|Titre||Expedition 311 synthesis: scientific findings|
|Auteur||Riedel, M; Collett, T S; Malone, M|
|Source||Proceedings of the Integrated Ocean Drilling Program vol. 311, 2010 p. 1-28, https://doi.org/10.2204/iodp.proc.311.213.2010 (Accès
|Séries alt.||Secteur des sciences de la Terre, Contribution externe 20100041|
|Éditeur||Integrated Ocean Drilling Program|
|Document||publication en série|
|Media||papier; en ligne; numérique|
|Province||Région extracotière de l'ouest|
|Lat/Long OENS||-127.0000 -124.5000 49.0000 44.5000|
|Sujets||hydrocarbures; capacité de production d'hydrocarbures; gaz; hydrate; méthane; combustibles fossiles; géologie marine|
|Illustrations||location maps; tables; seismic profiles; photographs; plots|
|Programme||Hydrates de gaz, Caractérisation des Hydrates de gaz|
|Diffusé||2010 07 10|
|Résumé||(disponible en anglais seulement)|
Integrated Ocean Drilling Program Expedition 311 was conducted to study gas hydrate occurrences and their evolution along a transect spanning the entire
northern Cascadia accretionary margin. A transect of four research sites (U1325, U1326, U1327, and U1329) was established over a distance of 32 km, extending from Site U1326 near the deformation front to Site U1329 at the eastern limit of the
inferred gas hydrate occurrence zone. In addition to the transect, a fifth site (U1328) was established at a cold vent setting with active fluid and gas expulsion, which provided an opportunity to compare regional pervasive fluid-flow regimes to a
site of focused fluid advection. In this synthesis, a revised gas hydrate formation model is proposed based on a combination of geophysical, geochemical, and sedimentological data acquired during and after Expedition 311 and from previous studies.
The main elements of this revised model are as follows:
1. Fluid expulsion by tectonic compression of accreted sediments at nonuniform expulsion rates along the transect results in the evolution of variable pore water regimes across the margin.
Sites closer to the deformation front are characterized by pore fluids enriched in dissolved salts at depth, where zeolite formation from ash diagenesis is dominant. In contrast, the landward portion of the margin shows a freshening of pore fluids
with depth as a result of the progressive overprinting of diagenetic salt generation with freshwater generation from the smectite-to-illite transition at greater depth.
2. In situ methane produced by microbial CO2 reduction within the gas hydrate
stability zone is the prevalent gas source for gas hydrate formation.
3. Some minor methane advection from depth is required overall to explain the occurrence of gas hydrate (and the associated downhole isotopic signatures of CH4 and CO2) within
the sediments of the accretionary prism and the absence of gas hydrate within the abyssal plain sediments. In contrast, methane migrating from depth is a dominant source for gas hydrate formation at the cold vent Site U1328 (Bullseye vent).
hydrate preferentially forms in coarser grained sandy/silt turbidites, resulting in very high local gas hydrate concentrations. Typically, gas hydrate occupies <5% of the pore space throughout the gas hydrate stability zone. Higher gas hydrate
saturations were observed in intervals with abundant coarse-grained sand layers and within fault-controlled fluid and gas migration conduits at the cold vent Site U1328.