|Titre||Seismic AVO for gas-hydrate-related reflections|
|Auteur||Chen, M -A P; Riedel, M; Dosso, S E|
|Source||Geophysical Characterization of Gas Hydrates; par Riedel, M (éd.); Willoughby, E C (éd.); Chopra, S (éd.); Society of Exploration Geophysicists, Geophysical Developments Series no. 14, 2010 p.
|Séries alt.||Secteur des sciences de la Terre, Contribution externe 20090398|
|Éditeur||Society of Exploration Geophysicists|
|Document||publication en série|
|Sujets||hydrocarbures; capacité de production d'hydrocarbures; gaz d'hydrocarbure; hydrate; milieux marins; pergélisol; modèles; établissement de modèles; combustibles fossiles|
|Illustrations||plots; graphs; profiles|
|Programme||Hydrates de gaz, Caractérisation des Hydrates de gaz|
|Résumé||(disponible en anglais seulement)|
This paper examines the usefulness of amplitude versus offset (AVO) analysis for marine and permafrost gas hydrate settings. In marine environments, AVO
analyses have traditionally focused on bottom-simulating reflections (BSRs) for estimating associated marine gas hydrate and free gas concentrations. A nonlinear Bayesian inversion is applied to estimate marginal probability distributions (MPDs) of
physical parameters at a BSR interface, which are related to overlying gas hydrate and underlying free-gas concentrations via rock physics modeling. The problem is further constrained by prior information and re-parameterization of inversion results.
Inversion of BSR AVO data from offshore Vancouver Island, Canada, shows that gas hydrate and free-gas concentrations are, respectively, 0-23% and 0-2% of the pore volume, at a 90% credibility level. However, it should be noted that these two
parameters are not independent in the analysis, but instead inversely related. Thus, 0% in gas hydrate concentrations requires some percent of free gas within the range stated (and vice versa).
This result indicates that the data do not provide
sufficient information to independently resolve gas hydrate and free-gas concentrations to useful accuracy.
The same Bayesian inversion method is applied to synthetic AVO data generated from well-log data obtained at the Mallik 5L-38 well in the
Mckenzie Delta, NWT. The model cases representing typical permafrost gas hydrate occurrences are investigated and include shale over gas hydrate-bearing sand, gas hydrate-bearing sand over shale, and gas hydrate-bearing sand over water-saturated sand
scenarios. The AVO inversion sufficiently constrains the shear-wave velocity for reliable quantitative analysis, only if the gas hydrate concentration exceeds ~40%.
The variable degree of model constraint obtained in this AVO study highlights the
need to include rigorous quantitative uncertainty analysis in all AVO studies.