Porosity and Permeability: Definition and Types

Porosity is the percentage of the bulk volume of a rock or soil that is occupied by interstices, whether isolated or connected.

Effective porosity is the percent of the total volume of a given mass of soil or rock that consists of interconnected interstices.

Types of Porosity

Primary porosity is the porosity that developed during the final stages of sedimentation or that was present within sedimentary particles at the time of deposition. “Primary porosity includes all predepositional and depositional porosity of a particle, sediment, or rock”.

Secondary porosity is the porosity developed in a rock after its deposition or emplacement, through such processes as solution or fracturing.

Permeabilty Defination

Permeability is the capacity of a rock to transmit a fluid. It varies with the fluid’s viscosity, hydrostatic pressure, the size of openings, and particularly the degree to which the openings are interconnected.

Rocks that commonly have high permeability are conglomerates, sandstones, basalt, and certain limestones. Permeability in sandstones and conglomerates is high because of the relatively large, interconnected pore spaces between the grains.

Basalt is permeable because it is often extensively fractured by columnar jointing and because the tops of most flows are vesicular.

Fractured limestones are also permeable, as are limestones in which solution activity has created many small cavities. Rocks that have low permeability are shale, unfractured granite, quartzite, and other dense, crystalline metamorphic rocks.

How can a rock be highly porous and still have low permeability?

If the pore spaces are very small, a rock can have high porosity but low permeability because it is difficult for water to move through small openings.

Important Books to Read

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Groundwater Hydrology by David K. Todd and Larry W. Mays