The lithosphere is a continuous composition of different rock masses, of which some include void spaces as well as solids whereas others consist of solids only. The void space provides a place where groundwater can be stored and kept under suitable conditions for use at times of demand. Any rock mass in the lithosphere consisting of mutually exclusive but complementary volumes of voids and solids will be referred to as a reservoir. It is, therefore, possible to consider the lithosphere under two very broad and mutually exclusive categories: reservoirs and nonreservoirs. A reservoir contains voids, allowing liquid into its main body. Of course, many sedimentary formations, such as sandstones, limestones, clay, etc., have the reservoir property whereas most igneous rocks are of the nonreservoir type.