The ease with which fluid is transmitted through a rock's pore space is called permeability. Although a rock may be very porous, it is not necessarily very permeable. Permeability is a measure of how interconnected the individual pore spaces are in a rock or sediment. A sandstone is typically porous and permeable. Shales are porous but have a lower permeability because the finer grain size creates smaller pore spaces. Igneous rocks tend to have low porosity and low permeability unless they are highly fractured by tectonic processes.

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