Impact of Diagenetic Processes and Dolomitization on the Dammam Formation in Central Iraq

This study examines the diagenetic processes that affect the Dammam Formation, with a focus on the dolomitization mechanism of limestone rocks within Al-Najaf Governorate, Central Iraq. The Dammam Formation is composed of dolostone, limestone, and dolomitic limestone. The results of XRD analysis show the presence of primarily non-clay minerals (calcite, dolomite, quartz, and feldspar) and clay minerals (palygorskite, montmorillonite, and kaolinite) as secondary minerals. In addition to the dolostone facies, six types of limestone microfacies are identified. Most of them indicate sparse to intense dolomitization. Three facies’ associations are distinguished in the Dammam Formation. Each represents a distinct depositional environment, including shallow restricted, shallow open marine, and shoal environments. Several diagenesis processes have affected the Dammam Formation, with dolomitization being the predominant one. Five major textural patterns of dolomite are distinguished: microcrystalline, planar-euhedral, planar-subhedral to euhedral, planar-porphyrotopic, and planar void-filling. Petrographic investigations, geochemical data, and isotope analysis indicate that the dolomitization of the Dammam Formation has formed in a mixing zone. The paleo-temperature of dolomite in Dammam Formation, measured based on δ¹⁸O, ranges from (42.2-67.1°C). Accordingly, its deposition by mixing meteoric-sea water associated with low temperature and shallow burial conditions becomes more probable. The recorded depletion in δ¹³C value of (-7.97‰). At depth 34 m in BH3, there is a sea-level fall, and exposure of the Dammam Formation was in the late Eocene. The diagenetic history of the limestones can be subdivided into early, middle, and late stages.