Petrography-Driven Zonation of Cretaceous Carbonates in the East Mediterranean: Enhancing Reservoir Models in Complex Carbonate Settings

Abstract

Characterizing carbonate reservoirs is notoriously difficult due to lateral heterogeneity, facies complexity, and diagenetic overprints. In the East Mediterranean, offshore Egypt, the Cretaceous carbonate reservoir has been traditionally divided into four levels (L1-L4) using seismic and well-log data. However, correlation inconsistencies between wells and the limited resolution of geophysical tools hinder detailed internal reservoir understanding. This study aims to overcome these limitations by applying high-resolution petrographical analysis across core, sidewall core, and cutting samples to refine reservoir zonation and facies classification. The work supports more accurate 3D reservoir models for exploration and development in analogous carbonate systems in the MENA region. A detailed petrographical workflow was applied to more than 70 thin sections from Well-A, a reference well with a thick and representative Cretaceous section. Samples included both vertical and horizontal plugs and ditch cuttings. Facies classification was performed using texture, grain type, and diagenetic overprints. These petrographic insights were integrated with core descriptions and wireline logs to recalibrate boundaries between reservoir levels. Particular focus was given to porosity-texture relationships, diagenetic features (e.g., dolomitization, brecciation), and pore type variation (vugs, molds, fenestrae). The refined facies scheme was correlated across other wells in the field and applied to improve the consistency of well-to-well reservoir correlations and unit definitions. The petrography revealed that the four reservoir levels represent distinct depositional environments, Level 1 contains pelagic and marly sediments; Level 2 is dominated by lagoonal facies; Level 3 is characterized by patch reefs and Level 4 by tidal flats. These facies assemblages were confirmed by the identification of boundstone, packstone, and wackestone textures. Porosity recalibration based on thin section texture led to boundary shifts of up to 10 meters between levels, particularly between L2-L3 and L3-L4. The refined zonation framework significantly improved correlation between wells, enhancing the geological input to static models. This methodology bridges the gap between seismic-scale resolution and sub-meter core-scale observations in complex carbonate platforms. This work emphasizes the underutilized potential of thin section petrography in carbonate reservoir segmentation and facies interpretation. While often overlooked in favor of geophysical tools, petrography offers critical ground truthing to improve seismic and log interpretations, especially in diagenetically altered systems. The workflow is scalable, low-cost, and applicable across other fractured or karstified carbonate reservoirs in the MENA region. It provides valuable input for facies modeling, flow unit definition, and development planning in geologically complex reservoirs beyond sturdy area.