Hydrostatic Pressure - fluid pressure at depth is due to the weight of overlying fluid. Nominal hydrostatic fluid-pressure gradients are usually about 10.5 MPa/km.

Lithostatic Pressure - pressure due to the weight of the entire overburden (fluid plus matrix). Lithostatic gradient is approximately 23 MPa/km.

Abnormal fluid pressures are above or below hydrostatic. Overpressures are more common than underpressures. Sedimentary basins with overpressures typically consist of 2-3 km of normal pressured sediments with a transition zone and then a deep section of high fluid pressures.

Static versus Dynamic Hypotheses

Static Hypothesis - abnormal pressures are maintained by seals, units that behave as if they have zero permeability over geologic time.

Dynamic Hypothesis - rate of pressure generation is sufficiently high so as to maintain abnormal pressures in the presence of low-permeability rocks over geologic time (e.g., very high sedimentation rates).

Topographic Recharge - In areas of recharge (high elevation and descending flow) the fluid pressures are below hydrostatic. In areas of discharge (low elevation and ascending flow) the fluid pressures are above hydrostatic.

Compaction disequilibrium - Porosity reduction is inhibited by the difficulty in expelling pore fluid from low-permeability shales and clay-rich sediments. Water is not very compressible. Thus, most of the overlying sediment's weight is borne by the pore water rather than the matrix creating high fluid pressures. Creation of overpressures by compaction disequilibrium requires high sedimentation rates (or erosion rates for underpressure) and a predominance of low-permeability sediments (e.g., Gulf of Mexico and Caspian Basin).

Aquathermal Pressuring - Water expands with temperature more than mineral grains do. Thus, as sediments are heated with burial, the expansion of water generates fluid pressure. This effect is much smaller than compaction disequilibrium.

Oil and Gas Generation - higher density kerogen is replaced by lower-density oil and gas which requires more volume for the same mass.

Illite/Smectite - Under high pressure and temperature, water is expelled from smectite as it is converted to illite. Illite and smectite are both clay minerals.