Facies Associations Related to Sequence Stratigraphy of the Middle Campanian-Maastrichtian Succession in Selected Oilfields, Southern Iraq

This study presents a detailed microfacies analysis of the Hartha and Shiranish formations in the Eridu and Nasiriyah oilfields, based on 232 thin sections of cores and cuttings from four boreholes (Er-X, Er-Y, Ns-X, and Ns-Y). The depositional environments were determined based on sedimentological and paleontological criteria. Eight microfacies were identified within these formations, revealing distinct paleoenvironmental settings ranging from shallow marine ramp to deep basin environments. The Hartha Formation exhibits facies transitions:open marine (Orbitoides wackestone-packstone, RMF 13and bioclastic floatstone RMF 15) to restricted lagoonal (Miliolids wackestone-packstone, RMF 16), shoal (Oolitic packstone-grainstone, RMF 29 and 30), and evaporitic lagoonal settings (Gypsum-anhydrite mudstone, RMF 19). The Shiranish Formation predominantly reflects deep depositional settings, represented by planktonic foraminifera wackestone-packstone (RMF 3) and argillaceous mudstone/wackestone (RMF 2), indicating conditions of normal wave base deposition. The stratigraphic framework of the succession comprises two third-order sequences, with three fourth-order cycles (H1, H2, and H3) in the Hartha Formation and two fourth-order cycles (Sh1and Sh2) in the Shiranish Formation. The sequence stratigraphic analysis highlights transgressive-regressive cycles that influenced facies distribution are marked by regional unconformities. High-resolution facies analysis and wireline log were used to support a refined understanding of the depositional cyclicity, basin evolution and enhancing the hydrocarbon exploration potential in the northern part of Arabian Plate.