Gas Hydrate Formation and Accumulation

Gas hydrate formation within fluid discharge areas takes place under conditions of localized high fluid flow. Infiltration of hydrate-forming gas-rich water or free gas into the widespread base of the hydrate formation zone from below takes place through sediment porosity and in faults or other secondary porosity. Infiltration models are controlled by the geological and oceanographic environment, which provides the pressure gradients. These gas rich fluids and/or free gas must migrate into the root and stem of a mud volcano in order to form high fluid flow features. The soft-sediment deformation associated with mud volcanoes may be attributed to triaxial stress caused by gas-flow (Ginsburg & Soloviev, 1998).

Our hydrate formation models describe the solubility of hydrate-forming gas in seawater and where gas hydrate is present. In the presence of gas hydrate, although the distance over which this condition acts is uncertain, solubility of the hydrate-forming gas does not depend solely on hydrostatic pressure, but is strongly influenced by the pressure of gas hydrate crystallization equilibrium. Since both the temperature and equilibrium pressure decreases in the direction of the sea floor as gas-rich waters ascend, the solubility of gas in the water in equilibrium with hydrate also decreases (mainly because of decreasing pressure). These two conditions cause the hydrate to form from the gas-bearing water.

For gas hydrate accumulation in the fluid discharge areas, two basic mechanisms are known. First, hydrate can form from precipitation of water where a solution saturated moves to a zone of lower temperature) (Ginsburg & Soloviev, 1997). Second, hydrate can form in static pore water in the hydrate formation zone by reaction with percolating free gas that has migrated into its presence from hydraulicly subjacent zones (Soloviev & Ginsburg, 1997). In the case of a biphasic infiltration (both water and gas), which is commonly found at fluid discharge locations in the mud volcanoes, both mechanisms can be active.

The HSZ is defined by the pressure of hydrate-forming gas and by temperature (Fig. 4). The gas pressure is usually equal to the external pressure, which is, as a rule, the sum of the hydrostatic and lithostatic pressure.

Figure 4. Upper and lower limits of HSZ. Water depth of 500m (left) and 1000m (right). The most overcooling (difference between actual and equilibrium temperature, AT) is near the seafloor and generally increases with water depth.

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Figure 4. Upper and lower limits of HSZ. Water depth of 500m (left) and 1000m (right). The most overcooling (difference between actual and equilibrium temperature, AT) is near the seafloor and generally increases with water depth.

The top and bottom limits of HSZ are defined by the intersections of an equilibrium curve of hydrate dissociation and a curve of temperature distribution with depth. The upper limit of the zone is usually situated in the water column well above the sea floor. Gas hydrate within the seafloor is often most common near the lower limit of the HSZ. Overcooling or oversaturation of the water by gas (relative to equilibrium values) is required to initiate hydrate formation and hydrate accumulation.

The kinetically most favourable conditions for hydrate formation and accumulation (the greatest possible oversaturation or overcooling) occur in uppermost part of the sedimentary cover and on the sea floor. Where there is extensive fluid discharge from the seafloor, gas hydrate accumulations can be anticipated because of gas hydrate saturation levels.

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