15. PACKER EXPERIMENTS ALONG THE DÉCOLLEMENT OF THE BARBADOS ACCRETIONARY COMPLEX: MEASUREMENTS OF IN SITU PERMEABILITY ¹

Andrew T. Fisher ² and Gretchen Zwart ²

ABSTRACT

The first in situ measurements of permeability and fluid pore pressure along the décollement zone (a low-angle detachment fault) between the North American and Caribbean Plates were completed during Ocean Drilling Program Leg 156. Measurements of properties within the fault zone were attempted at two sites using a drill string packer. Interpretation of the packer data was complicated by variations in the baseline formation fluid pressure during the tests, as well as temporal and spatial variations in formation properties. These measurements may be interpreted to reflect a consistent relationship between bulk permeability and vertical effective stress and between bulk permeability and the modified pore-pressure ratio. Measured bulk permeabilities from the most reliable tests varied from 6 ´ 10^–16 m² to 1 ´ 10^–13 m², whereas vertical effective stress varied from 1.7 to 0.1 MPa and the modified pore-pressure ratio varied from 0.45 to 0.95. These bulk permeability values and trends are consistent with a variety of independent, less direct estimates. These trends may help to explain the transient nature of prism dewatering and fluid flow along faults in this setting.

¹ Shipley, T.H., Ogawa, Y., Blum, P., and Bahr, J.M. (Eds.), 1997. Proc. ODP, Sci. Results, 156: College Station, TX (Ocean Drilling Program).
² Earth Sciences Department, University of California, Santa Cruz, Santa Cruz, CA 95064, U.S.A. afisher@earthsci.ucsc.edu