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.