Research Document - 2002/004

Physical Oceanographic and Geological Setting of a Possible Oil and Gas Industry in the Queen Charlotte Basin

By W. Cretney, W. Crawford, D. Masson and T. Hamilton

Abstract

Research, mainly by DFO physical oceanographers over the last decade and a half, has deepened our understanding of ocean currents in the Queen Charlotte Assessment Area. In this accomplishment, the Department has followed one of the recommendations of the 1986 West Coast Offshore Environmental Assessment Panel Report on offshore oil and gas issues. Much about the generation of currents and the formation of oceanic eddies has been explained.

Enhanced satellite images of oceanographic features, some with superimposed current vectors, are now available for examination by interested parties. Graphical representations help make understandable the results of complex mathematical equations. In this paper, extensive use of images is made to present some of the known physical oceanography of the region. Still, some phenomena require further explanation.

Our oceanographic knowledge is best for summer and worst for winter. Winter is the time when conditions can be expected to be especially severe for an industry that seeks to operate all year round. Giant waves, as have been observed in the Queen Charlotte Assessment Area, present a dangerous hazard to operations of an offshore oil and gas industry. Research provides some knowledge of the interaction of tide currents and waves to create treacherous conditions. The St. James Island area off the southern tip of Moresby Island has been the focus of studies in the 1990s.

Tsunamis also present the potential for harm by giant waves. The tsunami that 300 years ago devastated the southern B.C. and Washington coasts continues to hold the attention of researchers. Tsunami computer simulations are now available for the Cascadia region. Uncertainties remain in the computations, however. The major uncertainties lie in determining tsunami likelihoods.

Subsea earthquakes of the right types can generate tsunamis. A giant subduction earthquake along most of the length of the area where the Pacific plate moves under the North American plate off Southern B.C and Western U.S.A. is believed to have caused a giant tsunami 300 years ago. Having happened once, this giant subduction event seems likely to happen again, perhaps every few centuries. In the case of the Queen Charlotte Fault, which passes closely by the Queen Charlotte Islands, the question is can it happen at all. Landslides and underwater slumps can generate tsunamis as well. Giant tsunamis with wave heights up to 30 m generated by slides in the Hawaiian Islands likely hit our coast in ancient times.

The seabed of the Queen Charlotte Basin presents a complex topography, as might be expected from the topographical complexity of the bordering lands. Banks and troughs due to past glaciation dominate the seabed structure of the area. The area is replete with evidence of the strong currents that surge along the bottom. Large areas show sand ripples, sand waves and sand ridges. Some of these transitory features achieve heights of 6 m. A number of slopes around banks show evidence of instability. At shallow depth under the seabed are accumulations of biogenic gas and/or thermogenic gas. Throughout much of the area, especially in Hecate Strait, sediments are infused with gas, which renders them more susceptible to liquefaction than gas-free sediments. Although a significant amount is known about the seabed geomorphology, much remains to be learned.

The Queen Charlotte Islands display a large number of liquid petroleum seeps, which provide evidence of a number of source rocks. Submarine seeps of liquid petroleum surely must exist. Evidence of gas seeps appears in acoustic survey profiles. It seems plausible that the coverage of the seabed may mirror that of the land.

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