Integrated Formation Evaluation for Berriasian-Valanginian Reservoir in South Iraq: Integration of Core and Advanced Log Measurements

Abstract The Lower Cretaceous Formation "A" is a prolific reservoir in several fields across Southern Iraq. It is divided into four primary units from top to base: A1, A2, A3, and A4. The formation "A" has an average total thickness exceeding 400 meters. Based on the global eustatic sea level curve, it is believed to have been deposited during a highstand to a slightly regressive cycle. The original texture has been significantly altered by diagenetic processes, including cementation, dolomitization, and dissolution. This study synthesizes core analysis results, petrophysical logs, and borehole images from seven wells to refine lithofacies characterization and provide a comprehensive assessment of field-scale reservoir heterogeneity. Two key challenges were encountered: A) adapting the Dunham Classification to Borehole Images due to the lack of direct textural observations, and B) the absence of clear sedimentary structures for paleocurrent information. These challenges were exacerbated by extensive diagenesis, which significantly altered its original characteristics. To address the first challenge, several correlations were employed to guide the Dunham classification. These included utilizing high-resolution synthetic resistivity from borehole images and using porosity logs as a weak proxy for rock texture. Additionally, relic sedimentary structures observed on borehole images were used to infer the energy levels of depositional facies, thereby providing insights into the original rock fabric. For the second challenge, multiple indicators were applied, including lateral facies continuity, orientation of tidal channel lateral accretion, an increase in clay content as an indicator of proximity to terrestrial input, and formation variations in nearby fields as an indicator of regional facies distribution in southern Iraq. The Formation"A" in this field was interpreted to have been deposited in an outer to mid-ramp in the lower part, transitioning upward into an inner ramp. Lateral continuity is notably strong in the lower part but weakens upward in the inner ramp units. The highest clay content within the AL zone was interpreted as the most proximal facies. The tidal signature becomes evident in the uppermost unit. The depositional strike is inferred to align with the regional NW-SE depositional trend. High-quality reservoir shoal units were found to be correlatable between wells, suggesting that the facies extend several kilometres along the depositional strike. To further illustrate facies distribution, a three-dimensional conceptual model was constructed, incorporating well observations and providing insights into facies continuity both along and perpendicular to depositional strike. The study integrated multiscale datasets, ranging from microscopic observations to tens of meters in scale, with borehole images serving as the critical link. This comprehensive approach makes the best use of all available datasets andminimizes uncertainties. As a result, a robust facies model was constructed, providing a reliable framework to guide field development planning and reservoir modeling.