Microfacies of Shiranish Formation (Late Cretaceous) in Tikrit Oilfield, Central Iraq

The late Campanian-Maastrichtian cycle in TK.1 and TK.2 wells within Tikrit Oilfield in central Iraq is represented by basinal facies of the Shiranish Formation. The lithology consists of marly limestone in the lower part referred as unit (A) followed by unit (B) in the middle part, which is composed of shaly limestone with bands of shale and marl with detrital particles and dolomite rock fragments. The upper part unit (C) comprises marly limestone with less shale and carbonate rock fragments. Two biostratigraphic zones are distinguished; Globotrun-canita calcarata Total Range Zone referred to Middle-Late Campanian, and Globotruncana aegyptiaca Interval Zone referred to Early Maastrichtian. The Bioclasts Foraminiferal Wackestone Microfacies (Sh1), Foraminiferal Wackestone Microfacies (Sh2), Foraminiferal Wackestone-Packstone Microfacies (Sh3) and Mudstone Microfacies (Sh4) characterize the dominant microfacies. Those were affected by different diagenesis processes. In accordance to microfacies analysis and the biological components, the succession was deposited in basinal to toe of slope environments. The deeper part is located in the well TK.1 at the eastern side of Tikrit Oilfield, where it was responsible for depositional microfacies association of (Sh1) and (Sh4). The repetition of (Sh1, Sh2, Sh3 and Sh4) microfacies associations toward the west of the field within TK.2 well indicates that the shallowest part of depositional basin is located at the western side of the field due to the occurrence of quartz particles, rock fragments and benthonic foraminifera within unit (B) in this well. Those may be provided from the higher parts of the Rutba-Khleisia basin, which is not covered by the sea or partially covered.