Abstract

Continental margins of Canada represent vast potential for storing hydrocarbon gas (largely methane) in the form of gas hydrate. The theoretical stability field for gas hydrate encompasses some 120,000 km2 along the Scotian margin. Examination of >34,000 line-km of industry 2D seismic reflection data along this margin; however, resulted in identification of only one area with a clear bottom simulating reflector (BSR). This BSR forms over 280 km2 at 350–450 mbsf and lies within the stability field for methane gas hydrate.

Seismic velocity models derived from wide angle reflection and refraction analysis over this BSR include velocity inversions of 50–75 m/s between a 60–120 m thick high velocity zone (HVZ) and a 150–260 m thick low velocity zone (LVZ) at sub-bottom depths of 325–425 mbsf. Similarly these results indicate that hydrate concentrations at the study site range from 2–6% and free gas < 1%. Hydrate above the BSR is estimated to be 0.44–1.33 × 109 m3 in volume, as a result. Hydrates may exist elsewhere along the margin but lack of BSRs make them difficult to detect.

Introduction

Gas hydrates are believed to represent a large storehouse of methane gas in sedimentary basins around the world1 and especially in Canada because of extensive northern permafrost and offshore land areas.2,3,4 In Canada, hydrates have been known to exist for some time along the Cascadia margin, offshore Vancouver Island, initially with the recognition of bottom simulating reflections (BSR),5,6,7,8 followed by Ocean Drilling Program (ODP) sampling.9,10 They have also been recognized and sampled in the onshore MacKenzie delta region.11 Gas hydrates or clathrates have been recovered through drilling, coring and even fish trawling in the case of Cascadia.12,13 Estimates of the volume of hydrates in Canada indicate that there may be as much as 1012-1014 m3 of methane gas bound in hydrate.3

Despite occurrences on the west coast and northern regions of Canada, no physical and little geophysical evidence for hydrates had been published from Canada's east coast margin, although their presence has been surmised and the calculated stability zone is significant in size.3,4,14,15,16,17 It is the intent in this paper to document the first unequivocal observations of BSR's on the Nova Scotia margin of Canada (Fig. 1), indicating the possible presence of gas hydrates. Seismic reflection, wide-angle reflection and refraction data are used to characterize the zones around the BSR.

The possible existence of hydrate in this continental margin setting is significant for a number of reasons:

  1. hydrates may represent a potential hazard to hydrocarbon exploration and development,

  2. hydrates may be an indicator of an active petroleum generation system with fluid migration, and

  3. documentation of global occurrences of methane hydrate has important implications for carbon cycle and climate change investigations. In future, hydrates may also represent a significant hydrocarbon resource.

Geologic Setting

This content is only available via PDF.
You can access this article if you purchase or spend a download.