Mineralogic and Diagenetic Controls on Reservoir Quality of Paleozoic Sandstones, Gebel El-Zeit, North Eastern Desert, Egypt

Abstract

Studying the mineral composition and petrophysical properties of the Cambro–Ordovician sequence in Gebel El-Zeit indicates that it can be discriminated into four rock types, namely quartz arenite (RRT1 of the Cambrian Araba Formation, 35 m; RRT2 of the Ordovician Naqus Formation, 320 m), quartz wacke (RRT3) and mudstone (RRT4) of the Naqus Formation. Diagenesis played a major role in reducing or enhancing the studied microfacies. Cementation by silica, compaction and authigenic clay minerals are the main porosity-reducing factors, whereas dissolution and leaching out as well as fracturing are the most important porosity-enhancing factors. For the present study, reservoir quality was determined using reservoir quality index (RQI), flow zone indicator (FZI) and reservoir potentiality index (RPI) based on helium porosity and permeability values, in addition to the mercury injection capillary pressure (MICP) tests. The best reservoir quality was assigned to the quartz arenite microfacies, whereas the least quality was assigned to RRT4. Reservoir quality is mostly controlled by permeability, which in turn is controlled by the helium porosity and the pore radius of the studied samples (r35 of Winland and r10 of Schowalter). The Araba Formation at the bottom of the sequence is characterized by the best hydraulic flow units (best RQI, FZI, RPI and MICP, rank II). In addition, the middle and top parts of the Naqus Formation have good to very good reservoir quality (rank III).