DIAGENESIS AND POROSITY EVOLUTION OF THE PRE-CENOMANIAN SANDSTONES IN RAS EL NAQB AREA, EAST-CENTRAL SINAI, EGYPT ALAA M. SALEM Fac. Education, Kafr El Sheikh, Tanta Univ., 33516, Egypt The pre-Cenomanian (Cambro-Silurian and Early Cretaceous) quartzarenite and subarkose sand-stones of Ras ElNaqb area in east-central Sinai were subjected to cementation by hematite, infiltrated clays, calcite, quartz and kaolinite. This paragenetic sequence was controlled by meteoric diagenesis, sea water, and water-rock interaction. Meteoric diagenesis resulted in clay infiltration, dissolution/ kaolinitization of detrital feldspar and micas, and precipitation of hematite, quartz and kaolinite. The ð18O values of calcite cement (Type I calcite; - 4.4‰ to +l.8‰) indicate precipitation from mixed marine-meteoric waters at temperatures of ~20-300C, whereas the depleted ð18O values of Type II calcite (-l6.0‰ to -7.5‰) are exclusively related to meteoric waters and water-rock interaction at elevated temperatures. Early cementation by hematite, smectite and calcite, and the relatively limited burial (~ 1.5 Km) prevented strong compaction and preserved some primary intergranular porosity. The absence of early cements in some samples promoted compaction and porosity destruction. However, both compaction and cementation processes show similar effects in porosity reduction. Creation of secondary porosity was due to the dissolution of calcite cements and framework grains, although the former is more significant in secondary porosity enhancement than the latter.