DEFINITION OF POROSITY

The percentage of total volume of a body that

consists of open spaces. Sands and gravels typically have

about 20 percent open spaces, while clays have about 50 percent.

The sizes and shapes of grains determine the porosity,

which is also influenced by how much they are compacted,

cemented together, or deformed.

Porosity may be of several different types. Primary porosity

is the open space that forms during the deposition of sedimentary

rocks, such as the open spaces between individual

sand grains in a sandstone. Secondary porosity is the amount

of open space that forms in a rock by secondary processes,

such as dissolution of limestone. Fracture porosity measures

the amount of open space created by fracturing, jointing, and

faulting of a rock. Fracture porosity may be enhanced by dissolution,

and thus, fracture and secondary porosity typically

act together to increase the primary porosity of a rock.

Minerals that precipitate between grains, cementing

them together, may reduce the porosity of a rock. It may also

be reduced if high pressure and temperatures during compaction

or tectonic forces deform the grains.

See also AQUIFER; PERMEABILITY.

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