# Dictionary:Compaction

Loss of porosity with increasing differential pressure, usually in a nonelastic way, e.g., by grain deformation, repacking, recrystalization, etc. The normal porosity ${\displaystyle \phi }$ of shales given by the Hubbert-Rubey law is
${\displaystyle \phi =\phi _{0}e^{-cP}}$,
where ${\displaystyle \phi _{0}}$ is the initial porosity (the Atterberg limits, q.v.), c is a constant, and P is the differential pressure, the difference between the overburden pressure and the formation fluid pressure. This leads to a pressure-with-depth curve that is roughly logarithmic:
${\displaystyle P={\frac {1}{c}}{\text{ln}}{\frac {\phi }{\phi _{0}}}}$.