The concentration of deep sea gas hydrates from downhole electrical resistivity logs and laboratory data

The concentration of gas hydrate at an Ocean Drilling Program Site on the c ontinental slope off the coast of Vancouver Island has been estimated using a combination of downhole electrical resistivity logs, resistivity and por osity of recovered core, and core pore fluid salinity. From a depth of 100 m to the base of gas hydrate stability (bottom-simulating reflector, or BSR ) at about 225 m, the log resistivities have an average of about 2.0 ohm m, compared to 1.0 ohm m at a nearby deep sea reference site where no gas hyd rate is present. The downhole high resistivities result from a combination of (1) high-resistivity hydrate filling part of the pore space, and (2) low -salinity in situ pore water. The low-salinity fluids are probably produced at greater depths by hydrate dissociation as the base of the stability fie ld has moved upward with time. Both the hydrate concentration and the in si tu salinity can be calculated if the hydrate concentration vs resistivity r elation is known. Assuming that the effect of hydrate may be approximated b y porosity reduction, the relation is given by Archie's Law with exponent a bout two. The hydrate affects the core pore fluid salinity through the dilu tion of the pure water produced by hydrate dissociation upon core recovery. The core fluid salinity results from a combination of this dilution and of in situ freshening. The in situ pore fluid salinity in the hydrate zone ab ove the BSR is calculated to be about 80% of seawater whereas the measured salinity in the recovered cores is 60% of seawater. The computed hydrate co ncentration in the 100 m interval above the BSR is 25-30% of pore space (12 -15% of sediment volume), in general agreement with the 20% estimated from the velocity data. Crown Copyright (C) 1999 Published by Elsevier Science B .V.