Porosity in rocks refers to the open spaces within a rock mass. Primary porosity is distinguished from secondary porosity.[1]​ Primary porosity is that of the rock or sediment matrix and secondary porosity is that produced by fractures "Fracture (geology)") and dissolution of part of the rock or sediment.[1]​.

There are several measurements of porosity in rocks.[2]​ The most common, and the one discussed in this article unless otherwise indicated, is the ratio between the volume of the pores of the rock and the total volume of the rock.[2]​ Another measurement is the ratio between the volume of the pores and the volume of the grains of a sedimentary rock.[2]​ Porosity in rocks is usually expressed as a percentage.[2]​[3]​ In sediments and sedimentary rocks selection&action=edit&redlink=1 "Selection (sedimentology) (not yet written)") tends to increase porosity.[4]​ Something similar occurs in sediments and unconsolidated rocks where the greater the proportion of laminar minerals (e.g. clay minerals, micas) compared to round or angular particles, the greater the porosity.[4]​[1]​ In general, rocks and sediments have greater porosities as their size is smaller. grain.[5]​.

The shape of the interconnected pores in relation to fluid transit is called tortuosity so that a pore that meanders or makes a U-turn has high tortuosity.[6]​.

The porosity of the rocks can be a determining factor for their hydraulic conductivity.[4]​ The porosity and the type of fluid that fills the pores is related to the density of the rocks.[7]​.

In low-porosity fractured rocks, changes in the stress field can alter hydraulic conductivity.[4]​ Porosity in rocks decreases drastically at depths of the brittle-ductile transition in the Earth's crust.[8][9]​.

Fresh unweathered clay has porosities between 40% and 80%.[10]​ Peat has a porosity of around 70%.[11]​ Sand has a porosity of between 36% and 45% and clayey sand between 45% and 49%.[12]​[11]​ The porosity of gravel is between 25% and 30%.[11]​.

Sandstones that host oil or natural gas tend to have porosities between 5% and 40% (pores/total volume).[3] Limestone has estimated porosities in the range of 0.6% to 16.9%.[11] However, porosity in carbonate rocks, including limestone, can be much higher when considering the "Fracture (geology)" fractures that typically traverse them. rocks.[3]​ In fact, karst limestones have porosities of 5% to 50%.[1]​.