Capillary
Sealing Mechanism in the Southern Anadarko
Basin, Oklahoma, USA
Walcott, Dean M.1, Madisen
Nnaoji1, Constatin Cranganu2 (1) Brooklyn College, Brooklyn, NY (2) Brooklyn College, Brooklyn, NY
The Anadarko Basin located in southwestern
Oklahoma is modeled as
“overpressure” hydrocarbon province. The overpressure model suggests that
sections within the Anadarko Basin contain extensive units
of reservoir rock exhibiting abnormal pore-fluid pressures which exceed the
hydrostatic norm. We now suggest that the capillary force generated by
gas-water interface between fine- and coarse-grained clastic rocks, acts as a
zero permeability barrier that prevents the normal escape of excess pore-fluid.
This new hypothesis makes two specific predictions which can be tested. The
first is that anomalous pressures are associated with the presence of gas. The
second is that ambient fluid (or gas) pressures should undergo rapid changes
across capillary barriers. Detecting capillary seals and estimating the
magnitude of their pressure sealing in the Anadarko Basin implies two main
aspects: (1) measuring the pore throat radius on selected fine-and
course-grained rocks and, (2) identifying the presence of the gas layers using
a suite of geophysical logs. The average pore throat radius for these fine and
coarse rocks was determined to be 4.8 x 10-8 and 3.8 x 10-7 m, respectively.
Further calculation based on previous results indicated that a capillary
sealing mechanism in the Anadarko Basin could produce
approximately 0.9 x 106 Pa of pressure. If we determine that the presence of
gas between interfaces is associated with high capillary pressures, then we can
infer that a plausible cause of creating and maintaining the overpressures in
the Anadarko Basin may be due to the
capillary mechanism.