Pressure
Stratigraphy: A Useful Concept for Pressure Seal Analysis
Williams, Kenneth E.1, Joel
Gevirtz1 (1) Knowledge Systems, Inc,
Geopressure is a vertically-changing
phenomenon. Therefore, it is useful to describe it in stratigraphic terms. The
fundamental unit of pressure stratigraphy is a coupled pressure seal and
pressure compartment; and is a concept for improving accuracy and speed of
pressure seal analysis for well design, reservoir risk assessment, and prospect
selection.
Seal/compartment pressure units are
characterized by contrasting permeability. They are not necessarily dependent
upon lithology or traditional notions of a stratigraphic boundary. Instead,
they are the result of a complex interplay between the sources of overpressure
and the dissipation of that pressure over geologic time.
Pressure within seal intervals can
increase or decrease depending on the pressure in the adjacent compartments.
When pressure increases at a faster rate than overburden is added, the change cannot
be attributed to undercompaction disequilibrium. Instead, other unloading
phenomena, such as hydrocarbon generation or clay diagenesis, must be present.
The most common reason for this high rate of change across the seal may be
pressure dissipation from formation water draining out of the compartment via
bleed-off into a more permeable unit.
An example of a pressure stratigraphic
unit is a shale/sand pair, where the concept provides insight into pressure
mechanisms and the relationship between seal and compartment pressures.
Pressure in the sand can rise when fluids enter the sand at a rate faster than
the shales can accommodate. If the couple is reversed, then normal pressure is
maintained or released because the sand can accommodate the dewatering of the
shale.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California