Pore volume per unit gross volume. Porosity is determined from cores, sonic logs (see Wyllie relationship), density logs (q.v.), neutron logs, or resistivity logs (see Archie’s formula). See also movable oil plot. Primary porosity refers to the porosity remaining after the sediments have been compacted but without considering changes because of subsequent chemical action or flow of water through the sediments. Secondary porosity is additional porosity created by subsequent changes, especially fissures, fractures, solution vugs, and porosity created by dolomitization. Effective porosity is the porosity available to free (moveable) fluids, excluding unconnected porosity and space occupied by bound water and disseminated shale.