HYDRATE ON THE CASCADIA ACCRETIONARY MARGIN OF NORTH AMERICA

M. Riedel¹, R.D. Hyndman², G.D. Spence³, N.R. Chapman³, I. Novosel³, N. Edwards^4
1 Natural Resources Canada, Geological Survey of Canada-Atlantic, Dartmouth, NS, Canada
² Natural Resources Canada, Geological Survey of Canada – Pacific Geoscience Centre, Sidney, BC, Canada
³ School of Earth and Ocean Sciences, University of Victoria, Victoria, BC, Canada
⁴ Department of Physics, University of Toronto, Toronto, ON, Canada

Introduction

The presence of gas hydrate has been confirmed on Canada’s West Coast at the Northern Cascadia Margin by widespread BSRs in multichannel seismic (MCS) data, scientific ODP drilling (Leg 146) and electrical sounding. After the first discovery in 1985, the area offshore Vancouver Island has been the focus of many interdisciplinary studies. Studies include 2D and 3D single channel and MC conventional and high-resolution seismic studies, Ocean Bottom Seismometer (OBS) surveys, swath-bathymetry mapping, high-resolution subbottom profiling, heat-flow studies, piston coring with geochemical and geophysical sediment analyses, ODP scientific drilling Leg 146, and ocean bottom video surveying with the remotely operated vehicle ROPOS.

Recent studies focus on a cold-vent field near ODP Site 889/890. The vent field is characterized by several seismic blank or washout zones, representing faults that act as conduit for focused fluid and/or gas flow. The vent field was imaged by 3D SCS and MCS surveys and was investigated in detail by piston coring and ROV video observations. Gas hydrate was recovered at the main vent within the upper 2-8 mbsf.

Regional seismic data

A wide variety of seismic surveys have been used to map and characterize the gas hydrate and underlying free gas on the Northern Cascadia Margin. The seismic systems have frequencies ranging from 20 Hz to 650 Hz with additional seafloor acoustic imagery using 3.5 kHz subbottom profilers and 12 kHz echosounders. On regional conventional (low frequency) MCS lines, a BSR was observed in a 20-30 km wide band along much of the 250 km length of the Vancouver Island continental slope. On regional MCS lines a hydrate BSR is clearly observed as a generally symmetric wavelet with a reversed polarity relative to the seafloor suggesting a sharp negative impedance contrast.

Careful semblance velocity analyses were carried out down to depths of 2000 mbsf. Above the BSR, velocities increase to nearly 1900 m/s, indicating the presence of high-velocity hydrate within the pores. By extrapolating the deeper velocities upwards, a reference velocity of sediments unaffected by either hydrate or free gas was achieved. At the BSR, hydrate produces an increase in velocity of about 250 m/s. Downhole sonic logs from ODP Site 889 (Westbrook et al., 1994) provide detailed velocity information from about 50 mbsf to the BSR at 225 mbsf. Excellent agreement with the semblance velocity results was obtained. Constraints for low velocities below the BSR, due to the presence of small quantities of gas, come from vertical seismic profiles at Site 889 (MacKay et al., 1994). Multi-frequency seismic analyses showed a strong frequency dependence of the BSR reflectivity. Modeling of this behavior inferred a thickness of the BSR of about 6 – 8 m with a strong velocity decrease of 250 m/s.

Hydrate concentrations from seismic velocities and electrical resistivity

Estimates of gas hydrate concentrations were obtained using seismic velocity and electrical resistivity. Two different models have been used to relate seismic velocity to hydrate concentration: (a) porosity reduction due to hydrate filling the pore space (Yuan et al, 1996), and (b) a time-average approach with a mixture of hydrate, water and sediment matrix (Yuan et al., 1996, 1999). Both models yield similar concentrations of about 10 – 25 % hydrate in the pore space. An increase in electrical resistivity results from the formation of gas hydrate, which partially fills the pore spaces. A careful correction for lower insitu pore-fluid salinities had to be carried out prior to calculations (Hyndman et al., 1999). A simple Archie’s Law model of resistivity versus hydrate then yielded hydrate concentrations of up to 30% above the BSR, which is slightly higher than the velocity estimate but is in general agreement. Another independent constraint on hydrate concentrations was achieved from seafloor electrical sounding (Yuan and Edwards, 2001). Several surveys giving data to depths in excess of 300 m have been conducted in the area near ODP Site 889/890. Initial interpretation has yielded resistivities in agreement with those from the ODP downhole logs.

Cold vents

Recent research activity is concentrated around an active cold-vent field in close vicinity to ODP Site 889/890. The vent site has been investigated intensively over the last years including 3D seismic imaging, piston coring and ROV-based bottom video observations and seafloor sampling (Riedel et al., 2002). Several seismic blank zones were observed in the seismic data over a frequency range from 20 Hz to 4 kHz, where the degree of blanking increases with seismic frequency. The blank zones range from 80 m to several 100 m in width. The blank zones represent conduits for fluids and gas migrating upward. Blanking of the seismic energy is believed to be mainly the result of increased hydrate formation within the faults. One blank zone, almost circular with a diameter of about 400 m, has a distinct seafloor expression. It shows the characteristics of a mud/carbonate mound, and is probably associated with free gas expulsion. Massive hydrate was found at several sites by piston coring within this blank zone at depths of 2 – 8 m below the seafloor. Increased methane concentrations of up to 8 times the ocean background levels were measured in water samples taken above an active area. However, venting appears to be strongly episodic and localized.

Gas Hydrate mounds in Barkley Canyon

Massive gas hydrate outcrops at the seafloor were discovered in August 2002 in Barkley Canyon, offshore Vancouver Island at a water depth of around 800 m using the remotely operated vehicle ROPOS. At this site hydrates were accidentally dredged by a fishing boat in summer 2000, when an estimated 1.5 tons of gas hydrate were brought to the sea surface in the fishing net (Spence et al., 2001). ROV based seafloor observations revealed widespread hydrate outcrops several meters in height in conjunction with natural oil-seeps. The hydrate outcrops are covered with an about 1 – 5 mm thin veneer of mud and are surrounded by clam colonies. First geochemical analyses of recovered hydrate pieces indicate Structure II hydrate with abundant higher hydrocarbons, indicating a thermogenic source for the gases. This site will be focus of future research.

References

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