Loss of porosity with increasing differential pressure, usually in a nonelastic way, e.g., by grain deformation, repacking, recrystalization, etc. The normal porosity of shales given by the Hubbert-Rubey law is
where 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:
Compaction is nearly irreversible, i.e., relieving the pressure that compacted a rock does not uncompact it, but releaving the pressure is apt to produce a small change (unloading effect).