The stratigraphic sequence of Cenomanian-Early Turonian is composed of Ahmadi, Rumaila, and Mishrif formations in the Rifai, Noor and Halfaya Oil Fields within the Mesopotamian Zone of Iraq, which is bounded at top and bottom by unconformity surfaces. The microfacies analysis of the study wells assisted the recognition of five main environments (open marine, basinal, shallow open marine, Rudist biostrome, and lagoon); these microfacies were indicative of a normal lateral change facies from shallow water facies to deeper water and open marine sediments.
Ahmadi Formation (Early Cenomanian) is characterized by open marine sediments during the transgressive conditions, and would become deep basinal environment upward to deposition the Rumaila Formation. Rumaila Formation (Middle Cenomanian) was deposited in the deeper part of the intrashelf basin; it comprises basinal sediments mainly, and includes an abundant of open marine fauna supportive of Middle Cenomanian age. Rumaila Formation is represented as time equivalent basin to the Mishrif Formation, where they were deposited during highest and system tract (HST). The Cenomanian- Early Turonian sequence can be subdivided into three cycles displaying coarsening upward cycles :Mishrif A, Mishrif B, and Mishrif C; which comprises a highest and system tract dominated by rudistid packstone to grainstone or rudistid biostrome facies separated by transgressive units (CR I and CR II).
