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Data report: a downhole electrical resistivity study of northern Cascadia marine gas hydrate1

Marc-André P. Chen,2, 3 Michael Riedel,4 George D. Spence,2 and Roy D. Hyndman2, 3


Downhole electrical resistivity measurements can be exploited for gas hydrate concentration estimates. However, to do so requires that several assumptions be made, in particular about in situ pore water salinity and porosity. During Integrated Ocean Drilling Program Expedition 311, electrical resistivity was measured in four boreholes along a transect across the northern Cascadia margin, offshore Vancouver Island, Canada. Logging-while-drilling and conventional wireline logging data of resistivity, density, and neutron-porosity are used together with measurements of porosity and pore water salinity of the recovered core to systematically estimate gas hydrate concentrations at these four sites. Using Archie's law, empirical parameters a and m are determined from gas hydrate–free zones by means of a Pickett plot. The in situ salinity baseline trend for each site was estimated directly from the measured pore water salinity values, as well as indirectly by calculating the trend using Archie's law and simultaneously solving for gas hydrate saturation and in situ salinity. Results showed that the in situ salinities at Sites U1325, U1326, and U1329 were well determined from a smooth trend through the highest measured salinity values of the recovered core. Only Site U1327 exhibits strongly decreasing pore water salinity, reaching 22‰ at the base of the gas hydrate stability field. This regional low salinity requires special analyses to estimate gas hydrate concentration from resistivity and introduces a large uncertainty. It is suggested that the decreased salinity results largely (90%) from a deeper fresh water source with the remaining freshening being the result of dissociation of pervasive gas hydrate (~3% of the pore space). Considering estimates from density porosity to be the most accurate, gas hydrate saturations average ~9% ± 7% at Site U1326, ~10% ± 7% at Site 1325, and 11% ± 7% at Site U1327 over the entire range of gas hydrate occurrence. No significant gas hydrate is inferred at Site U1329, although small amounts may be present just above the bottom-simulating reflector. In two localized zones at Site U1326 (60–100 mbsf [meters below seafloor]) and Site U1327 (120–150 mbsf), significantly higher gas hydrate concentrations of >30% of the pore space were encountered.

1Chen, M.-A.P., Riedel, M., Spence, G.D., and Hyndman, R.D., 2008. Data report: a downhole electrical resistivity study of northern Cascadia marine gas hydrate. In Riedel, M., Collett, T.S., Malone, M.J., and the Expedition 311 Scientists, Proc. IODP, 311: Washington, DC (Integrated Ocean Drilling Program Management International, Inc.). doi:10.2204/iodp.proc.311.203.2008

2School of Earth and Ocean Sciences, University of Victoria, PO Box 3055 STN CSC, Victoria BC V8W 3P6, Canada.

3Geological Survey of Canada, Pacific Geoscience Centre, PO Box 6000, 9860 West Saanich Road, Sidney BC V8L 4B2, Canada.

4Department of Earth and Planetary Sciences, McGill University, 3450 University Street, Montréal QC H3A 2A7, Canada.

Initial receipt: 3 May 2008
Acceptance: 30 July 2008
Publication: 21 November 2008
MS 311-203