Lithofacies Modeling and Depositional Environment of the Carbonate-Clastic rocks: A Case Study from the Asmari Formation, Southeastern Iraq

The mixed Oligocene–Miocene succession of the Asmari formation constitutes an important clastic-carbonate petroleum reservoir in southeastern Iraq, where a detailed understanding of depositional settings and reservoir evolution is essential for optimizing exploration and development strategies. This study aims to improve reservoir characterization by establishing a refined lithofacies classification through the integration of core data, well logs, petrographic analyses, and seismic information. A multidisciplinary approach was applied to interpret the vertical and lateral facies architecture of the formation, and Sequential Indicator Simulation (SIS) was employed to construct a three dimensional lithofacies model capable of capturing both vertical stacking patterns and lateral facies variability within the reservoir. The results indicate that depositional environments and subsequent diagenetic processes play a fundamental role in controlling reservoir heterogeneity. Depositional settings were interpreted through the integration of lithofacies characteristics, textural attributes, and seismic impedance patterns, allowing the identification of deltaic, mixed shallow marine, and restricted lagoonal environments. These environments reflect the interaction between a delta influenced siliciclastic system and a carbonate platform, governed by repeated relative sea level fluctuations that influenced the distribution of mixed sediment types. Reservoir quality varies spatially from poor to good and is significantly affected by diagenetic alterations, particularly the development of authigenic clays and cementation, which reduce porosity and permeability. Member A is characterized by low porosity and low to moderate permeability, whereas Member B exhibits moderate porosity and moderate to high permeability, indicating more favorable reservoir conditions. Overall, the application of integrated lithofacies modeling and advanced geological analysis provides a more reliable representation of reservoir architecture and heterogeneity. The results enhance the understanding of the Asmari formation as a complex carbonate clastic reservoir and support improved reservoir evaluation, modeling, and heterogeneous oilfield development.