If a rock has high porosity but low permeability, which statement is true?

• A. It has high porosity but low permeability.
• B. It has high porosity and high permeability.
• C. It has low porosity and low permeability.
• D. It has low porosity but high permeability.

Answer: (A)

Porosity is about how much empty space a rock has to hold fluids, while permeability is about how easily fluids can flow through that rock. A rock can have lots of pore space but if those pores aren’t connected well, or the connections are too narrow, fluids can’t move through it easily, so permeability is low. So a rock with high porosity but low permeability can store a lot of fluid but won’t let it flow through quickly. If both porosity and permeability were high, fluids would flow readily; if both were low, there wouldn’t be much space to hold fluids; and if porosity were low, there wouldn’t be much space to store fluids even if flow paths existed.