The sediments of the Hartha Formation were deposited during the Upper Campanian- Maastrichtian cycle. Due to the importance of this sequence in terms of stratification and economics in the oil industry, it has been focused on in this study. The present study includes three oil fields in central of Iraq within the Mesopotaminan Zone, East Baghdad, Balad and Kifl oil fields. This study was accomplished by describing 190 thin sections and interpreting the response of the available well logging data. Seven major microfacies were diagnosed in the Hartha succession at studied oil fields, they are; Orbitoidal wackestone - packstone, Orbitoidal and miliolids wackestone, Rotaliidae and Siderolites with echinodermata wackestone - packstone, Peloidal - echinoderm packstone - grainestone, Pellets - echinodermata packstone - grainestone, Planktonic foraminifera wackstone - packstone and Evaporitic mudstone microfacies. These microfacies were deposited in lagoon, restricted, shallow open marine, shoal, mid ramp and outer ramp. The Hartha Formation in the studied oil fields represents one cycle third order with three cycles fourth order (A1, A2 and A3). These cycles are asymmetrical and starts with cycle A where the open marine facies in the Lower Hartha represents the TST underly the HST, which bounded below by a TS which coinside with the unconformable surface (SB1) with the underlying Sadi Formation. The highstand systems tract of cycle A1 is reflected by the open marine facies of the Hartha Formation, A2 is reflected by the open marine and mid ramp facies represent thick TST and then appear the HST which reflected by the open marine facies bounded by a transgressive surface (TS) reflected mid ramp and outer rampfacies in the TST which represents A3. The second cycle A2 is disappeared in Kifl Oil Field with appeared evaporitic lagoon facies.
