HYDROGEOCHEMICAL AND DIAGENETIC EVOLUTION OF THE SAFA SANDSTONE FORMATION, NORTH SINAI, EGYPT. RIFAI, RI. (1) and EL-FIKY, A.A. (2) Institute of Desert Environmental Researches, Minufiya University, Egypt1, Department of Environmental Sciences, Faculty of Science, Alexandria University, Egypt2 The Middle Jurassic Safa Formation of northern Sinai, Egypt is mainly formed of siliciclastic sequence with few carbonate strata. The studied siliciclastic sequence represents shore-face of sand bar accumulation deposited under fairly high-energy condition. This sequence was subjected to cementation by authigenic illite and intergranular calcite cements at shallow burial depths. Basinal brine has played an important role in the diagenesis evolution of the sequence and resulted in the formation of quartz over-growths, calcite, kaolinite ferroan calcite, siderite, ferroan dolomite and hematite cements. The recorded porosity is extremely low ( ~ 2%) in the studied sandstone facies except that of the subarkose facies (~ 9%). Isotopic and burial history data suggest that the calcite cement precipitated at ~760-1360 m and ~ 48-66oC from a mixed meteoric-marine pore-water. The groundwaters in the Safa Formation are highly mineralized with salinity values up to 10.3 g/L. These waters are considered as paleowater resulted from simple mixing of meteoric and saline connate waters. The Safa groundwaters are mainly of a Na-Ca-Mg-Cl-SO4 type and some of them show predominance of Mg over Ca. The concentrations of both dissolved SiO2 and Fe decrease substantially along the flow path. Mass-balance considerations have been applied to diagenetic history of the study sequence. The prevailing processes controlling carbon, iron, and silica cycling in groundwater are the dissolution and precipitation of calcite, dolomite, siderite, and quartz.