The Yamama Formation represents a part of the Late Berriasian-Aptian sequence, deposited during the Early Cretaceous period within the main shallow marine depositional environment. The studied area covers three oil fields; Sindbad oil field, Halfaya and Ad'daimah oil field, located in southeastern Iraq. Six major microfacies were recognized in the succession of the studied area represented by the Yamama Formation to determine and recognize depositional paleoenvironments. These microfacies are; Peloidal Packstone, Algal Wackestone to Packstone, Bioclastic Wackestone – Packstone, Foraminiferal Bioclastic Wackstone, Packstone, Peloidal – Oolitic Grainstone and Mudstone Microfacies. These microfacies are classified into three standard microfacies and three Facies Zones MFS-18/FZ- 8, SMF 15/FZ-6 and SMF-10/FZ-7, representing the restricted marine, shoal and shallow open marine associations facies, respectively.
The Yamama succession in the studied area is divided into three cycles representing three depositional stages of highstand system tracts. These three cycles are characterized by shallowing upward depositional mode where they are deposited in the shallow open marine shoal with semi-restricted associations facies for each cycle.
To the northeast of the study area near Halfya-5 well, the shoal association appeared in the lower and middle part of the Yamama succession, and in Da-1 and Sn-2 were three cycles. This case suggests that the paleo high was developed north of the study area, while the open marine was extended to the southern part. The presence of shallow open marine association facies between the Sulaiy and Yamama successions refers to continuous deposition during the same depositional stage in all studied wells. It may mark the end of the Sulaiy succession with the maximum flooding surface (MFS). This first depositional stage is started with the shallow open marine association facies underlain by shoal facies association and then semi-restricted association. The repetition of this cycle in the studied sections symmetrically indicates the harmonic oscillation in sea level during the second and third depositional stages of this sequence.
The Halfaya oil field within the Mesopotamian Block and Ad'daimah and Sindbad oil fields within the Basra Blok explain the significant variation in the thickness for this succession between the Halfaya oil field and the other oil fields. This may suggest the occurrence of high depositional subsidence in Basra Blok and uplifting during the Yamama depositional stages. The fact that the studied sections are distinguished by the same cycles, albeit roughly, indicates that the lifting and sit-down processes synchronized sedimentation.
