Gas Movement In Sediments Mechanisms And Models 31 Driving forces for flow

The concentration of methane in pore waters generally increases with depth in the sediment due to a combination of in situ and deep thermogenic sources, methane recycling at the base of the hydrate zone and the thermodynamic equilibrium in the two phase region (Fig. 4). Free methane is strongly buoyant: even beneath 3-4 km of water its density is only 200-300 kg rrf3 compared with 1024 kg m"3 for seawater. In most shallow gas systems these two factors contribute to a dominant flux of methane and associated gases towards the seabed. Water saturated with dissolved methane is also lighter than normal pore water (Park et al. 1990), and overpressure may exist at depth as a consequence of disequilibrium compaction or through the process of hydrocarbon generation itself (Hedberg 1974, Hunt 1997), so that additional mechanisms can drive methane-charged waters from depth.

Methane solubility in pore water, moles per litre x 1000

Methane solubility in pore water, moles per litre x 1000

Mechanisms Disequilibrium Compaction

Figure 4. Gas solubility in the hydrate zone from Tohidi et al. (1997). Within the hydrate stability zone, methane is partitioned from the liquid phase into the hydrate phase (Handa 1990). Penetrating further upwards into the hydrate stability zone progressively less methane remains in solution. Thus there is a concentration gradient which exerts a dominant control on methane fluxes and distributions (Rempel and Buffet 1998, Xu and Ruppel 1999).

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