Core description, well logs data, petrographic analysis and scanning electron microscopetechnique were conducted to unravel factors controlling the preservation of high porosity upto 25% in deeply-buried sandstones > 4 km of the Lower Cretaceous Nahr Umr reservoir,southern Iraq. The Nahr Umr Formation composed of sandstone interbedded with shale,minor siltstones and streaks of limestone. The sandstones are arenites and range from fine tocoarse, and poor to moderate sorted. Parallel lamination, planer cross-bedded and lenticularbedding are common sedimentary structures found in the formation, suggesting that the NahrUmr deposited in fluvial-deltaic to the shallow-marine environment. Cementation by syntaxialquartz overgrowth was retarded by the presence of illite coats, which was formed by theillitization of the infiltrated smectite that formed during the deposition around the quartzgrains. Microquartz coats in the form of quartz crystals probably further prevented the quartzovergrowths. Thus, the porosity of Nahr Umr preserved by the illite coats and microquartzcrystals, whereas, the process of K-feldspar dissolution has created secondary porosity. Thestylolite formation and the quartz-calcite replacement are the main sources of silica for theprecipitation of quartz overgrowth.