HR: 11:50h
AN: OS42A-07    [Abstracts]
TI: Cohesive Strength of Gas-hydrate-bearing Marine Sediments
AU: * Cook, A E
EM: acook@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Borehole Research Group 61 Rt. 9W, Palisades, NY 10964 United States
AU: Goldberg, D
EM: goldberg@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Borehole Research Group 61 Rt. 9W, Palisades, NY 10964 United States
AB: We examine the relationship between gas hydrate saturation and the cohesive strength of marine sediments in a variety of continental margin settings. The cohesive strength (cohesion) is a fundamental physical property controlling sediment resistance to compressive failure. The cohesion (C_o), is typically defined by the uncompressive rock strength and the friction angle, but it can also be related to the dynamic Young's modulus (E_D), where: C_o = 1.5*10-3 E_D. The dynamic Young's modulus is computed using in situ V_p, V_s, and bulk density borehole logs. The C_o profiles are compared to estimates of the in situ hydrate saturation, S_h, calculated using electrical resistivity logs and the modified Archie formula: S_h = 1 - (aR_w/RΦ^m)^1/n.
We will present results of these comparisons from data collected during Ocean Drilling Program Legs at Cascadia margin (204 & 168) and Blake Ridge (164), the JIP gas hydrate drilling project in the Gulf of Mexico, and Malik permafrost wells. In general, at all the sites investigated, C_o steadily increases downhole as sediments compact due to overburden. In marine sediments, cohesion ranges from 500-2000kPa above the BSR, with a baseline gradient usually between 5 and 10 kPa/m. Preliminary results show at Cascadia margin that sediments with S_h > 15%, C_o increases dramatically, at least 200kPa greater than the general trend of the downhole gradient. This suggests that C_o is affected directly by S_h, and may be related to the rate of change in S_h (e.g. gradual or sharp) as a function of depth. Further study on the relationship between C_o and S_h may provide information on the growth habit of gas hydrates in sediment pore spaces.
DE: 0915 Downhole methods
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3004 Gas and hydrate systems
DE: 3022 Marine sediments: processes and transport
DE: 3036 Ocean drilling
SC: Ocean Sciences [OS]
MN: Fall Meeting 2005