Which statement is true about porosity?

• A. It measures the ability to transmit water (permeability)
• B. It is the ratio of the volume of solids to the total bulk volume
• C. It is the fraction of void volume to the total bulk volume
• D. It is always equal to the compressibility of the rock

Answer: (C)

Porosity is the fraction of a rock’s bulk volume that is void space, i.e., the portion made up of pores that can store fluids. This is exactly the ratio of the volume of voids to the total bulk volume. It tells you how much storage space is available, not how easily fluids move through the rock. Permeability, which describes fluid flow, depends on how connected the pores are and their sizes, not just how much void space exists. The ratio of solids to total bulk volume would be the solid (or grain) fraction, which is 1 minus porosity for a simple rock—porosity isn’t that ratio. Porosity also isn’t a measure of compressibility, which relates to how the volume changes under pressure. So the statement that porosity is the fraction of void volume to the total bulk volume is the true one.