The Early-Middle Miocene succession in Iraq is represented by the Serikagni, Euphrates and Dhiban formations, which deposited during the Early Miocene. The Jeribe and Fatha successions were deposited during Middle Miocene age. This study includes microfacies analysis, depositional environments, sequence stratigraphy and basin development of Early – middle Miocene in Hamrin and Ajeel oil fields and Mansuriyha Gas Field. The study area includes four boreholes in three oil fields located in central Iraq: Hamrin (Hr-2) and Ajeel (Aj-13, and 19) oil feilds, and Mansuriyha (Ms-2) Gas Field. Five facies associations were distinguished within the studied fields: deep marine, slop, platform-margin, open marine, restricted interior platform, and evaporitic interior platform. The facies associations interpreted were based on texture and fauna. Deep marine facies association consists of planktonic foraminifera wackestone, it was observed in well Ms–2. Slope environment association is observed in Euphrates Formation in Hamreen-10, and it represented by bioclastic – planktonic mudstone – wackestone. The open marine association is observed in all studied wells in particular in Jeribe and Euphrates formations, and it includes bioclastic mudstone-wackestone, bioclastic peloidal wackestone-packestone and bioclastic foraminiferal wackestone- packestone. While the restricted interior platform association is dominated in all studied wells, it consists of ooidal packstone, foraminiferal peloidal packstone, dolomitic lime mudstone, dolomititic and evaporitic peloidal wackestone-packstone. In Ajil Oil Field the studied succession is consisting of two sequences (C1 and C2). The fist sequence (C1) was started with transgressive system tract (TST) reflected by open marine occupies the lower part of Euphrates Formation, the TST bounded below by SB and above by MFS. The second sequence (C2) was started with TST which is observed in the lower part of Jeribe Formation, represented by evaporitic and restricted interior platform. In Hamrin Oil Field, this succession is started with thin transgressive system tract (TST) as C1 reflected by toe of slope and it occupied a bit of lower part of Euphrates Formation. The highstand system tract (HST) is divided to five cycles (CE1, CE2, CE3, CE/Dh and CDh). Sequence C2 was started with relatively thin transgressive system tract (TST) occupies the lower part of Jeribe Formation represented by open marine environment. Two sequences are observed in Mansuriya Gas Field (C1 and C2). Sequence C1 started with thin transgressive system tract (TST) bounded below by SB1 and above by MFS which is corresponding with conformable boundary that separated between Serikagni Formation and Dhiban. Contrary, the second sequence (C2) was started with relatively thin transgressive system tract (TST) occupies the lower and middle parts of Jeribe Formation represented by restricted interior platform environment.
