ABSTRACT: Geopressures: Two Distinctly Different Kinds of Conditions

GRETENER, P. E., University of Calgary, Calgary, Alberta, Canada, and G. BLOCH, NAM, Assen, The Netherlands

Published geopressure curves indicate that there exist two distinctly different types of geopressure environments.

In type "A" curves, the geopressures (pressures above the normal hydrostatic value) are limited to the aquitards. Maximum deviation normally occurs near the middle of the aquitard resulting in dewatering gradients both upward and downward into the adjoining aquifers (aquifers are identified by a fluid pressure gradient, dp/dz, ~ 10.5 kPa/m, 0.45 psi/ft). These aquifers are always at or near hydrostatic pressure (open aquifers). This situation has been described by

Chapman in his 1973 textbook as a perfect flow barrier preventing any long-range upward or downward fluid flow. Examples of this situation have been reported from the Anadarko and Williston basins.

In type "B" curves, successively deeper aquifers are under increasing excess pressure (confined aquifers). Within the aquitards the pressure gradient is unidirectional upward and may reach values where dp/dz is in the order of 100 kPa/m (5 psi/ft). When this pressure gradient reaches a critical value, the aquitard may be fractured, permitting periodical upward fluid releases as suggested by Hunt and others. Curves of this type are known from the east coast of Canada, the U.S. Gulf coast, and other places.

The difference of these curves is caused by the absence (A) or presence (B) of effective lateral seals. Where such seals are present, even the aquifers are externally confined and their normal pressure gradient (dp/dz ~ 10.5 kPa/m or 0.45 psi/ft) is displaced toward higher than normal values. Lateral seals may take the form of sealing faults, facies changes, and/or salt (shale) ridges.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)