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THE ABIOD FORMATION IN THE ENFIDA AREA: PARTICULAR SEDIMENTARY FEATURES AND RESERVOIR PROPERTIES IMPLICATIONS

MELKI Seddik, NEGRA Mohamed Hédi, 2008

Tunisian Petroleum Exploration and production Conference, 1, 27, 10, Octobre 2008

Résumé

In the Enfida area located in northern central Tunisia, the Abiod Formation which is

represented by an only one carbonatic member shows frequent variations in thickness and facies. From North to South, the four sections respectively surveyed in Jebel Bou Safra, Mdheker, Garsi and Fadhloun show that the whole thickness of Abiod Formation changes from 80m (in Jebel Bou Safra and Garsi) to 110m (in Jebel Mdheker and Fadhloun). In addition, the Abiod Formation locally includes several intercalations of resedimented limestones which are interbedded within the thinly bedded micritic chalky limestones (classic Abiod facies). Resedimented limestones are only identified within subsident areas such as the Jebel Mdheker and Jebel Fadhloun in which the Abiod Formation is relatively thick.

The resedimented facies are expressed by relative coarse-grained siliciclastic and bioclastic sediments showing graded bedding and corresponding to turbiditic and/or tempestitic sequences. They are identified within subsiding and thick series preferentially deposited in depressed areas. On contrary, on highs, Abiod Formation facies are clearly thinner and do not include resedimented sediments. Rapid variations in thickness and facies appear to be insured by NW-SE normal faults. Highs occupying the Jebel Bou Safra and Garsi areas correspond to horsts while depressed areas occupying Jebel Mdheker and Fadhloun could correspond to grabens and/or half-grabens.

The main source of resedimented sediments could correspond to the highs and horsts immediately neighbouring the "depressions". Quartz grains and/or bioclasts are transported from highs to depressions on slopes as suggested by the frequent identified slump marks. resedimentation processes occur during tectonic and/or seismic instabilities related to upper Cretaceous tectonic and/or volcanic events. In terms of reservoir properties, the coarse-grained resedimented facies constitute particular layers in which porosity and permeability are higher than those of the micritic chalky limestone. Relative porous and permeable resedimented bioclastic deposits could constitute conduits favouring fluids circulation within the Abiod fine-grained carbonates.