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Article

THE BOU DABBOUS FORMATION IN THE GROMBALIA AREA OUTCROPS. SEDIMENTOLOGICAL CHARACTERS AND DIAGENESIS

NEGRA Mohamed Hédi, HAFSIA-MARDASSI Besma, 2000

Tunisian Petroleum Exploration and production Conference, 1, 7, 19, Avril 2000

Résumé

In the Jebel El Ghorfa outcrop (Grombalia area), the Bou Dabbous Formation (Fournié, 1978) is characterised by vertical and lateral facies changes. Vertically, the Bou Dabbous Formation globally consists of wackestones to packstones rich in planktonic microfauna. However, detailed petrographic analyses show that within the lower part, limestones are mainly rich in Globigerinids while limestones of the summital part are essentially rich in Radiolarians. In addition, lateral variations in facies and thickness are very rapid. From the Southern to Northern part of the Jebel El Ghorfa, only on a distance of 1 km, massively bedded and relatively grained limestones (1m thick) laterally change to more than 60m thick of well bedded and fractured limestones. These changes appear to be linked to the existence of a high occupying the Southern part of Jebel El Ghorfa, near Sidi Rhilane. Syntectonic distensive movements may, at least, facilitate sinking and thickening of facies toward the North. In addition, tectonic movements appear to be the main phenomena responsible of the Bou Dabbous limestones fracturing.

Diagenetic features also are laterally variable and appear to be initially related to sedimentation. On highs, certain diagenetic modifications such as early cementation and solution are relatively developed. On contrary, compaction features are rather obvious toward the North in a depressed area.

In terms of porosity and permeability development, despite the creation of secondary porosity by solution, fracturing constitutes, as shown in the Sidi el Kilani field (Ben Ferjani et al., 1990; Ben Brahim, 1993) within the Abiod Formation (Negra,1994 ; Negra et al., 1994 ; Negra, 1998) the main process enhancing the reservoir potential.

In Grombalia subsurface, the equivalent of the Jebel El Ghorfa outcrop may constitute an interesting prospect because of the availability of source rocks, especially in grabens, fractured reservoir rocks, marly to argillaceous cap rocks and horst-graben structures.