Diagenetic overprint on porosity and permeability of a combined conventional-unconventional reservoir: Insights from the Eocene pelagic limestones, Gulf of Suez, Egypt

Lower to Middle Eocene organic-rich deep-water limestones of the ∼335-m-thick (1100 ft) Radwany (Thebes-equivalent) Formation represent a source rock and a potential reservoir unit at the October Oil Field in the Gulf of Suez. However, in spite of recent exploration advances, the pore system and diagenetic history of the formation are still poorly understood. This study aims to discriminate porosity types and their vertical distribution, assess the diagenetic processes controlling porosity evolution, and evaluate the unit's overall reservoir potential. To achieve these goals, we utilized an integrated petrographic, mineralogical, geochemical and petrophysical dataset from four offshore wells. At October Oil Field, the Radwany Formation is informally divided into three intervals (from the base up: C, B and A) which are composed of planktonic foraminiferal wackestone, wackestone-packstone and packstone microfacies. Pores include both fabric selective (interparticle, intraparticle and moldic) and non-fabric selective (fracture) types. Visible porosity reaches up to 11.6%, and the highest porosity values are associated with interparticle pores in foraminiferal packstone in interval C. The two overlying intervals have lower porosities but have TOC values of up to 6 wt %. The Nuclear Magnetic Resonance (NMR) analysis yields low amplitudes of short T2 components (max. 52 ms), which suggests the presence of small pores (intraparticle, moldic, fracture, and fissure). Based on the electrical quality index (EQI) rock classification, nine groups were identified, suggesting a high heterogeneity of the formation. The highest EQI values positively correlate with high porosity (visible and estimated) in the lower part of Interval C which is dominated by interparticle/intraparticle porosity, thus reflecting the low tortuosity values of this microfacies. Dominantly low EQI values in Intervals A and B indicate low porosity efficiency and higher tortuosity. Carbonate sediments of the Radwany Formation are interpreted to have undergone a complex series of diagenetic processes. These processes modified the primary pore system either by enhancing the reservoir properties (dissolution and fracturing), or by reducing or destroying porosity (cementation, mechanical and chemical compaction, and minor pyritization, chertification, and asphaltene precipitation). The study highlights the unconventional reservoir potential of the middle and upper parts of the Radwany Formation (intervals B and A) at October Oil Field, and the conventional reservoir potential of interval C. It emphasizes the complexity of diagenetic controls on the porosity evolution in deep-water limestones of the Gulf of Suez, and may provide valuable insight into similar deep-water carbonate systems in rift basin reservoirs elsewhere.