HR: 0830h
AN: OS51C-0874    [PDF]
TI: Stress Orientations and Constraints on the Magnitudes of In Situ Strength of Hydrate Bearing Sediments - Evidence from Borehole Breakouts at ODP Leg 204 Sites, Hydrate Ridge
AU: Goldberg, D
EM: goldberg@ldeo.columbia.edu
AF: Lamont Doherty Earth Observatory, BRG, Palisades, NY 10964
AU: * Janik, A
EM: ajanik@ldeo.columbia.edu
AF: Lamont Doherty Earth Observatory, BRG, Palisades, NY 10964
AU: Moos, D
EM: moos@geomi.com
AF: Geomechanics International, Main Headquaters, Palo Alto, CA 94306
AU: Sheridan, J
EM: judith@geomi.com
AF: Geomechanics International, Main Headquaters, Palo Alto, CA 94306
AU: Flemings, P
EM: flemings@geosc.psu.edu
AF: Pennsylvania State University, 307 Deike Building, University Park, PA 16802
AU: Chevalier, J
EM: johanna_chevallier@yahoo.com
AF: Oregon State University, COAS, Corvallis, OR 97331
AU: Johnson, J
EM: jjohnson@coas.oregonstate.edu
AF: Oregon State University, COAS, Corvallis, OR 97331
AU: Weinberger, J
EM: jlweinbe@ucsd.edu
AF: Scripps Institution of Oceanography, UCSD, San Diego, CA 92093
AU: Germaine, J
EM: jgermain@mit.edu
AF: Massachusetts Institute of Technology, Dept. of Civil and Envi. Engg., Cambridge, MA 02139
AU: Tan, B
EM: brain@mit.edu
AF: Massachusetts Institute of Technology, Dept. of Civil and Envi. Engg., Cambridge, MA 02139
AB: Borehole breakouts are observed in the boreholes drilled in gas hydrate-rich marine sediments near Hydrate Ridge off the coast of Oregon. These stress-induced features are expressed as elongations in the cross sectional shape of the borehole, caused by compressive failure of the formation under differential horizontal stresses that exceed the formation's in situ strength. Breakouts were detected using resistivity-at-the-bit tool (RAB) in images of the borehole wall. Breakouts occur at ODP Sites 1244 and 1245, located close to the summit on the west and east flank of the ridge, respectively; and at ODP Site 1251 to the east of the ridge. No breakouts were observed at sites west of Hydrate Ridge. Breakouts indicate that the direction of the maximum horizontal stress, SHmax, at Site 1244 is NE-SW (ridge sub-parallel) and NNW-SSE at Site 1245 (also ridge sub-parallel). These directions correspond to the strike of mapped subsurface normal faults within 2 km of the holes and are aligned with the strike of Hydrate Ridge. The SHmax direction at Site 1251, located further away from the ridge, is ESE-WNW and coincides with the direction of the strike of the regional Alvin Canyon Fault and is roughly perpendicular to the front of the subduction backstop to the east (Siletz Terrain). In the vicinity of the ridge summit (Sites 1244 and 1245), the presence of breakouts in the RAB images and laboratory tests on core samples provide constraints on SHmax and Shmin (minimum horizontal stress) at the depths where breakouts occur. Assuming that the SHmax/Shmin ratio remains constant in shallow seafloor sediments above the breakouts, a lower limit of in situ formation strength can be estimated at these shallower depths; this interval contains gas hydrate. Understanding the in situ strength of the host formation and the impact of hydrate dissociation within the gas hydrate stability zone is critical to establishing the stability of the slope and possible triggering of landslides in some submarine settings.
DE: 0915 Downhole methods
DE: 8168 Stresses--general
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting