Northern Cascadia marine gas hydrate: constraints from resistivity, velocity, and AVO




Chen, Marc-André Paul

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This thesis presents estimates of marine gas hydrate distribution and concentration obtained from various geophysical methods. The study area is located in the accretionary prism of the Northern Cascadia subduction zone, offshore Vancouver Island. Canada. The primary objective of this study was to assess the applicability of a suite of geophysical methods in estimating marine gas hydrate distribution and concentration. The measurements tested are downhole log electrical resistivity and seismic velocity, multi-channel seismic (MCS) velocity, and seismic amplitude vs. offset (AVO) of a gas hydrate-related bottom-simulating reflection (BSR). The downhole log data are from Integrated Ocean Drilling Program Expedition 311, along a transect of four wells, and the seismic data are from a conventional 2-D MCS line along the well transect. Gas hydrate distribution and concentration estimates along the well transect exhibit high spatial variability, both from site to site, and within any given site. On average. estimates from electrical resistivity measurements give 5-15% gas hydrate pore space saturation. whereas velocity-based estimates are 15-25%. Some intervals in both cases show concentrations over 40%. Nonlinear Bayesian inversion of seismic AVO data yields a gas hydrate concentration estimate of 0-23% of the pore space. These results lead to the conclusion that resistivity and velocity data are effective tools for estimating marine gas hydrate concentration. The main uncertainty in the resistivity analysis is the in situ pore fluid salinity, whereas the main uncertainty in the velocity study is the magnitude of the bulk sediment velocity increase associated with gas hydrate occurrence (related to how gas hydrate forms). It is shown here that AVO of a gas hydrate BSR is not a useful method to estimate marine gas hydrate concentration. The method lacks the shear-wave velocity resolution necessary to add useful constraints to what is already known from compressional-wave velocity information.



natural gas, submerged lands, hydrates, Vancouver Island