The ratio of void space to the bulk volume of rock containing that void space. Porosity can be expressed as a fraction or percentage of pore volume in a volume of rock.

(1) Primary porosity refers to the porosity remaining after the sediments have been compacted but without considering changes resulting from subsequent chemical action or flow of waters through the sediments. See primary porosity.

(2) Secondary porosity is the additional porosity created by chemical changes, dissolution, dolomitization, fissures, and fractures. See secondary porosity.

(3) Effective porosity is the interconnected pore volume available to free fluids, excluding isolated pores and pore volume occupied by adsorbed water. In petroleum engineering practices, the term porosity usually means effective porosity.

(4) Total porosity is all void space in a rock and matrix whether effective or noneffective. Total porosity includes that porosity in isolated pores, adsorbed water on grain or particle surfaces, and associated with clays. It does not include water of crystallization wherein the water molecule becomes part of the crystal structure.