Authigenesis of clay minerals occurs during rock-fluid interactions in the subsurface, and inevitably occludes the pore spaces of clastic reservoirs. The presence of authigenic clay minerals defines the pore geometry of clastic reservoirs, which could impact on drilling and production of hydrocarbons by affecting subsurface pressure, and reducing hydrocarbon recovery, respectively. The latter has made clay diagenesis study a major component of reservoir quality assessment studies, and consequently an important area of research. Here we explore how clay diagenesis affects the basic petrophysical properties (porosity, permeability and saturation) of lower Cretaceous sandstone reservoirs of two wells (AU-1 and KH-1) along the west coast of South Africa. We use petrographical (SEM and thin-section analysis), geochemical (Pore water chemistry, XRD and CEC), and geophysical (wireline logs interpretation) tools in our study. Clay minerals present include kaolinite, illite, and montmorillonite. Illite occurs as the grain coating mineral, and kaolinite as pore filling mineral. Average porosity values of 8.5% (9.6%), effective porosity of 3.3% (9.5%), permeability range of 0.001 to 0.03 mD (0.005 to 0.2mD), and average water saturation of 51% (27%) from wells AU1 and KH1, respectively, were obtained. The low resistivity, poor porosity and permeability values recorded are attributed to the presence of the dominant clay minerals (illite, kaolinite and montmorillonite) within the reservoir intervals. Generally, the dominant cement in the sediment is quartz with calcite and pyrite in traces. The presence of calcite and glauconite in the reservoir rocks is an indication that the sediments were deposited in shallow marine conditions. It is concluded that all these minerals likely contributed to the low values encountered for porosity and permeability, as well as the moderate water saturation levels in the reservoir intervals.