Microfacies Analysis and Depositional Model of the Hartha Formation (Late Cretaceous) in Selected Fields, Southern Iraq

The Hartha Formation, dated to the Late Cretaceous period, is one of the major carbonate reservoirs in southern Iraq, specifically in the Mesopotamian Basin. Although some stratigraphic and sedimentological studies exist, they are limited and mainly focus on microfacies analysis and depositional modeling in the Dewan and Mufaiqia exploration fields. This work aims to analyze core samples from the Majnoon oilfield, as well as rare core from the Dewan and Mufaiqia fields, to identify microfacies types, depositional environments, and diagenetic processes of the Hartha Formation. Three main microfacies types and eight submicrofacies were identified, including lime mudstone, which reflects a shallow interior platform. The Wackestone (Planktonic wackestone, Foraminiferal Bioclastic Wackestone, and Miliolids Wackestone) microfacies reflects restricted and lagoon environments, while the Bioclastic Wackestone/Packstone (Bioclastic wackestone-packstone, Dolomitized bioclastic wackestone-packstone, Bioclastic foraminifera wackestone-packstone, Peloidal wackestone/packstone and Echinoderm Bioclastic wackestone-packstone) reflects deeper water, presented by the mid-ramp and outer ramp. These depositional environments indicate accumulation along a ramp homoclinal system influenced by sea-level fluctuations. Diagenetic processes, including dolomite formation, cementation, and dissolution, were also observed, occurring in both early and late stages, which significantly modified the original pore network. The results highlight differences between Majnoon Field, which shows more planktonic facies and oil-stained thin sections suggesting higher reservoir potential, and the more studied Dewan and Mufaiqia fields, which are considered more confined and localized.