Petroleum Geology of East-Central Tunisia

TLDR: In this article, the authors focused on the offshore shelf of Tunisia, where two carbonate intervals contain proven hydrocarbon reservoirs: the Metlaoui Formation of earliest Eocene (Ypresian) age and the Zebbag Formation of Late Cretaceous (Turonian) age.

Abstract: The tectonic features of Tunisia are complex and include folds, all types of faults, evaporite diapirs, and the Saharan flexure, which separates a stable Paleozoic province on the south from a subsidence zone of Mesozoic and Cenozoic strata of the offshore Ashtart and Tripolitania basins. The remainder of the offshore region is mostly stable shelf of the Pelagian craton, which also extends onshore. The present study focused on this shelf, where two carbonate intervals contain proven hydrocarbon reservoirs: the Metlaoui Formation of earliest Eocene (Ypresian) age and the Zebbag Formation of Late Cretaceous (Turonian) age. Regionally, well-defined belts of Metlaoui carbonates trend northwest. On the northeast are open-marine deep-water micrites and marls with abundant planktonic foraminifers. Thick bars of nummulitid packstone/grainstone were deposited in shallow water at an angle to the paleoshelf. The reservoir is confined largely to the bars, and visible effective porosity is best developed in those areas among the foraminifers filled with sand-size nummulitid debris, where secondary solution enlargement has occurred. This lithology tested oil in two recent wildcat wells and is a commercial reservoir at Sidi El Itayem and Ashtart fields. Distribution of Zebbag carbonates is more complex. A northwest-trending platform was bounded on three sides by deep water, where shale and micrite with planktonic foraminifers were deposited. Predominately back-reef and lagoonal foraminifer/rudist wackestones and packstones occur in narrow belts, apparently controlled at least locally by block faulting. They tested oil in two recent discoveries. The most significant porosity is interparticle, generally enlarged by solution, in foraminifer packstones, but intraparticle voids in foraminifers and rudists commonly contribute to the porosity. Analyses of surface and subsurface samples identified the Bahloul (basal Turonian) and Bou Dabbous (Ypresian) formations as source rocks. Fluorescence spectra of several oils were compared with extracts from these samples and indicate the Bahloul to be the source of oils in recent Metlaoui and Zebbag discoveries.