Multi-scale characterization of carbonate reservoir quality, diagenesis, and microfacies in the upper cretaceous khasib formation, South Iraq

Despite the significance of the Khasib Formation as a carbonate reservoir in the Halfaya oil field, comprehensive characterization integrating microfacies architecture, depositional environments, diagenetic processes, reservoir pore systems, and synergistic microfacies-electrofacies relationships remains absent in this area. This study presents an integrated reservoir characterization approach utilizing thin section petrography, scanning electron microscopy, X-ray diffraction, electrofacies analysis, Formation Micro Imager, and thorium/uranium ratio to evaluate the Khasib Formation reservoir properties. Six distinct microfacies (MF) were identified, representing deposition across inner, middle, and outer ramp settings. MF2 (allochems-calcispheres packstone) and MF5 (molluscan packstone) exhibit superior reservoir quality, deposited in high-energy shoal and flank shoal environments. These findings align with electrofacies analysis, where EF2 and EF5 demonstrate enhanced petrophysical characteristics. Diagenetic processes significantly influenced Khasib reservoir quality, with dissolution substantially improving porosity in MF2 and MF5, while calcite cementation, pyritization, stylolitization, and dolomite cementation negatively impacted reservoir performance. Vuggy and moldic pores constitute the primary contributors to enhanced porosity in these optimal reservoir intervals. The integrated FMI-SEM approach proved effective in characterizing vuggy pore systems, and offers significant applications for carbonate reservoir development through improved reservoir quality prediction and enhanced flow unit identification. This multi-scale characterization provides a better understanding of Khasib reservoir in the studied field and the adjacent areas, leading to a better reservoir development plans and optimized production strategies.