 by Houston Geological Society. All rights reserved.