N. R. Chapman

Decreased stability of methane hydrates in marine sediments owing to phase-boundary roughness

Below water depths of about 300 metres, pressure and temperature conditions cause methane to form ice-like crystals of methane hydrate. Marine deposits of methane hydrate are estimated to be large, amassing about 10,000 gigatonnes of carbon, and are thought to be important to global change and seafloor stability, as well as representing a potentially exploitable energy resource. The extent of these deposits can usually be inferred from seismic imaging, in which the base of the methane hydrate stability zone is frequently identifiable as a smooth reflector that runs parallel to the sea floor. Here, using high-resolution seismic sections of seafloor sediments in the Cascadia margin off the coast of Vancouver Island, Canada, we observe lateral variations in the base of the hydrate stability zone, including gas-rich vertical intrusions into the hydrate stability zone. We suggest that these vertical intrusions are associated with upward flow of warmer fluids. Therefore, where seafloor fluid expulsion and methane hydrate deposits coincide, the base of the hydrate stability zone might exhibit significant roughness and increased surface area. Increased area implies that significantly more methane hydrate lies close to being unstable and hence closer to dissociation in the event of a lowering of pressure due to sea-level fall.

Fishing trawler nets Massive “Catch” of methane hydrates

In November 2000, the commercial fishing vessel Ocean Selector recovered over 1000 kg of gas hydrate in a trawl net. The catch occurred in 800 m near the head of the deeply-incised Barcley Canyon off Vancouver Island (Figure 1). This is probably the largest recovery of gas hydrate ever reported from a marine setting. Although methane hydrate is known to be stable at the sea floor for water depths greater than 500–600 m at temperate latitudes, observed outcrops of hydrate at the sea floor are rare and poorly understood. There is a very large disequilibrium between the concentration of methane in gas hydrate and in seawater. Therefore, one would expect that gas hydrate would sublime and dissociate when it comes in contact with seawater. Nevertheless, massive hydrate outcrops have been observed and sampled, most prominently at the Hydrate Ridge off Oregon [Suess et al., 1999] and in the Gulf of Mexico [MacDonald et al., 1994; Sassen et al., 2001].

Deep-sea gas hydrates on the northern Cascadia margin

This article describes recent work on marine hydrates off western Canada. Natural occurrences of gas hydrates have been reported in the geoscience literature since the early 1970s. Gas hydrates are solid ice-like structures in which gas molecules (largely methane) are trapped within cages of water molecules. Some 50 regions of hydrate occurrence have been reported in marine, permafrost, and lake environments worldwide. Interest in gas hydrates is due to the fact that the total amount of hydrocarbons locked up in them probably exceeds other known fossil hydrocarbon resources.

Cascadia Margin, Northeast Pacific Ocean: Hydrate Distribution from Geophysical Investigations

Natural gas hydrate was first recognized on the Cascadia margin in 1985 through the characteristic bottom-simulating reflector (BSR) on conventional multichannel seismic data (Davis and Hyndman, 1989, Davis et al., 1990). Since then, the Cascadia accretionary margin has received the most intensive studies of any convergent margin for determination of the in-situ properties of marine gas hydrate. Key control for understanding the properties and formation processes of hydrate has been derived from drill holes of the Ocean Drilling Program (ODP) Leg 146, carried out in 1992. Estimates of hydrate concentration were provided through analysis of downhole seismic and resistivity logs and through measurement of chlorinity in pore fluids from recovered sediment core samples.

North Cascadia Deep Sea Gas Hydrates

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.