Clays with Abnormal Interstitial Fluid Pressures

Abstract

Abstract 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 (ze) is reexpressed. z e = ( 1 − λ ) ( 1 − λ e ) z in which the ratio (1 – λ)/(1 – λe) 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.