Thermodynamic state of hydrate-bearing sediments on continental margins around the world

In situ salinities within hydrate-bearing sediments in the gas hydrate stability zone (GHSZ) were calculated from Archie-derived water saturations at ODP Site 1249 at Hydrate Ridge, NGHP Site 01-10 in the Krishna-Godavari Basin, and IODP Site U1328 offshore Vancouver Island. At these sites, in situ salinities are higher than those of seawater (~550 mM Cl-) and reach maximum values of 2520, 2190, and 2160 mM Cl- at Sites 1249, 01-10, and U1328, respectively. At Sites 1249 and 01-10, the in situ salinities reach the three-phase equilibrium boundary, as defined by the in situ pressures and temperatures for each site, and tend to increase towards the seafloor. These hydrate system are near three-phase equilibrium for a large portion (50 - 85 percent) of the GHSZ. As Site U1328, however, calculated in situ salinities are not elevated to the three-phase boundary, indicating that this system is not near three-phase equilibrium. We determined the in situ hydrate saturation using Logging-While-Drilling data in an iterative application of Archie's Law. The in situ salinities were calculated through a volumetric relationship between water saturation and the core-derived salinities. The salinity required for three-phase equilibrium was determined using an equilibrium thermodynamic model for methane hydrate. We examined the in situ salinities of hydrate-bearing sediments around the world to gain understanding into the connection between thermodynamic state and the possibility of hydrate dissociation as a result of fluctuating in situ conditions. The in situ salinities at the study sites indicate that Sites 1249 and 01-10 would be more sensitive to changing in situ conditions than Site U1328 and therefore more prone to mass dissociation of hydrate.