Subsurface Fluid Compartments

POWLEY, DAVID E., Tulsa, OK

In most deep sedimentary basins in the world there is a layered arrangement of at least two superimposed hydraulic systems. The shallowest hydraulic system can extend to great depths; however, in many basins it extends from the surface down to about 10,000 ft (greatest historical depth of burial) in normal geothermal gradient basins and to slightly greater depths in cool basins.

The shallow hydraulic systems are basin wide in extent and exhibit normal pressures. The pore water apparently is free to migrate; however, the usual rate of movement, below the uppermost few hundred feet, is so slow that motion is surmised rather than detected.

The deeper hydraulic systems usually are not basin wide in extent and exhibit abnormal pressures. They generally consist of a layer of individual fluid compartments that are sealed off from each other and from the overlying system.

The individual compartments in the compartmented layer are like huge bottles. Each one has a thin, essentially impermeable, outer seal and an internal volume that exhibits effective internal hydraulic communication. The rate of increase in pressure with increasing depth within the internal volume is in direct accordance with the density of the internal fluids.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)