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doi:10.2204/iodp.proc.311.109.2006

Conceptual model of gas hydrate formation

The formation of substantial amounts of gas hydrate requires large amounts of methane. Geochemical and isotope analyses have shown the methane to be mainly of biogenic origin on the Cascadia margin. Gas hydrate may be formed from biogenic methane that is generated locally at depths where pressure and temperature conditions are within the gas hydrate stability zone (GHSZ). However, there appears to be insufficient organic carbon in the Cascadia sediments in the section above the BSR to produce the estimated amounts of hydrate. Therefore, a model has been developed where the methane is generated over a great depth interval and gas hydrate is formed through removal of the methane from upward-moving pore fluids as they pass into the GHSZ (Hyndman and Davis, 1992) (Fig. F4). In this fluid expulsion model, the required biogenic methane may be generated throughout a thick sediment section extending well below the BSR. In its simplest form for regional gas hydrate, the upward fluid expulsion model predicts the highest concentration of gas hydrate just above the BSR, decreasing upward.

In the mid-slope region where the sediments are ~5 km thick, the rapid thickening of the sedimentary wedge results in sediment elements being moved to greater depths with little immediate porosity loss and associated velocity change. The sediments are therefore underconsolidated for their new depth, and high pore pressures are generated that are slowly dissipated through upward fluid expulsion (Hyndman et al., 1993). Fluid expulsion rates that reach ~1 mm/y appear to be sufficient to carry enough methane upward to the GHSZ to form as much as several tens of percent gas hydrate saturation. The second important process is that, subsequent to initial gas hydrate formation on the lower continental slope, the sediments are carried landward into the prism where there is shallower water depth and therefore pressure. As a result, the stability field must move upward because of pressure reduction from tectonic uplift and ongoing sediment deposition. The gas hydrate just above the BSR is therefore progressively moved out of the stability field and dissociated to free gas. The free gas in turn will migrate upward or be carried upward by the fluid expulsion to be reincorporated as gas hydrate above the newly re-formed, shallower BSR (e.g., Paull et al., 1994). This progressive accumulation of methane just above the BSR results in the maximum regional concentration of gas hydrate being expected not far above the BSR (e.g., Zatsepina and Buffett, 1998; Xu and Ruppel, 1999). The thickness and concentration of the free gas layer are expected to depend upon the permeability of sediments to rising free gas and the rate of fluid expulsion that carries the gas upward. For northern Cascadia, the permeability and fluid expulsion rate appears sufficient, in that there is only a thin layer a few tens of meters thick of free gas with average concentration estimated to be <1% (Desmons, 1996). In contrast, in the Blake Ridge area off eastern North America, the estimated gas concentrations are much higher (~5%) and much thicker (~100 m) (e.g., Paull et al., 1996). In that region, the hydrate is in finer grained sediments with probably lower permeability, and upward fluid expulsion rates are much lower. The free gas produced by the upward-moving base of the gas hydrate stability field, therefore, will remain beneath the BSR for a much longer time.

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