Methane Hydrate Accumulation along the Western Nankai Trough

Abstract

A one‐dimensional gas hydrate accumulation model is developed to simulate the existence of hydrate in the Nankai Trough margin, where the total organic carbon content is extremely low. In order to study the accumulation of gas hydrate along a seismic profile, our model assumes an initial 1000 m thick turbidite sedimentation at the trench axis, and then the sediment experiences tectonic uplift to form the Nankai accretionary prism. Methane generated by microbial processes during the sedimentation is partly trapped in the pore space and partly migrated upwards due to compaction and subsequent porosity decrease at deeper levels. Upward methane gas flux from the deeper part (below 1000 m) is predicted by the model because the maximum thickness of the accreted sediment exceeds ten kilometers in the Nankai Trough accretionary prism. The basic geological parameters input to the model are those obtained from the ODP site 808, which was drilled at the lower inner trench slope. The methane hydrate stability zone is essentially controlled by bottom water temperature, regional heat flow and hydrostatic pressure. The simulation shows that methane hydrate will not accumulate in the Nankai Trough margin if only microbial methane generation is considered because the organic carbon content in the Nankai Trough area is as low as 0.75%. However, if we assume an upward flux of 5 kg of methane per square meters per 10,000 years, it will cause approximately 10 to 25% hydrate saturation under current physical conditions. The origin of the methane may be thermogenic and/or microbial. The resultant distribution of methane hydrate shows very good correlation with the distribution of the BSR.

Methane hydrate stability and anthropogenic climate change

“…Pore water flow can be focused by layers of high permeability in sediments (Hovland et al, 1997;Aoki et al, 2000;Flemings et al, 2002). Lateral flow steered by sediment permeability predicts expulsion of fluid near the base of the continental slope off New Jersey, consistent with the observed patterns of porewater seeps, and leading to nucleation of landslides from the base of the slope, consistent with the observation of submarine canyons on continental margins (Dugan and Flemings, 2000).…”

“…Seismic studies at Blake Ridge have observed the presence of bubbles along faults in the sediment matrix (Taylor et al, 2000). Faults have been correlated with sites of methane gas emission from the sea floor (Aoki et al, 2000;Zuhlsdorff et al, 2000;Zuhlsdorff and Spiess, 2004). Seismic studies often show "wipeout zones" where the bubble zone beneath the hydrate stability zone is missing, and all of the layered structure of the sediment column within the stability zone is smoothed out.…”

Numerical Studies on the Geomechanical Stability of Hydrate-Bearing Sediments

“…The host sediment is assumed to be weak and unconsolidated sediment, with S H = 0.1. Such HBS are typical of a widely occurring type of hydrate deposits (Moridis and Kowalsky 2006;Aoki at al. 2000), both in terms of texture (soft and structurally weak muds and clays are common at the ocean floor, with the cementing hydrates providing the only structural strength of the HBS) and hydrate saturation (Klauda and Sandler 2005).…”

Numerical Studies on the Geomechanical Stability of Hydrate-Bearing Sediments

“…Such a HBS is typical of a widely-occurring type of hydrate deposits 14,24 both in terms of texture (soft and structurally weak muds and clays are common at the ocean floor, with the cementing hydrates providing the only structural strength of the HBS) and hydrate saturation 25 . In this case we investigate the geomechanical behavior of oceanic HBS at or near the ocean floor, in which anchors or foundation of structures (such as offshore platforms) are installed.…”