Petrographic and petrophysical analyses of core samples that are obtained from selected deep wells of the Sauk Sequence (Cambro-Ordovician) in New York State indicate that the lithologies of the Sauk intervals were subjected to severe diagenetic modifications. The intensity of these diagenetic modifications varied throughout the lithofacies. These diagenetic processes produced intervals that are varied in their petrophysical characteristics including porosity, permeability, recovery efficiency, pore diameters, and fluid saturation. Injection of brines or waste liquids under high pressure into these intervals may cause technical failure of the pre-existing geologic formations and structures. This technical failure may lead to a major contamination of the underground and surface water supplies. Consequently, careful geologic investigation of possible brine disposal sites in deep geologic formations is, therefore, a prerequisite to avoid any negative impacts to humans in particular and to the ecosystem in general.