Porosity

<math>\phi = \frac{V_p}{V_m}</math>

where V_p is the non-solid volume (pores and liquid) and V_m is the total volume of material, including the solid and non-solid parts.

It is common to express porosity as a percentage, by multiplying the above ratio by 100.

The porosity of a rock, or sedimentary layer, is an important consideration when attempting to evaluate the potential volume of hydrocarbons it may contain. Sedimentary porosities are a complex function of many factors, including but not limited to: rate of burial, depth of burial, the nature of the connate fluids, the nature of overlying sediments (which may impede fluid expulsion). One commonly used relationship between porosity and depth is given by the Athy[?] (1930) equation:

<math>\phi(z) = \phi_0 \exp^{-kz}</math>

where φ₀ is the surface porosity, k is the compaction coefficient (m^-1) and z is depth (m).