High-frequency cycles drive the cyclical enrichment of oil in porous carbonate reservoirs: A case study of the Khasib Formation in E Oilfield, Mesopotamian Basin, Iraq

Porous carbonate reservoirs of the Middle East commonly combine high porosity with low permeability, resulting in pronounced heterogeneity in hydrocarbon distribution. This study investigates the mechanisms behind the cyclicality of oil-bearing intervals in the Upper Cretaceous Khasib Formation at the E Oilfield, Iraq. A multiproxy dataset – including detailed core descriptions, thin-section petrography, conventional well logs, and laboratory measurements of porosity, permeability, pore-throat radius, extractable organic matter, and oil saturation – was used to define seven fourth-order depositional sequences (SSQ1–SSQ7), whose lithofacies architectures closely follow high-frequency sea-level oscillations. An effective porosity threshold of 15% was established for hydrocarbon charging. Within these reservoirs, permeability and wettability emerge as the primary controls on oil saturation: permeability dominates below 3 mD, while wettability exerts greater influence at higher permeabilities. Variations in lithofacies and episodic exposure driven by sea-level fluctuations generate heterogeneous reservoir properties and drive the cyclic distribution of oil intervals. Transgressive system tracts predominantly produce poor- or non-oil facies, early regressive tracts host medium- to good-quality oil layers, and oil content declines in late regressive tracts. These results offer critical insights for assessing hydrocarbon potential and predicting the spatial distribution of oil intervals in analogous porous carbonate systems.