We present an equilibrium model of methane venting through the hydrate stability zone at southern Hydrate Ridge, offshore Oregon. Free gas supplied from below forms hydrate, depletes water, and elevates salinity until pore water is too saline for further hydrate formation. This system self-generates local three-phase equilibrium and allows free gas migration to the seafloor. Log and core data from Ocean Drilling Program (ODP) Site 1249 show that from the seafloor to 50 m below seafloor (mbsf), pore water salinity is elevated to the point where liquid water, hydrate and free gas coexist. The elevated pore water salinity provides a mechanism for vertical migration of free gas through the regional hydrate stability zone (RHSZ). This process may drive gas venting through hydrate stability zones around the world. Significant amount of gaseous methane can bypass the RHSZ by shifting local thermodynamic conditions.

We present a discrete element model for the simulation, at the grain scale, of gas migration in brine-saturated deformable media. We account rigorously for the presence of two fluids in the pore space by incorporating grain forces due to pore fluid pressures, and surface tension between fluids. The coupled model permits investigating an essential process that takes place at the base of the hydrate stability zone: the upward migration of methane in its own free gas phase. We elucidate the way in which gas migration may take place: (1) by capillary invasion in a rigid-like medium; and (2) by initiation and propagation of a fracture. We find that the main factor controlling the mode of gas transport in the sediment is the grain size, and show that coarse-grain sediments favor capillary invasion, whereas fracturing dominates in fine-grain media. The results have important implications for understanding hydrates in natural systems. Our results predict that, in fine sediments, hydrate will likely form in veins that follow a fracture-network pattern, and the hydrate concentration in this type of accumulations will likely be quite low. In coarse sediments, the buoyant methane gas is likely to invade the pore space more uniformly, in a process akin to invasion percolation, and the overall pore occupancy is likely to be much higher than for a fracture-dominated regime. These implications are consistent with field observations of methane hydrates in natural systems.

PUBLISHER’S NOTES This report was prepared from shipboard files by scientists who participated in the cruise. The report was assembled under time constraints and does not contain all works and findings that will appear in the Initial Reports of the ODP Proceedings. Reference to the whole or to part of this report should be made as follows:

this report, the possible impact on global climate is one of several key issues related to methane hydrate currently being studied. Other issues include the emerging resource potential of methane hydrates, and the implications of methane hydrate on the safety of offshore facilities and on seafloor stability

Abstract not found

DOE Award DE-FC26-06NT43067

Multi-phase fluid flow is critical to the formation and concentration of gas hydrate in marine sediments. A transient, multi-phase (hydrate, gas and liquid) fluid and heat flow model is presented to describe hydrate formation in porous media. Fluid flux and physical properties of sediment largely control the dynamics of gas hydrate formation and free gas migration. In fine-grained sediments, hydrate formation leads to rapid permeability reduction and capillary sealing. Free gas accumulates below the hydrate layer until a critical gas column builds up, thereby forcing gas upward to the seafloor. In coarse-grained sediments, large volumes of gas are transported into the hydrate region to produce a significant change in salinity. An interconnected three-phase zone with high hydrate concentration and elevated salinity develops from the base of hydrate stability to the seafloor. Both processes may drive gas venting through the hydrate stability zone. We also extend these models to demonstrate that the likely impact of climatic warming events on marine hydrate reservoirs. If hydrates are originally formed in the two-phase region, dissociated methane cannot be released to the ocean until the warming at the seafloor exceeds a critical value. However, all of hydrates formed within the threephase

Gas hydrates and free gas, as indicated by the seismic proxy known as the BSR, are widespread on the Oregon continental margin. A number of geological, geochemical and geophysical studies have been conducted here in the past several years in preparation for deep drilling during Leg 204 of the Ocean Drilling Program, scheduled for summer 2002. In this presentation, we first present an overview of the seafloor morphology and reflectivity. We then discuss recent results from samples and measurements made at the seafloor. Finally, we discuss the subsurface plumbing as determined from a recent 3-D seismic survey and objectives of the planned drilling. Hydrate Ridge, known as Second Ridge prior to discovery in 1996 of abundant massive methane hydrate at the seafloor, is a 15-km-long northeast-trending accretionary ridge 80 km west of Newport, OR, that was formed by subduction of the Juan de Fuca plate beneath North America. The northern summit of the ridge is covered by a carapace of authigenic carbonate whereas the southern summit is mostly covered by sediment, with the exception of a single spectacular carbonate pinnacle on the SW flank (Clague et al., 2001; Torres et al., 1999, Johnson and Goldfinger, in prep). This has been interpreted to indicate that the northern summit is at a more mature stage in the evolution of a hydrate-bearing accretionary ridge (Trehu et al., 1999), a

This report was prepared from shipboard files by scientists who participated in the cruise. The report was assembled under time constraints and does not contain all works and findings that will appear in the Initial Reports of the ODP Proceedings. Reference to the whole or to part of this report should be made as follows: