Porosity alone does not guarantee that fluid can flow. Permeability is the capacity of a rock or sediment to transmit fluids. It depends on the size, shape, and connectivity of pore spaces and fractures.

Gravel and well-sorted sandstone typically have high permeability; shale and unfractured crystalline rock have very low permeability. Permeability is measured in darcys or millidarcys and can vary by many orders of magnitude among common rocks. Anisotropy is common—flow may be easier parallel to bedding than across it.

Aquifer performance, petroleum reservoir behavior, and contaminant transport all depend critically on permeability. Laboratory measurements on cores and field tests in wells are used to quantify it. Secondary processes such as fracturing or cementation can dramatically increase or decrease the permeability of a rock unit.

• Ability of rock to transmit fluids
• Depends on pore connectivity and size
• Measured in darcys
• Controls groundwater and petroleum flow