North Cascadia Deep Sea Gas Hydrates

Abstract

Abstract: The Cascadia accretionary margin off Vancouver Island is one of the best studied margins world‐wide for the determination of in situ properties of marine gas hydrates. Most quantitative information has come from cores and downhole logs of the Ocean Drilling Program (ODP) Leg 146 and from extensive seismic and other geophysical surveys. As part of the ODP site surveys, large‐offset multichannel seismic lines outlined the regional distribution of hydrate, and seismic velocity analyses coupled with full waveform inversion provided estimates of the vertical distribution of hydrate and gas. High resolution single‐channel seismic surveys indicate correlations between hydrate/gas concentrations and topographic highs, and between geothermal flux and topography; these correlations provide insight into fluid and methane flow through the sediments. A deep‐towed multichannel seismic survey (DTAGS), together with other data from 20–600 Hz, provide constraints on gradients at the base of the hydrate and gas layer. Extensive heat flow measurements, from probes and from depths of the bottom simulating reflector (BSR), have been modelled numerically, including the regional effects of sediment thickening and advective fluid flow in the accretionary prism. Other geophysical surveys include several seafloor electrical sounding experiments and seafloor compliance measurements of hydrate. ODP drilling has provided valuable downhole log data, core physical property data, and detailed pore fluid chemistry and isotopes. Several semi‐independent estimates of hydrate and gas concentrations have been obtained; all are dependent on reference sediment properties for which no hydrate and no gas is present. From a vertical seismic profile and other seismic data, the velocity increase in a hydrated region is consistent with hydrate concentrations of 20–30% of the pore space in a 100‐m interval above the BSR. Similar values are determined from log resistivity data and from core pore fluid chlorinity.