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TitreCompressional and shear-wave velocities from gas hydrate bearing sediments: Examples from the India and Cascadia margins as well as Arctic permafrost regions
AuteurRiedel, M; Goldberg, D; Guerin, G
SourceMarine and Petroleum Geology vol. 58, 2014 p. 292-320, https://doi.org/10.1016/j.marpetgeo.2014.07.028
Année2014
Séries alt.Secteur des sciences de la Terre, Contribution externe 20140039
ÉditeurElsevier BV
Documentpublication en série
Lang.anglais
DOIhttps://doi.org/10.1016/j.marpetgeo.2014.07.028
Mediapapier; en ligne; numérique
Formatspdf
ProvinceTerritoires du Nord-Ouest; Colombie-Britannique
SNRC107; 117; 92; 102
Lat/Long OENS-124.0000 75.0000 75.0000 0.0000
Sujetssismo-sondages; vitesse des ondes sismiques; congélation du sol; glace fossile; pergélisol; gaz; hydrate; géologie marine; combustibles fossiles
Illustrationslocation maps; tables; plots
ProgrammeÉvaluation des hydrates de gaz, Géoscience Énergie
Résumé(disponible en anglais seulement)
Shear wave velocity data have been acquired at several marine gas hydrate drilling expeditions, including the India National Gas Hydrate Program Expedition 1 (NGHP-01), the Ocean Drilling Program (ODP) Leg 204, and Integrated Ocean Drilling Program (IODP) Expedition 311 (X311). In this study we use data from these marine drilling expeditions to develop an understanding of general grain-size control on the P- and S-wave properties of sediments. A clear difference in the downhole trends of P-wave (Vp) and S-wave (Vs) velocity and the Vp/Vs ratio from all three marine regions was observed: the northern Cascadia margin (IODP X311) shows the highest P-wave and S-wave velocity values overall and those from the India margin (Expedition NGHP-01) are the lowest. The southern Cascadia margin (ODP Leg 204) appears to have similar low P-wave and S-wave velocity values as seen off India. S-wave velocity values increase relative to the sites off India, but they are not as high as those seen on the northern Cascadia margin. Such regional differences can be explained by the amount of silt/sand (or lack thereof) occurring at these sites, with northern Cascadia being the region of the highest silt/sand occurrences. This grain-size control on P-wave and S-wave velocity and associated mineral composition differences is amplified when compared to the Arctic permafrost environments, where gas hydrate predominantly occurs in sand- and silt-dominated formations. Using a cross-plot of gamma ray values versus the Vp/Vs ratio, we compare the marine gas hydrate occurrences in these regions: offshore eastern India margin, offshore Cascadia margin, the Ignik-Sikumi site in Alaska, and the Mallik 5L-38 site in the Mackenzie Delta. The log-data from the Arctic permafrost regions show a strongly linear Vp eVs relationship, similar to the previously defined empirical relationships by Greenberg and Castagna (1992). P- and S-wave velocity data from the India margin and ODP Leg 204 deviate strongly from these linear trends, whereas data from IODP X311 plot closer to the trend of the Arctic data sets and previously published relationships. Three new linear relationships for different grain size marine sediment hosts are suggested: a) mud-dominated (Mahanadi Basin, ODP Leg 204 & NGHP-01-17): Vs ¼ 1.5854 Vp 2.1649 b) silty-mud (KG Basin): Vs ¼ 0.8105 Vp 1.0223 c) silty-sand (IODP X311): Vs ¼ 0.5316 Vp 0.4916 We investigate the relationship of gas hydrate saturation determined from electrical resistivity on the Vp/Vs ratio and found that the sand-dominated Arctic hosts show a clearly decreasing trend of Vp/Vs ratio with gas hydrate saturation. Though limited due to lower overall GH saturations, a similar trend is seen for sites from IODP X311 and at the ash-dominated NGHP-01-17 sediment in the Andaman Sea. Gas hydrate that occurs predominantly in fractured clay hosts show a different trend where the Vp/Vs ratio is much higher than at sand-dominated sites and remains constant or increases slightly with increasing gas hydrate saturation. This trend may be the result of anisotropy in fracture-dominated systems, where Pand S-wave velocities appear higher and Archie-based saturations of gas hydrate are overestimated. Gas hydrate concentrations were also estimated in these three marine settings and at Arctic sites using an effective medium model, combining P- and S-wave velocities as equally weighted constraints on the calculation. The effective medium approach generally overestimates S-wave velocity in high-porosity, clay-dominated sediments, but can be accurately used in sand-rich formations.
Résumé(Résumé en langage clair et simple, non publié)
Dans cette étude, nous utilisons des données sur la vitesse de propagation des ondes de cisaillement, obtenues à partir de carottes de sédiments prélevées lors de trois expéditions internationales de forage en mer, afin d'examiner l'influence de la taille des grains sur la présence d'hydrates de gaz. Les résultats tirés des occurrences d'hydrates de gaz en milieu marin ont été comparés à ceux de sites arctiques en Alaska et dans le delta du Mackenzie. Nous avons également estimé les concentrations d'hydrates de gaz. En outre, nous présentons une nouvelle approche statistique dans laquelle nous combinons les vitesses de propagation des ondes P et S comme étant des contraintes égales.
GEOSCAN ID293937