Effect of Volcanics on the Reservoir Quality of Coniacian Rudist-Rich Carbonates in the Gulf of Gabes, Tunisia

Abstract

In the Gulf of Gabes, the petrophysical properties and reservoir potential of the Coniacian Douleb rudist-rich carbonates appear directly to be influenced by sediments composition, texture, and diagenetic fabrics. Volcanic material, especially breccia, interbedded within some layers of the reservoir, initially formed of fine-grained micritic to bioclastic limestones, appears to transform the initial sediment texture and clearly enhance their reservoir potential. Similar Cretaceous features were described in China (the Songliao and the Er'Lian Basins). On the other hand, the main diagenetic processes affecting the reservoir quality are usually dolomitization, compaction, and silicification. In fact, dolomitization is the main diagenetic process which improves the porous system, whereas silicification and cementation tend to decrease it. In terms of reservoir properties, the best values of helium porosity and nitrogen permeability (reaching 16.8% and 4.2 mD, respectively) were measured in bioclastic-rich limestones (carbonates rich in rudists, peloids, echinoderms, and oncoliths) and in some volcano-sedimentary facies intercalated in the micritic facies of the Douleb Member. These beds with volcanic facies show porosity values ranging from 4 to 21.15% and negligible to low permeability values (<0.01 to 1.4 mD).