Consideration of the stresses in clay rocks, and their compaction processes, leads to the conclusion that abnormally pressured clay sequences generally have been abnormally pressured since burial to a shallow depth--2,000 ft (600 m) or even less--in areas of gravity loading under sediments.

Rubey and Hubbert's concept of equilibrium depth (z_e) is reexpressed.

[EQUATION]

in which the ratio (1 - ^lgr)/(1 - ^lgr_e) is a dimensionless measure of the expulsion of fluids from the formation.

Zones of abnormal fluid pressure are caused more by low hydraulic conductivity in a rock unit or sequence than by load or rate of loading. Hence the transition zone from normal to abnormal pressures at the top of a relatively impermeable sequence will have its mirror image at the bottom of the unit, if it overlies a sequence of sufficient hydraulic conductivity. The petroleum potential below an abnormally pressured clay sequence is similar to that above it.