Using Scanning Electron Microscope SEM technics with XRD analyses, the authors have studied the influence and control of post-sedimentation changes on the upper Triassic (Mulussa F formation), and lower Cretaceous (Rutbah formation) reservoirs quality at Euphrates graben area. The detrital components occupy an average of 72% of the rock volume represented by sandstone quartz arenite, the authigenic minerals occupy an average of 20% represented by overgrowth quartz, kaolinite, chlorite, illite, illite-smectite mixed layers, dolomite, siderite, pyrite, and anhydrite. The history of post-sedimentation changes is subdivided into 2 stages; diagenesis-misocatagenis, and mesocatagenesis-apocatagenesis. The porosity of the reservoirs decreased due to the detrital clay, and the authigenic minerals which cause blocking of pore spaces, and reduce the diameter of the inter-pore channels and leads to a significant effect on the reservoir porosity. Quartz cement reduced the porosity of the upper Triassic reservoir to a lesser extent due to its low content 3%, while the other cements which consist mainly of kaolinite and siderite serve as the main factor in reducing the porosity since their content is higher 7.4%. In the lower Cretaceous reservoirs, quartz cement with the other cements equally contributed to decreasing the porosity due to their presence in equal volumes 2.8 to 3.6%. Houseknecht diagram shows that the initial porosity of the upper Triassic reservoir was reduced by compaction by 47.5%, while reduced by cementation by 26%. In the lower Cretaceous reservoir, the initial porosity was reduced by compaction by 59.5%, while reduced by cementation by 19%.