Diagenesis of the Safaniya Sandstone Member (mid-Cretaceous) in Saudi Arabia

Safaniya Member of the mid-Cretaceous Wasia Formation was deposited as fluvio-marine deltaic sediments in northeastern Saudi Arabia and the northwestern Arabian Gulf. After its deposition and initial burial, the mid-Cretaceous sequence was uplifted during Late Cretaceous structural movements. Structural traps in the region also started forming at this time, but structural growth continued during the Eocene, and new structures developed during the Plio-Pleistocene. The diagenetic mineral assemblage in the Safaniya sandstones includes kaolinite, illite, glauconite, pyrite, quartz and carbonate cements. Kaolinite and illite occur both as authigenic and detrital minerals. Authigenic kaolinite exists as skeletal and euhedral plates forming vermicules and aggregates of booklets infilling the pore spaces. Authigenic illite occurs as overgrowths of projecting laths. Quartz cement is present as syntaxial overgrowths and as microcrystalline quartz. Carbonates occur as patches and show replacement textures. Early diagenetic events include the formation of calcite, siderite, glauconite and pyrite, at or close to the sediment/water interface. Vermicular kaolinite and microcrystalline quartz probably formed by meteoric-water diagenesis during the Late Cretaceous uplift. Quartz overgrowths and the bulk of the euhedral kaolinite may have formed during the Late Cretaceous-Early Tertiary burial. Illite and ankerite were precipitated during deep burial, probably from fluids expelled from the basinal Kazhdumi shales to the east and northeast. The precipitation of authigenic minerals in the clean quartz sandstones has reduced the primary interparticle porosity by an average of about 9% of the rocks. Because the clays occur mainly as pore filling cements, their precipitation has caused only a small reduction in the permeability of these rocks.