gas hydrate - CCOP

Marine Gas Hydrate off W. Canada and Some Comparisons
With the East Sea off Korea
R.D. Hyndman
on behalf of the west coast Canada hydrate team
Pacific Geoscience Centre, Geological Survey of Canada, Sidney, B.C.
and School of Earth and Ocean Sciences, Univ. Victoria
ABSTRACT: This presentation firstly provides a short summary of the Geological
Survey of Canada Natural Gas Hydrate Program, and then describes the marine
hydrate studies that have been carried out off the Canadian west coast. Several
comparisons with hydrate occurrences in the East Sea off Korea are also given.
There have been extensive geophysical and geological field studies off the west
coast of Canada as well as two programs of ocean drilling (ODP/IODP) that
provide constraints on the occurrence, distribution, and concentration of gas
hydrate and underlying free gas. Most geophysical information comes from a
variety of different seismic systems, including mapping the characteristic BSR,
determining the depth distribution of hydrate and underlying free gas, and the
geological controls of hydrate formation. BSRs occur beneath about half of the
mid-continental slope off Vancouver Island. Special seismic studies include BSR
reflection coefficients, frequency dependence, and amplitude-versus-offset (AVO),
as well as full waveform inversions, and 3-D high-resolution structure mapping.
Electrical resistivity and measurements of seafloor compliance provide important
additional information on hydrate concentration. Detailed heat flow surveys have
determined the thermal regime that controls the depth to which gas hydrate is
stable, the fluid chemistry of piston cores have helped define the upwelling methane
regime, and seafloor surveys have mapped and photographed exposed hydrate.
Gas plumes above the seafloor also have been mapped.
ODP Leg 146 and IODP Expedition 311 drilled a series of holes across the margin,
a reference hole in the adjacent deep sea Cascadia basin, and into a seismic
blanking upwelling zone on the mid-continental slope. Downhole geophysical logs
and core analyses have constrained the concentrations of hydrate and free gas at
the drill sites, and provided calibration of the regional geophysical data. The
hydrate distribution and concentrations are highly variable, including, (1) Regional
hydrate located at and at variable levels above the BSR, up to 50 m thick; it is
usually associated with a BSR. The hydrate is inferred to be generated, especially
in sandier sediments, from methane carried by upward fluid transport from
biogenic generation deeper in the sediment section. The hydrate from both
geophysical and borehole data is locally estimated to be up to 20% of the pore
space (10% of total volume), with a gas layer underlying the BSR that is tens of
metres thick with concentration less than ~1%. (2) Localized cold vents or
chimneys. There is nearly massive hydrate in the upper ~50 m in at least one of
these structures that has been sampled by IODP drilling, and by piston coring. At
another site extensive seafloor hydrate has been photographed and sampled by the
Canadian deep sea remotely operated vehicle, ROPOS.
New Energy Resources in the CCOP Region - Gas Hydrates and Coalbed Methane 33