Which statement correctly describes clays in terms of porosity and permeability?

• A. High porosity but low permeability
• B. Low porosity but high permeability
• C. Low porosity and low permeability
• D. High porosity and high permeability

Answer: (A)

Clays illustrate the difference between porosity and permeability. Porosity is the amount of void space in a material, while permeability is how easily fluids can move through that pore network. Clay particles are extremely fine and create a lot of tiny pores, including interlayer spaces, so their total porosity can be relatively high. But the pathways through these pores are very small and poorly connected, making fluid flow extremely slow. That combination—lots of void space but very restricted flow—means clays typically have high porosity yet low permeability. The other scenarios would imply easier flow or less void space, which doesn’t match the way clay pores are structured.