Abstract

The Cambrian basin of the mainland Northwest Territories has been of considerable economic interest since gas was first discovered in Mount Clark Formation sandstones in 1974. A recently completed assessment by the Geological Survey of Canada has estimated that the Cambrian Clastics play contains almost a billion barrels of oil and 10.7 TCF of gas. The play’s reservoirs, seals and source rocks are all of Cambrian age, making the understanding of the basin’s evolution and contents critical to any exploration effort.

The basin is a semi-enclosed, epicontinental marine basin, bordered by positive elements and open to a subsiding continental shelf and margin, and represents the eastern portion of a much larger deposystem that developed after the breakup of Laurentia. A multi-disciplinary study of a large regional reflection seismic grid, supported by borehole and potential field data, shows the basin to have evolved in three stages: 1) Early Cambrian pre-rift regional subsidence of the area lying inland of the Mahony and Bulmer arches, and a newly identified high that pre-dated the Mackenzie Trough. It was during this period that the Mount Clark Formation was deposited; 2) A middle Cambrian rift phase during which a graben system, about 50 km wide and over 400 km long, developed in the basin’s core. The grabens are deepest in south Mackenzie Trough where rifting began, and are progressively shallower and younger to the north and northeast. Mount Cap was deposited during this phase and it is expected that the grabens received finer-grained clastics than did the shallower areas on their flanks; 3) A post-rift subsidence phase that began with deposition of the Saline River Formation during the Late Cambrian and continued, with interruptions, until the middle of the Devonian. Throughout Cambrian time, sediment onlapped the basin’s flanks and its internal arches, each layer advancing further than its predecessor. It was not until the latest Cambrian that the internal arches were completely covered by beds of the lower Franklin Mountain Formation.

A previously undocumented Cambrian age graben (herein named McConnell Graben) lies in the hanging wall of the unnamed Laramide thrust fault that emerges from the east flank of the McConnell Range. The fault began as a Cambrian normal fault that separated the Bulmer and Mahony arches on the east from McConnell Graben on the west. The present day ‘St. Charles’ range developed in a similar manner.

A preliminary exploration risk analysis, based on the predicted distribution of reservoir, seal and source rocks, reveals two attractive areas for exploration. The larger one, centred on Lac des Bois, is about 150 km wide and extends on a NW-SE trend for about 300 km across the Aubry and Good Hope depocentres. The smaller area (less than 50 × 100 km in extent) lies within Mackenzie Plain depocentre.