Constructing Depositional Models of the Cretaceous Reservoirs in Southeastern Iraq: Integration of Borehole Images, Petrophysical Logs and Core Data

The Cretaceous period in southern Iraq was characterized by significant facies variations driven by a combination of significant climate swings, eustatic sea-level fluctuations, and basin configuration. These factors influenced the deposition of major carbonate and clastic reservoir units, including the formations A, B and C, which are key hydrocarbon-bearing intervals in the region. Facies architecture strongly governs initial reservoir quality, with high-energy shoals, channel sands, and rudist-rich carbonates forming the most prolific reservoir units. Fine-grained lagoonal and distal marine deposits contribute to reservoir heterogeneity and flow barriers. The identification of these depositional units and their orientation assist in understanding extension of reservoir flow units and flow barriers. This study aims to analyze the depositional environments and reservoir heterogeneity of the reservoir units in the a study area in southern Iraq. The research specifically focuses on the Valanginian formation A, Barremian formation B, and Cenomanian-Turonian formation C, characterized by their complex depositional and diagenetic histories. The study incorporates borehole imaging, core samples, stratigraphic correlations, and advanced petrophysical logs from eight wells, offering a comprehensive assessment of facies distribution, depositional cyclicity, and reservoir properties. The primary objectives of this study are to characterize facies distribution, identify depositional environments and cyclic patterns, and assess reservoir heterogeneity and overall quality. By examining these factors, the study provides valuable insights into the key controls on hydrocarbon accumulation in this study area and comparable geological settings. Formation A, which represents a regressive carbonate ramp system, exhibits a diverse range of lithofacies that vary in composition and reservoir quality. These lithofacies range from mudstone and wackestone in the lower, more laterally extent units, to more complex and porous packstone and grainstone facies in the upper units. The good reservoir quality is associated with the inner ramp grainstone and packstone facies, which exhibit high levels of porosity due to moldic and vuggy texture. Formation B is characterized by its deposition within a mixed clastic-carbonate ramp setting, influenced by depositional cyclicity. This cyclicity is observed as a transition between a transgressive lower part and a highstand upper part of the formation, which reflects changes in relative sea level and depositional environment over time. The lower part of the formation is associated with a transgressive phase, where sea levels rise, leading to the deposition of more distal carbonates. The upper part, in contrast, corresponds to a highstand phase, characterized by stable or slightly falling sea levels, with the deposition of more proximal clastic and carbonate lithofacies. The formation exhibits a transition from clastic deposits in more proximal settings (closer to land) to carbonates in more distal, deeper-water areas. The sand-rich, unit-F of formation B is found to be comprised of two depositional packages with different sand body orientation. Formation C was deposited during the Cenomanian-Turonian highstand on a shallow-water carbonate platform. The shoals and back-shoal areas are typically characterized by high-energy environments where grainstone and packstone accumulate. The presence of rudist build-ups indicates the formation of bioherms or reef-like structures, which act as excellent reservoir rocks. Carbonate tidal channel and bars possess significant reservoir potential. Back-shoal facies is characterized by moderate reservoir quality and is often found to be intercalated with Lagoonal mudstones that exhibits lower reservoir quality. A regional stratigraphic boundary was encountered in the upper part of formation C and is characterized by signs of subaerial exposure and karstification. Formation C is blanketed by a supratidal tight facies creating a field scale seal rock. This comprehensive study enhances the understanding of the depositional and diagenetic complexities of these Cretaceous reservoirs, offering valuable insights into depositional history with a sequence stratigraphic framework. This study explained the reservoir potential and contributed to more effective hydrocarbon exploration and production strategies in this study area.