AAPG HEDBERG CONFERENCE
"Mobile Shale Basins - Genesis, Evolution
and Hydrocarbon Systems"
June 5-7, 2006 -- Port-of-Spain, Trinidad and
Tobago
THE CAPILLARY SEALING
MECHANISM IN THE ANADARKO BASIN
OKLAHOMA, USA
Dean Mark Walcott
Brooklyn College of
the City University
of New York
The
Anadarko Basin
located in southwestern Oklahoma
is modeled as a “geopressure” or “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. Past research in the Anadarko
Basin attempting to
explain the origin and dissipation of overpressurized
fluids over extended timescales has resulted in the evaluation of two leading
hypotheses or models. The first model proposed that the present day abnormally
high pore-fluid pressures in the Anadarko
Basin were a remnant of
Paleozoic compaction disequilibrium preserved for 250 million years. A second
hypothesis proposed to account for overpressures in the Anadarko Basin
is the hydrocarbon or “gas” generation model. Both hypotheses failed to explain
calculations and observations made in the field. In this study, we propose to
test a new model based on gas capillary sealing to account for observed geopressure regimes in the present-day Anadarko Basin.
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 (Gamma Ray, Neutron Porosity, Density
Porosity, Caliper and Photoelectric logs) and other
recorded data. Mercury Intrusion Porosimetry (MIP)
measurements were conducted on 62 fine- and course- grained rock samples from
the northern overpressure sections of the deep Anadarko Basin.
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. In the studied area, five wells containing
multiple gas-bearing layers were identified based on interpretation and
correlation of geophysical logs from the Roger Mills County, Oklahoma. 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. This figure represents
pressure confined to only a single gas-water interface. The proposed model
requires the presence of gas-bearing layers interbedded
into shale/sandstone rock interfaces. 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.