HR: 0800h
AN: OS21A-1507    [Abstracts]
TI: In-situ pressure measurements in the Ursa Basin, Northeast Gulf of Mexico
AU: * Long, H
EM: hlong@geosc.psu.edu
AF: Penn State University, Dept. of Energy & Geo-Environmental Engineering, University Park, PA 16802 United States
AU: Flemings, P B
EM: flemings@geosc.psu.edu
AF: Penn State University, Department of Geosciences, University Park, PA 16802 United States
AU: Dugan, B
EM: dugan@rice.edu
AF: Rice University, Dept. of Earth Sciences 6100 Main St., Houston, TX 77005 United States
AU: Germaine, J T
EM: jgermain@mit.edu
AF: Massachusetts Institute of Technology, Dept. of Civil & Environmental Engineering Massachusetts Ave., Room 1-353, Cambridge, MA 02139 United States
AU: Behrmann, J H
EM: jan.behrmann@geologie.uni-freiburg.de
AF: Albert-Ludwigs-Universitat Frieburg, Geologisches Institut Albertstrasse 23b, Frieburg, 79104 Germany
AU: John, C M
EM: cjohn@pmc.ucsc.edu
AF: University of California - Santa Cruz, 1156 High Street, Santa Cruz, CA 95064 United States
AB: In-situ pressure measurements document shallow overpressures within mudstones approximately 150 meters below the seafloor (mbsf) in the Ursa Basin. Two penetrometers were used to measure the in-situ pressure: the Davis Villinger Temperature and Pressure Probe (DVTP-P) and the Temperature 2 Pressure (T2P) probe. We made 21 T2P and 18 DVTP-P penetrations at Sites U1322 and U1324 that were drilled to 234.5 and 608 mbsf, respectively. Interpretation of the good and fair deployments reveal that the overpressure is 40-50% of the vertical hydrostatic effective stress at Site U1322 (below 135 meters), and at Site U1324 (below 200 meters). The estimated coefficient of consolidation is 2.5x10^-7 to 7.8x10^-7 m²/s. Pore pressures induced by the probes did not dissipate to in-situ pressure by the end of deployment (within 90 minutes) for either tool. The T2P tip pressure declined much faster, and converged much closer to in-situ pressure than either the T2P shaft pressure or the DVTP-P pressure did. Theoretical analysis of the process of probe insertion reveals that the tool geometry dictates the spatial distribution of the induced pressure. The subsequent dissipation successfully predicts the observed behavior and suggests that, as consolidation proceeds, the tip pressure dissipation is retarded and forms a bench on the dissipation curve. Its long-term dissipation converges to the dissipation behavior of its above tapered section. The residual excess pressure on the bench is approximately 10% of the initial excess pressure at the tip of T2P. For the T2P deployments, in-situ pressure and the coefficient of consolidation are estimated by comparing the incomplete pressure dissipation with the model results.
DE: 1895 Instruments and techniques: monitoring
DE: 3045 Seafloor morphology, geology, and geophysics
DE: 3094 Instruments and techniques
DE: 5114 Permeability and porosity
DE: 8045 Role of fluids
SC: Ocean Sciences [OS]
MN: Fall Meeting 2005