Stratigraphic Sequence and Basin development of the Mishrif Formation in Selected Oil fields in the Mesopotamian Zone, Southeastern Iraq

The Mishrif Formation represents an important succession in the southern Iraq and has extensive distribution in the Middle East. The present study is focused upon the stratigraphic sequence and basin development of Mishrif Formation in three important oil fields in the Mesopotamian Zone of Iraq are:- Halfaya oil field (Hf-1, Hf-272, Hf-316), Noor oil field (No-1) and Buzurgan oil field (Bu-2, Bu-3, Bu-4). There are several types of microfacies were recognized in the succession of the Mishrif Formation. Their characteristic grain types and depositional texture enabled the recognition of six facies associations (depositional environments) were distinguished in this Formation, they are: Basinal, Slope, Shoal, Biostorm, Back Shoal (restricted) and Lagoon associations facies. The microfacies analysis and reconstructed the paleoenvironments of the Cenomanian-Early Turonian basin in the studied area; there are three stages of the deposition: - First stage:- during this stage the basin of Rumaila Formation was continued to deposition the lower part of Mishrif within the basinal environment. The end of this basin (Rumaila basin) marked by appeared the open marine associated facies to the northwest basin which represent the mfs surface. The Highstand system distinguished by sequential the open sea facies to the shoal facies and biostorm. The first stage was finished by widespread of shoal facies in all studied area to marked a sequence boundary type II (prograde stage A). Second stage:- is represented by the developed the basin from the shoal to biostorm dominated facies with slow sea level rise. The presence of the open marine associated facies within the biostorm-shoal sequence marked the mfs surface. The final step of this stage was shown the shallowing up-ward by appeared the lagoon/restricted association facies overlying the biostorm. At the end of this period, the lagoon/restricted facies were spread in the studied area to mark the prograde stage B as sequence boundary type II. Third stage:- the sea level was raised from the northwest direction as open sea association facies, while to the southeast the biostorme and shoal facies was dominated. The first appeared for the shoal facies upon the open sea facies marked the mfs surface to start the final highstand deposition within the restricted and biostorm/shoal facies. This stage is represent the prograde stage C for the Mishrif Formation, where the deposition has ended to mark the unconformable surface (SBI) with Khasib Formation.