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The early Tournaisian - Vis茅an Banff Formation which accumulated as shallowing upward basinal, slope, shelf margin, and protected shelf deposits of a carbonate ramp system extends throughout the Western Canadian Sedimentary Basin (WCSB). In northeastern BC, the Banff formation was divided by Richards (1993) into the informal members A and D. Basinal shale layers of Member A grade upward and laterally into interbedded layers of supratidal and shallow marine limestones and shales of member D. Well-log correlation and mapping of a 1700 km2 area just northeast of Fort Nelson revealed a 5 m thick, potential carbonate reservoir unit, informally known as the Banff C which is particularly well developed over the Beatton High in the underlying bedrock. A cored interval within well d-A3-E/94-I-14 centered is upon the Banff C. Lithological interpretations of this core suggest that it consists of an upper, middle, and lower unit, each of which represents a different, relatively shallow marine environment.

The lower unit is an open-marine, mixed carbonate and siliciclastic deposit that consists of interbedded shale and massive carbonate rich in brachiopods and microfossils, including foraminifera and radiolarians. The unit represents sediments transported basinward by storms. The middle unit is a tidally influenced carbonate sand. It consists of stacked successions of tight skeletal and peloidal grainstones that coarsen upwards into porous oolitic grainstones. This unit correlates with the potential Banff C reservoir unit observed in the subsurface. The middle unit is interpreted as a sand-shoal barrier complex that formed on a tidally influenced shallow shelf margin. The upper unit formed in a protected setting landward of the shoal, and is represented by peloidal wackestones and mudstones with pelmatozoan and bryozoan fragments but generally lacking siliciclastic grains. Thin green paleosols represent brief periods of exposure during deposition of the upper unit. The upper unit accumulated in normal salinity conditions in a quiet, back-barrier environment.

Porosity is negligible in both the lower and upper units and ranks from neglibible to good (0.1 to 18.5%) in the middle unit. Highest porosity levels are associated with the oolitic grainstone, where large moldic pores are prominent, along with ineffective micropores within micritized grains. Permeability levels rank from negligible to poor (< 2.1 mD), and connectivity is generally poor. The middle unit sand belt is of low to moderate reservoir quality and may be capable of producing light oil or gas. Gas shows observed in strip logs are as high as 51 times background levels, indicating that the Banff C is a hydrocarbon-charged reservoir which probably belongs to a closed Late Devonian-Early Carboniferous petroleum system. Hydrocarbons probably originated from source rocks in the underlying Exshaw Formation and lower Banff correlatives.

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Pages: 127
Supervisor: Martin Gibling