Abstract:
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 (sub o) ) can be related to the dynamic Young's modulus (E (sub D) ), where: C (sub o) = 1.5*10 (super -3) E (sub D) . The dynamic Young's modulus is computed using in situ Vp, Vs, and bulk density borehole logs. The C (sub o) profiles are compared to estimates of the in situ hydrate saturation, S (sub h) , calculated using electrical resistivity logs and the modified Archie formula: S (sub h) = 1 - (aR (sub w) /Rp (super m) ) (super 1/n) . We will present results of these comparisons from data collected during: Ocean Drilling Program Legs 204 (at Cascadia margin) and 164 (at Blake Ridge); the JIP gas hydrate drilling project in the Gulf of Mexico; and Mallik permafrost wells. In general, C (sub o) steadily increases downhole as sediments compact due to overburden. In the marine sediment environments, cohesion ranges from 500-2000kPa above the BSR, with a baseline gradient between 5 and 10 kPa/m. Preliminary results at Cascadia margin show that in sediments with S (sub h) > 15%, C (sub o) increases dramatically, at least 200kPa greater than the general trend of the downhole gradient. This suggests that C (sub o) is affected directly by S (sub h) , and may be related to the rate of change in S (sub h) (e.g. gradual or sharp) as a function of depth. Further study on the relationship between C (sub o) and S (sub h) may provide information on the growth habit of gas hydrates in sediment pore spaces.