Unraveling factors controlling petrophysical properties and reservoir quality in a multi-facies carbonate system: case of the Cenomanian–Turonian reservoir in Southern Tunisia (Dahar Cliff) carbonates and evaporites

The present study is dealing with the characterization of 67 m thick Cretaceous carbonate sequences. The study relied on petrophysical laboratory core analysis, sedimentological and fracture investigations. Four facies were recognized based on field and petrographic analysis i.e., stromatolitic limestone, fossiliferous limestone, rudist-rich dolomite, and dolomite with stromatolitic intercalations. Throughout the studied deposits, the measured data showed relatively high porosities. The stromatolitic limestone revealed the most important porosity reaching 30%. The bioclastic limestone, the rudist-rich dolomite, and the dolomite with stromatolitic intercalations showed an average porosity of 8.9%, 11.5% and 10.4%, respectively. The diagenetic processes affecting petrofabric and pore types are the main factors controlling the evolution of porosity, fluid flow and acoustic properties of the studied deposits. Several diagenetic features were identified and seem to enhance or reduce the pore space. The dissolution and micro-fracture processes are responsible for enhancing porosity, whereas, compaction, neomorphism and cementation processes contribute on its reduction. Dolomitization has a mixed-impact by improving and decreasing reservoir quality. Cemented rocks filled with calcite may have considerable influence on enhancing P-wave propagation; however, connected open micro-fractures and channel pores showed a significant decrease in velocities. The fracture analysis revealed a clear anisotropy of permeability being mostly represented by WNW-trending in the upper part of the Zebbag Formation and the lower part of the Bahloul equivalent, whereas NW–SE to NNW-trending predominate the remaining series. Consequently, the Upper Cenomanian–Turonian deposits represent a petroleum potential and hydrogeological interest where the fracture intensity is the main factor enhancing reservoir qualities.