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 continental slope off the coast of Vancouver Island has been estimated using a combination of downhole electrical resistivity logs, resistivity and porosity 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 hydrate 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 field has moved upward with time. Both the hydrate concentration and the in situ salinity can be calculated if the hydrate concentration vs resistivity relation is known. Assuming that the effect of hydrate may be approximated by porosity reduction, the relation is given by Archie's Law with exponent about two. The hydrate affects the core pore fluid salinity through the dilution 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 above 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 concentration 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.