Implications For Hydrate Habitat 41 Bottom Simulating Reflectors

One of the key questions relating to prediction of hydrate distribution and quantity is the nature of the interface at the base of the hydrate stability zone. Wherever a strong a bottom simulating reflector occurs, it is generally considered that free gas is present below, and that this marks the position of the three phase boundary (Pecher et al. 1998 and refs therein); though other models have been considered Brown et al. (1996).

The free gas at low saturations in the pore space will most likely be trapped as a mist of isolated bubbles, that will be buried with the sediment. Henry et al. (1999) suggest that a threshold saturation necessary for gas movment would typically be about 20%, which represents the percolation threshold for buoyant separate phase movement. This obviously impedes recycling of gas into the hydrate zone (Paull et al. 1994). Pecher et al. (1998) among others note that the free gas responsible for accentuating most geophysically mappable BSRs probably comes from melting of prexisting hydrate rather representing a trapped pool that is sourced from below. This inference is based on the finding that on the Central and South American margins BSRs are largely confined to geological units that are downwarping tectonically or subsiding due to continued sedimentation. Very importantly, in this scenario the amount of gas is tied to the amount of hydrate that has melted to source the gas (see Dickens et al., 1997; Henry et al. 1999).

In the (perhaps rare) instances that gas does come from below, it will form a mist rather than buoyant column of high saturation except where the flux is concentrated. This makes for an overall prevision of low saturations of gas and small, discontinous pockets rather than extensive BSRs in such settings (e.g. the

Niger Delta, Hovland et al. 1997). Indeed if the gas flux is small, the pore fluid may never reach supersaturation at any depth below the hydrate stability zone. Hydrate will only precipitate higher in the section as "solubility" drops due to increasing thermodynamic stability of the clathrate.

Rempel and Buffett (1998) and Zhu and Ruppel (1999), using computation reaction-transport models of different flavours showed how important was the flux of methane from below to the location and position of both the BSR and to the concentration profile. These works show that if there is a small gas flux then a gas- BSR will not form for the reason given above, and the greatest depth of hydrate occurrence will be encountered at a higher level in the sediments than the base of thermodynamic stability that is predicted from the phase diagram if methane is abundant.

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