The amount of natural gas and its viability depends on two main factors: the porosity and permeability of the formation rock. Porosity is defined as the percentage of void space in a rock; it is the volume of the voids or pore space relative to the total volume of the rock. The reason porosity is important is that it is a measure of the capability of the reservoir rock to contain and store oil, gas, and water. The higher the capability of a reservoir rock to contain hydrocarbon, the greater the potential oil and gas production. The highest porosity possible in a rock is 40%; sand rocks, which are considered highly porous rocks, typically have a porosity of 5–25%. Fig. 1.13 is an image of a porous rock with a porosity of around 20%.