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UPPER CRETACEOUS AND LOWER EOCENE HYDROCARBON PROSPECTS IN GROMBALIA AREA. LATERAL EXTENSION AND RESERVOIR PROPERTIES

MELKI Seddik, HAFSIA-MARDASSI Besma, NEGRA Mohamed Hédi, 2004

Tunisian Petroleum Exploration and production Conference, 1, 25, 10, Novembre 2004

Résumé

In Tunisia, the Grombalia area constitutes an interesting site providing hydrocarbon prospects. Oil and gas were discovered in several fields such as in Zinnia, El Menzah and Belli (Moody et al., 1990; Ben Brahim, 1993; Touati et al.,1994; Loocks et al., 1996; Fourati et al, 1998). The lateral extension of Upper Cretaceous and Lower Eocene prospects and reservoir properties could be in relation with sedimentary characters and tectonic activity during this period. Outcrop studies have been carried out to focus on sedimentary characters of Abiod and Bou Dabbous carbonates (Burollet, 1956; Fakhfakh-Ben Jemia, 1996), especially, the linkage between diagenesis, fracturing and reservoir properties (Negra et al., 1991). Concerning the structural setting, the Grombalia area is dominated by a distensive tectonic system mainly expressing horsts, half grabens and grabens and a faulting system globally oriented North-South to North West to South East (Turki, 1988 ; Ouahchi et al., 1993).Tectonic activities could be responsible, at least partly, of the irregular paleotopography which is obvious since the Lower Cretaceous. In fact, in Jebel El Ghorfa, Campanian Abiod limestones are unconformably overlying Aptian shales. The irregular paleotopography is mainly expressed by the existence of highs surrounded by relatively depressed areas. The most spectacular high occupies the southern part of Jebel El Ghorfa in which thickness of both Abiod and Bou Dabbous Formations are very reduced (about 4m, each). In this area,

Abiod and Bou Dabbous Formations are constituted of massively bedded micritic limestones which consist of packstones (Dunham, 1961) rich in planktonic foraminifera (Negra et al.,2000; Mardassi, 1998; Mardassi-Hafsia, 2004). In addition, the Abiod Formation top exhibits a bored hard ground suggesting an early lithification of Abiod micritic limestones, probably in relation with an emersion of this sector during Upper Cretaceous (Melki, 1999). From Jebel El Ghorfa South (constituting a high), to North, South West and South East, lateral changes in thickness and facies are variable. lt means that changes which are progressive to North are, in contrast, rapid, sometimes abrupt to South West and South East. The progressive or abrupt changes appear closely related to a variable inclination degree of slopes. To the North, progressive changes of thickness and facies rather deal with a relatively gentle slope. In fact, massively bedded micritic limestones progressively change to thicker well bedded banks admitting frequent intercalations of argillaceous limestones and marls.

In contrast, to South West and South East, the Abiod Formation is obviously thicker (90m).

It fossilises slump marks which rather deal with steeper slopes.