KOREA

Since the report of the presence of brines associated with massive gas hydrate deposits on Hydrate Ridge (Oregon), there have been additional observations of pore fluids highly enriched in dissolved chloride at sites of massive gas hydrate occurrence in northern Cascadia accretionary margin (Canada), the Krishna-Godavari Basin (India) and the Ulleung Basin (Korea). Dissolved chloride (up to 1440 mM in the Ulleung Basin) generally extends from near-seafloor (~1 mbsf) to depths of ~100 mbsf. Below the depth of chloride maxima, chloride values approach concentrations that are lower or equal to seawater values, with minor negative chloride anomalies superimposed on baseline that reflect discrete gas hydrate bearing horizons. The Ulleung Basin and northern Cascadia sites were all drilled on seismic acoustic chimneys, indicative of methane transport in the gas phase. None of these sites, however, show any evidence for the formation of a salinity front that can shift the thermodynamic equilibrium and sustain gas transport through the gas hydrate stability front, as postulated by current models. More likely, hydrofracturing and critical gas pressures below the GHSZ support the gas transport. The composition of gases sampled after gas hydrate dissociation from massive shallow deposits is similar to those o