Bai Hassan Oil field is considered one of the important north Iraq oil fields. The giant oil field of Bai-Hassan is located near Kirkuk City in North Iraq. The field consists of two domes (in SE – NW direction) Kithka Dome and Dauod Dome separated by a narrow saddle called Shashal saddle.

The current study is focused on microfacies analysis to interpret the stratigraphic sequence of the Palani Formation (early – lower Oligocene age), Baba, Bajawan and Tarjil Formations (middle Oligocene age).

The Bajawan Formation consists basically of five microfacies easily recognizable throughout the thin section. These microfacies are Lime Mudstone Microfacies, Miliolid Packestone Microfacies, Miliolid Grainstone Microfacies and Dolostone Microfacies. The Baba Formation comprises four microfacies: Rutalia - Nummulites Packstone Microfacies, Coral Boundstone Microfacies and Dolostone Microfacies. Two microfacies have been recognized in Tarjil Formation, these are Lime Mudstone Microfacies and Planktic foraminiferal packstone microfacies. In addition to two microfacies within Palani Formation are Lime Mudstone and Planktonic foraminifera Wackstone Microfacies.

The depositional environment of the studied formations and the analysis of their facies components showed that they were deposited within the three parts of the ramp environment, where the Palani Formation was deposited in the deeper part from the outer ramp during TST and HST stages, while the Tarjil Formation was deposited at the distal part of upper slope area from middle ramp environment during HST stage. The Baba Formation was deposited on the swelling parts of the approximal part for the middle ramp at the lower part (TST), and it was reached separately by MFS from the distal part of inner ramp sediments which were deposited during HST as two cycles. The Bajawan Formation with its two parts was deposited at the approximal part from the inner ramp environment during the early and late HST stage.