HR: 0830h
AN: NS41B-13    [Abstracts]
TI: Capillary Sealing and Overpressure Regime in the Anadarko Basin
AU: * Cranganu, C
EM: cranganu@brooklyn.cuny.edu
AU: Villa, M A
EM: mava1978@hotmail.com
AB: The Anadarko Basin in southwestern Oklahoma is known to contain today areas of extensive overpressures (pressures higher than hydrostatic pressure). Explaining the origin and maintenance of overpressured pore-fluids in the basin over long periods of time cannot be achieved by invoking classical, common causes, such as compaction disequilibrium or gas generation. We propose a capillary sealing mechanism that is responsible for both generating and maintaining almost all overpressure observed today in the Anadarko Basin. Capillary sealing occurs in a sedimentary basin when capillary forces act at gas-water interfaces between coarse- and fine-grained clastic rocks. Detecting capillary seals and estimating the magnitude of their pressure sealing implies two main aspects: (1) measuring the pore throat radius of coarse- and fined-grained clastic rocks, and (2) detecting the presence of gas-bearing layers using geophysical logs and other data. Measurements by injecting mercury into rock pores allow estimation of the pore throat radii controlling the capillary sealing. 21 fine-grained rock samples from the Anadarko Basin were thus measured and the average pore throat radius was found to be 2.5 x 10^-8 m. The proposed model also requires the presence of gas-bearing layers interbedded into shale layers. Using a suite of geophysical logs from more than 100 wells, we were able to identify such gas-saturated layers in more than 50 wells. Further calculation indicates that a capillary sealing mechanism in the overpressured area of the Anadarko Basin may produce ~40 MPa of pressure, or ~80% of the maximum observed overpressure in the basin.
DE: 5199 General or miscellaneous
SC: Near-Surface Geophysics [NS]
MN: 2005 Joint Assembly