Rocks and sediments contain open spaces that can hold fluids. Porosity is the fraction of bulk volume occupied by those voids, expressed as a percentage. It includes spaces between grains, fractures, and cavities formed by dissolution.

Primary porosity originates during sediment deposition or rock solidification; secondary porosity develops later through fracturing or weathering. Well-sorted sandstones may have porosities of 20–30 percent; crystalline igneous rocks often have porosities below 1 percent unless fractured.

Porosity determines how much water or hydrocarbon a rock can store. Combined with permeability it controls fluid flow. Geologists and engineers measure porosity with laboratory techniques on cores and with borehole geophysical logs. Compaction and cementation progressively reduce porosity with depth and time.

• Percentage of void space in rock or sediment
• Controls fluid storage capacity
• May be primary or secondary
• Reduced by compaction and cementation