This paper dealing with Digenesis System & Its Relationship With Porosity Development of Yamama Formation in the Tithonian – Hautervian sequence within the Stratigraphic column of Lower Cretaceous in Ratawi oil Field of Basra city in South of Iraq. Diagenesis Processes are among the most important factors affecting carbonate rocks, especially reservoir rocks, such as the Yamama Formation Rocks, through their varying role in improving or reducing the porous system, which plays a major role in controlling the behavior of fluid movement in the reservoir, as well as its role in influencing the original tissue of the facies or changing it completely by affecting the skeletal and non-skeletal components of facies to varying degrees, which in turn affect the petrophysical properties of the reservoir (porosity and permeability).