Sedimentological and diagenetic effects on hydraulic Flow Units of Ramp Carbonates: Khasib formation, Mesopotamian Basin, Iraq

Research on the inhomogeneity of carbonate reservoirs emphasizes macroscopic sedimentary phases, although it lacks a comprehensive examination of the microscopic pore-fluid flow mechanisms. A quantitative model of the core data-fluid flow unit was developed using the Khasib Formation of the Iraqi B field as a case study, integrating cast thin section analysis, capillary pressure curves, and sedimentary microphase analysis. The hydraulic flow unit is categorized into four types using the normal probability distribution model. HFU-1 (High-Medium Energy Shoal) comprises Arenaceous packstone, recognized for its high porosity and permeability, attributed to the dissolution of fibrous calcite colluvium. HFU-2 (High-Medium Energy Shoal and Medium-Low Energy Shoal) is characterized by green algae-foraminifera assemblages, exhibiting enhanced vertical pore connectivity correlating with the abundance of bioclasts. Conversely, HFU-3 (Medium-Low Energy Shoal/Back Shoal) displays patchy, non-homogeneous characteristics resulting from significant biological disturbance, with permeability improvements confined to the fault sparsity zone. In contrast, HFU-4 (Distal/Proximal) exhibits the poorest reservoir quality, primarily due to elevated grey mud content and suboptimal hydrodynamics. The research indicates that the sedimentary zone (characterized by clastic composition and abundance) predominantly influences the initial pore architecture, while diagenetic dissolution (selective dissolution of aragonite/high-mg calcite) enhances reservoir capacity. Additionally, tectonic activity (fault channelling) exhibits minimal activity, impacting seepage capacity only in localized areas. Collectively, these factors facilitate the progressive alteration of reservoir quality from the high-energy zone of the shoal to the low-energy zone at the shoal margin. The findings offer a pragmatic direction for recognizing flow units in analogous reservoirs.