Collett, Timothy S.1, David S. Goldberg2, Aleksandra Janik2,
(1) U.S. Geological Survey, Denver, CO
(2) Lamont Doherty Earth Observatory, Palisades, NY
ABSTRACT: Occurrence of Gas Hydrate and Associated Free-Gas Accumulations on Hydrate Ridge, Offshore Oregon, USA
Logging while drilling (LWD) tools were deployed at eight (Site 1244-1150) of the nine sites drilled on Hydrate Ridge during Ocean Drilling Program (ODP) Leg 204. The LWD tools used during Leg 204 included Schlumberger’s resistivity-at-the-bit (RAB-GVR) tool, the magnetic resonance while drilling (geoVision) 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. 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. Thus, the Archie derived gas hydrate saturations at the crest of the ridge exceed 90%. Combined Archie and porosity (NMR, density, and neutron) log analysis also reveal the occurrence of free-gas below the gas hydrate stability zone on Hydrate Ridge, with free-gas saturations exceeding 50% in the “Horizon A” turbidite sequence.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.