The Santonian-Campanian Sadi Formation carbonate reservoir of the late Cretaceous in H Oilfield located in SE Iraq has been proved to be an oil-bearing stratigraphic unit with medium porosity and low to ultra-low permeability, which increases difficulties in producing oil contained in carbonate rocks. In order to forecast the favorable reservoir with higher production rate, it needs to study the characteristics and genesis of this reservoir. Data used includes a total of 236m cores from 4 wells. Firstly, microfacies interpretations were supported by log interpretations and core descriptions. Then, diagenesis processes were analyzed according to thin sections. Next, relationship between microfacies, diagenesis process and reservoir characteristics such as porosity, permeability, pore types and pore-throat characteristic were investigated. Finally, favorable reservoir characters and the genesis of reservoir in this formation were obtained. Sadi Formation represents a third-order depositional cycle composed of limestones and marlstones containing planktonic foraminifera and bioclastic debris together with extensively bioturbated argillaceous rocks, depositing on a ramp setting. Foraminifera limestone in middle ramp experienced syngenetic diagenesis and burial diagenesis, ultimately evolved into low and ultra-low permeability reservoir with intrafossil pores and micro pores. Shapes of pore throat distribution of middle ramp rocks represent thin and single peaks. Bioclastic limestone in inner ramp shoals experienced marine pore-water diagenesis, meteoric fresh water dissolution and burial diagenesis, ultimately evolved into low permeability reservoir with dissolution pores, mouldic pores and micro pores. Shapes of pore throat distribution in inner ramp shoal rocks represent irregular peaks, with wide range distribution. The favorable reservoir exists in intraclastic shoal sediments deposited in inner ramp. Results of this paper contribute to further understanding of the sedimentary and reservoir characters of Late Cretaceous Formation in SE of Iraq, and benefit for favorable reservoir predicting and oil producing in this oilfield.