Sensitivity to Gas Hydrate Related Deformation

The potential for gas hydrate to alter the mechanical properties of sediment is not uniformly distributed with water depth. Obviously, the proper temperature and pressure conditions and the presence of gas are required. Gas hydrates are typically stable in continental margin sediments in a zone that can be viewed as a seaward-thickening prism (Figure 1). This prism has a distinct up-slope limit. The water column temperature structure associated with most mid- and low-latitude regions is such that the upper limit of methane hydrate stability occurs at 500-700 m water depths. Below this level methane hydrates are stable at the seafloor and remain stable throughout a zone that increases in thickness with increasing water depth. However, where there is the presence of even small amounts of other gas hydrate-forming gases that leak up from great depths (e.g. ethane, propane, hydrogen sulfide, carbon dioxide, etc.), significant shallowing of the minimum hydrate forming depth is possible. Such chemically driven shallowing of the gas hydrate occurrence zone is particularly relevant in known petroleum generating areas like Gulf of Mexico.

The sediments that are close to the up-dip limit of the gas hydrate occurrence zone have the greatest potential to be altered by the effects of gas hydrate decomposition. Gas hydrates in this area are sensitive to short term transient thermal perturbations in bottom water temperature, and thus more likely to have experienced repeated episodes of gas hydrate formation and decomposition. However, as the water depths increase, the thickness of the sediments that overly the gas hydrate phases boundary also increases (figure 1). Because temperature changes are primarily transferred through the sediments by thermal conduction, it takes thousands of years for seafloor-warming events to propagate through the sediments and alter the position of the phase boundary. Thus, the effects of transient thermal events become dampened and/or cancelled with increased water depth.

The sediments that are at the up-dip end of the gas hydrate stability field, near where they outcrop at the seafloor, are also subject to the largest potential pore pressure alterations when gas hydrate decompose. This is because the ambient pressure controls the relative volume of gas that will be generated when gas hydrate decomposes (Figure 1). For example, at a water depth of 650-m along the US Atlantic seaboard, which is close to the seafloor intercept of the gas hydrate phase boundary, there is a 2.84-fold net volume increase going from gas hydrate to gas bubbles plus water. In contrast, when gas hydrate decomposes at a total depth of 4-km below sea level, the net increase in volume is only 1.16 fold.

If gas hydrate decomposition is a significant factor in generating slope instabilities, we predict that the frequency of sediment failures should be focused at or just below the up-dip limit of gas stability within seafloor sediments for three reasons: (1). Neither gas hydrate nor gas hydrate-induced deformation will occur at shallower depths. (2) The sediments near and below the seafloor intercept of the gas hydrate phase boundary may have experienced the greatest number of gas hydrate growth and decomposition events because they are most susceptible to seafloor temperature changes, and (3) The potential volume increases associated with gas hydrate decomposition increases toward the shallow water limit of the gas hydrate occurrence zone.

Figure 1. Diagram showing the distribution of gas hydrate stability beneath the ocean floor. The dark line indicates the seafloor. The lighter line is the position of the gas hydrate phase boundary, (which is presumably coincident with the BSR if one is present). The shaded area between the two lines indicates that region where gas hydrate is stable within the sediments. Boxes show the relative volume change associated with a fixed volume of gas hydrate decomposing into water and methane gas(also see Max and Dillon, 1998). Note that the relative volume change decreases with increasing water depth. Diagram is constructed assuming a 30° C/km geothermal gradient and hydrography similar to the U.S. Atlantic margin north of Cape Hatteras.

Oil Well Geothermal Gradient

Figure 1. Diagram showing the distribution of gas hydrate stability beneath the ocean floor. The dark line indicates the seafloor. The lighter line is the position of the gas hydrate phase boundary, (which is presumably coincident with the BSR if one is present). The shaded area between the two lines indicates that region where gas hydrate is stable within the sediments. Boxes show the relative volume change associated with a fixed volume of gas hydrate decomposing into water and methane gas(also see Max and Dillon, 1998). Note that the relative volume change decreases with increasing water depth. Diagram is constructed assuming a 30° C/km geothermal gradient and hydrography similar to the U.S. Atlantic margin north of Cape Hatteras.

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