BOTTOM SIMULATING REFLECTORS AND HYDRATE OCCURRENCES BENEATH THE SCOTIAN SLOPE, OFFSHORE EASTERN CANADA

John Shimeld¹, David Mosher¹, Keith Louden², Chris LeBlanc², Kirk Osadetz³
¹ Geological Survey of Canada (Atlantic), Natural Resources Canada, Bedford Institute of Oceanography,
Dartmouth, Nova Scotia
² Department of Oceanography, Dalhousie University, Halifax, Nova Scotia
³ Geological Survey of Canada (Calgary), Natural Resources Canada, Calgary, Alberta

Widespread natural gas hydrate accumulations exceeding 20 m in thickness were inferred, in the mid 1980s, beneath vast regions of the eastern Canadian continental margin. The assumed pressure-temperature conditions support their presence but, apart from sparse examples, there is little direct borehole or geophysical evidence that can be used to identify and map the hydrates. The 1980s reconnaissance-scale studies identified 26 east coast offshore wells that may have penetrated hydrates (Fig. 1), but the nature and quality of the available borehole geophysical data were poorly suited to hydrate identification and these results require a careful re-assessment.

A new phase of east coast offshore oil and gas exploration, targeting deep-water prospects, is spurring acquisition of extensive 2-D and 3-D seismic datasets over areas where hydrates should theoretically be present. These data are of significantly better quality than data from earlier exploration rounds. Conducted as part of the Geological Survey of Canada gas hydrate research program, a systematic study using 34,000 km of 1998/9 2-D multi-channel seismic data has determined that bottom simulating reflectors (BSRs) are not generally observed beneath the Scotian Slope and Laurentian Fan. These results were found despite estimates of a 500 m-thick stability zone for much of the continental slope in area. The only unambiguous exceptions are found in the regions of Haddock Channel and Mohican Channel, where strong BSRs cover areas of 345 km² and 260 km² respectively (Fig. 1). These BSRs occur in water depths ranging between 1330 and 2150 m, and at sub-seafloor depths of approximately 330 to 440 m.

Recent interpretation of a 2002 3-D seismic dataset has revealed the Mohican Channel BSR is intersected by a system of small-scale faults that form a polygonal pattern in plan view (Fig. 2). These polygonal faults sole into a unit of incoherent, high-amplitude reflections that exhibit seismic attributes suggestive of gas charging. It is possible that the overlying gas hydrate manifested by the Mohican Channel BSR is charged from this unit through the polygonal fault system. Indeed, there are vertical pipes that are cylindrical in plan view that are interpreted as gas or fluid escape structures associated with some of the fault intersections (Fig. 2). In several localities, these pipes intersect the seafloor and may contain hydrate, as they do on the Cascadia margin, for example.

Wide-angle seismic reflection/refraction data were acquired at three ocean bottom seismometer stations in 2002 near the Mohican Channel BSR (Fig. 3). The experiments were conducted both where a strong BSR is present and where no BSR is discernible on conventional reflection data; the velocity profiles derived from joint travel-time inversion of reflected and refracted events were similar in both regions. Each profile contains a low velocity anomaly consistent with the presence of free gas beneath a hydrate zone and, although the absence of local heat flow measurements creates some uncertainty, the anomaly occurs near the base of the hydrate stability zone. It is possible, therefore, that hydrate is present in the vicinity of Mohican Channel even where there are no clear BSRs.