Permeability, Fluid Pressure, and Effective Stress in an Active Plate Boundary Fault Zone: Observations and Models

Andrew T. Fisher and Gretchen Zwart

Hydrologic and geologic processes are inextricably linked in tectonically-active environments. This paper presents results of the first in-situ measurements of permeability and fluid pore pressure along the decollement zone (a low-angle detachment fault) between the North American and Caribbean plates. These measurements define a quantitative permeability-effective stress relationship for this plate-boundary fault zone, with changes in permeability of several orders of magnitude as fluid pressure varies. This relationship may help to explain the dynamics of fluid-fault interactions and the transient nature of hydrologic processes during deformation at convergent margins.

Measurements were completed at two sites at the toe of the Barbados accretionary complex using a mobile drilling platform. Following penetration of the decollement and emplacement of casing and screen, an inflatable drill-string packer was used to determine the hydrologic properties within this dynamic system. Interpretation of the tests is complicated because the background pressure within the isolated zone rose throughout the experiments. Analyses of the test results, following application of simple, linear pressure-time corrections to the raw data, reveal bulk permeabilities (k_b) within the decollement zone of 1 to 100 mDarcies, varying inversely with effective stress. Actual permeabilities along thin parts of the decollement zone may be considerably higher. Additional i sight is provided by models of the formation surrounding the borehole.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California