Abstract:
ODP Leg 164 will be devoted to refining our understanding of the in situ characteristics of natural gas hydrates. The program involves drilling four sites to 750-m depths on the Blake Ridge that extend down through the zone where gas hydrates are stable and into the sedimentary section below. Short holes (50 m) will also be drilled at three sites on the crests of two diapirs on the Carolina Rise where gas hydrate-bearing sedimentary sections have been disturbed by the intrusion of diapirs. Because of the ephemeral nature of the gas hydrates, emphasis will be placed on downhole measurements and sampling strategies that allow the in situ conditions of the gas hydrates to be reconstructed. The objectives of Leg 164 include 1) assessing the amounts of gas trapped in extensively hydrated sediments, 2) contributing to an understanding of the lateral variability in the extent of gas hydrate development, 3) refining the understanding of the relationship between bottom-simulating reflectors and gas hydrate development, 4) investigating the distribution and in situ fabric of gas hydrates within sediments, 5) establishing the changes in the physical properties (porosity, permeability, velocity, thermal conductivity, etc.) associated with gas hydrate formation and decomposition in continental margin sediments, 6) determining whether the gas captured in gas hydrates is produced locally or has migrated from elsewhere, 7) investigating the role of gas hydrates in the formation of authigenic carbonate nodules, 8) refining our understanding of chemical and isotopic composition of gas hydrates, 9) determining the gas composition, hydration number, and crystal structure of natural gas hydrates, 10) determining the role of gas hydrates in stimulating or modifying fluid circulation, 11) investigating the potential connection between major slumps and the breakdown of gas hydrate, and 12) establishing the influence of the Carolina Rise diapirs on the gas hydrates as well as the origin of the diapirs themselves.