HR: 0830h
AN: OS51C-0876    [PDF]
TI: Downhole Log Assessment of Gas Hydrate and Free-Gas Concentrations on Hydrate Ridge
AU: * Collett, T S
EM: tcollett@usgs.gov
AF: U.S. Geological Survey, Denver Federal Center, Box 25046, MS-939, Denver, CO 80225 United States
AU: Goldberg, D S
EM: goldberg@ldeo.columbia.edu
AF: Lamont Doherty Earth Observatory, Columbia University, RT 9W, Palisades, NY 10964 United States
AU: Janik, A
EM: ajanik@ldeo.columbia.edu
AF: Lamont Doherty Earth Observatory, Columbia University, RT 9W, Palisades, NY 10964 United States
AU: Guerin, G
EM: guerin@ldeo.columbia.edu
AF: Lamont Doherty Earth Observatory, Columbia University, RT 9W, Palisades, NY 10964 United States
AB: The downhole-logging program on ODP Leg 204 was designed to assess the occurrence and concentration of gas hydrates on Hydrate Ridge. Logging while drilling (LWD) tools were deployed at eight (Site 1244-1150) of the nine sites drilled on southern Hydrate Ridge. LWD tools measure in-situ formation properties with instruments located in the drill collars immediately above the drill bit. The LWD tools used during Leg 204 included the resistivity-at-the-bit (RAB-GVR) tool, the magnetic resonance while drilling (MRWD) tool, and the azimuthal density-neutron (VDN) tool. The downhole log inferred distribution of gas hydrate beneath Hydrate Ridge and the adjacent slope basin is heterogeneous, with downhole RAB images showing gas hydrate occupying fractures and occurring as a disseminated pore-filling material within flat lying stratigraphic units with thicknesses varying from several centimeters to as much as 10 m. It has been shown that it is possible to obtain gas-hydrate saturations (percent of pore space occupied by gas hydrate) by using the Archie relation to analyze the electrical resistivity of gas-hydrate-bearing sediments. In this study, the Archie relation was used with resistivity data from the GVR tool and porosity data from the VDN density tool to calculate water saturations at all eight LWD log sites on Hydrate Ridge. The Archie relation yielded water saturations ranging from low values near the crest of the ridge, of about 10% in Hole 1249A, to a more common value of about 90% along the flanks of the ridge. It is generally accepted that with the zone of gas hydrate stability, gas hydrate saturations (Sh) are the mathematical complement of Archie derived water saturations (Sw), with Sh=1-Sw. Thus, the Archie derived gas hydrate saturations at the crest of the ridge exceed 90%. It should be noted that the Archie relation cannot distinguish between the occurrence of free-gas and gas hydrate. But, density, neutron, and NMR log derived porosity data can be used to differentiate gas-hydrate from free-gas-bearing sediments. At Site 1249, the density and neutron log-derived porosities are generally higher than the core-derived porosities. However, the NMR porosities are lower than the core-derived porosities. The apparent increase in density and neutron log derived porosities can be attributed to the occurrence of gas hydrate, as can the apparent decrease in NMR porosities. The affect of gas hydrate on density and neutron-derived porosities is relatively small. But NMR porosities are more significantly affected by the presence of gas hydrate. Neutron porosity logs are also affected by the presence of free-gas (yielding apparent low porosity measurements), but the downhole neutron porosity logs did not reveal the presence of any free-gas above the depth of the BSR on Hydrate Ridge. However, the combined Archie and porosity log analysis did reveal the occurrence of free-gas below the BSR on Hydrate Ridge, with free-gas saturations exceeding 50% in the "Horizon A" turbidite sequence.
DE: 4255 Numerical modeling
DE: 4806 Carbon cycling
DE: 4820 Gases
DE: 4825 Geochemistry
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting