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"Mobile Shale Basins - Genesis, Evolution and Hydrocarbon Systems"

June 5-7, 2006 -- Port-of-Spain, Trinidad and Tobago





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.

AAPG Search and Discovery Article #90057©2006 AAPG/GSTT Hedberg Conference, Port of Spain, Trinidad & Tobago