Janik, Aleksandra¹, D Goldberg¹, D. Moos², J. Sheridan², P. Flemings³, J. Germaine⁴, B. Tan⁴ 
(1) Lamont Doherty Earth Observatory, Palisades, NY 
(2) GeoMechanics International, 
(3) Pennsylvania State University, PA
(4) Massachusetts Institute of Technology, 

ABSTRACT: Constraints on the Strength of the Gas Hydrate-Rich Sediments from Borehole Breakouts - Implications for Slope Stability Near Hydrate Ridge on the U.S. Continental Margin Off-Shore Oregon

To understand the potential impact of the gas hydrates dissociation on the slope stability along the north-west continental margin of the United States we investigate the borehole breakouts observed in the boreholes drilled by the Ocean Drilling Program (ODP) in gas hydrate-rich formation near Hydrate Ridge off the coast of Oregon. Breakouts are the stress-induced features 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. Borehole breakouts were detected using an electrical resistivity logging-while-drilling tool (RAB) in images of the borehole wall, and they occur at two ODP sites located close to the summit on the west and east flanks of the ridge, respectively; and at one site to the east of the ridge. No breakouts were observed at sites west of Hydrate Ridge. 
Based on the orientation of the breakouts, the direction of the in situ maximum horizontal stress (SHmax) can be determined. Moreover, in the vicinity of the ridge summit, the precise measurements of the breakouts width in conjunction with the preliminary results of the laboratory tests on core samples provide constraints on SHmax and Shmin (the minimum horizontal stress) magnitudes at the depths where breakouts occur. Assuming that the SHmax/Shmin ratio remains constant in seafloor sediments above the breakouts, a lower limit of in situ formation strength can be estimated at these shallower depths, where the gas hydrate occurs. Additional one-dimensional consolidation and consolidated-undrained triaxial tests on whole core samples are underway to further constrain the strength of the hydrate bearing sediments. 
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.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.