Log and core data document gas saturations as high as 90 % in a coarse-grained turbidite sequence beneath the gas hydrate stability zone (GHSZ) at south Hydrate Ridge, in the Cascadia accretionary complex. The geometry of this gassaturated bed is defined by a strong, negative-polarity reflection in 3D seismic data. Because of the gas buoyancy, gas pressure equals or exceeds the overburden stress immediately beneath the GHSZ at the summit. We conclude that gas is focused into the coarse-grained sequence from a large volume of the accretionary complex and is trapped until high gas pressure forces the gas to migrate through the GHSZ to seafloor vents. This focused flow provides methane to the GHSZ in excess of its proportion in gas hydrate, thus providing a mechanism to explain the observed coexistence of massive gas hydrate, saline pore water and free gas near the summit.

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.

Gas phase pressure at the base of the gas hydrate zone (GHZ) exceeds overburden stress at ODP Site 997 of the Blake Ridge, offshore South Carolina. Pressures predicted from porosity range from hydrostatic in the shallow sediments to significantly overpressured beneath the GHZ. The GHZ traps a free gas column of ~100 m at Site 997. A two-phase, steady-state flow model with pressure-dependent fracture permeability describes the relation between flux, pressure, stress and gas column height. In the model, both water and gas pressures are predicted to follow the lithostatic gradient. As water flux increases, water pressure converges on overburden stress, fractures dilate and the amount of free gas that can be trapped below the GHZ declines. The model is insensitive to changes in gas flux. The geochemically-estimated fluid fluxes (qw = 0.2 m 3 /m 2 /ky and qg = 6.8×10-4 m 3 /m 2 /ky) and the porosity-predicted pressure predict that a closely spaced fracture network (S = 0.3 mm) traps the gas column observed at Blake Ridge. 1

We conducted uniaxial compression tests on whole core samples from the Ursa Basin, deepwater Gulf of Mexico, to obtain the consolidation properties of mudstones. We showed that specific volume (v = 1+e) (where e is void ratio) declines as an exponential function of effective stress and we used this relationship to successfully predict in-situ pressure from void ratio at IODP Sites U1322 and U1324. This confirmed high overpressures near the seafloor at Ursa. Rapid sediment compression near the seafloor drives fluid flow, impacts overpressure, and ultimately controls land subsidence. A better understanding of this behavior will allow us to better image the subsurface acoustically, better predict subsurface pressure, and more completely understand the global water cycle.

Multi-phase flow modeling has the potential to illuminate the processes by which hydrocarbons are emplaced during secondary migration. An analytical and numerical model of multi-phase flow is presented to describe the relationship between secondary migration through fault zones and the concurrent charging of reservoirs in juxtaposition with these faults. The reservoir hydrocarbon column heights that result from this process are controlled by the petrophysical properties of the fault zone, the flux of water and hydrocarbons in the fault zone, and the geometry of the reservoir. At steady state, the capillary pressure between the fluid phases must be equal in the fault zone and the reservoir. This capillary pressure determines the hydrocarbon column height in the reservoir. Key results of the modeling are: 1) For a given oil flux, the presence of a dynamic water phase in the fault zone will decrease the oil column height in the reservoir. The amount of decrease will be directly dependent on the water potential gradient in the fault. 2) Three phase oil and gas column heights are a dynamic function

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

Geologists have invoked fault zones as both migration pathways and seals (Figure 1). For example Smith (1966) proposed that hydrocarbons are trapped within sands juxtaposed against low permeability fault zones. Schowalter (1979) suggested that the capillary properties of sealing material could be used to predict the hydrocarbon column height of

May 2002I grant The Pennsylvania State University the non-exclusive right to use this work for the University’s own purposes and to make single copies of the work available to the public on a not-for-profit basis if copies are not otherwise available.

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