Abstract

The Upper Ordovician Bad Cache Rapids and Churchill River groups and Red Head Rapids Formation of the Hudson Bay Basin are 180–300 m thick. These units are dominated by carbonate, but contain thin shales informally named “Boas River shale” and “Sixteen Mile Brook shale”. The stratigraphy and hydrocarbon potential of the shale units are poorly understood.

The organic-rich “Boas River shale” and “Sixteen Mile Brook shale” were discovered on Southampton Island about 40 year ago. However, there has been considerable debate surrounding these oil shales, which focused on fundamental issues such as: one, two or three oil shale intervals within the Ordovician sequence, their precise stratigraphic positions, their extension in the Hudson Bay offshore area, and their hydrocarbon potential.

Field studies of Upper Ordovician and lowest Silurian strata on Southampton Island, with a focus on the new discovery of the three oil shale intervals in the Cape Donovan area of eastern Southampton Island, “Boas River shale” and “Sixteen Mile Brook shale”, unequivocally demonstrated: 1) the three oil shale intervals in the Cape Donovan area are within the lower Red Head Rapids Formation; and 2) the Cape Donovan lower oil shale interval can be correlated to the “Boas River shale”, and the Cape Donovan middle or upper oil shale interval to the “Sixteen Mile Brook shale”, respectively.

Rock-Eval⁶ data from samples of the three oil shale intervals in the Cape Donovan sections lead to the recognition of Type I–Type II kerogen and much higher yield and TOC than previously reported. Nineteen samples from middle and upper oil shale intervals have average and maximum yields of 136.5 kg HC/tonne and 230 kg HC/tonne, and average and maximum TOC of 20% and 34%; 21 samples from lower oil shale interval have mean and highest yields of 58.5 kg HC/tonne and 112.5 kg HC/tonne, and mean and highest TOC of 9.8% and 17.3%.

The three oil shale intervals are recognized in the Hudson Bay offshore area. This is supported by three lines of evidence in the lower Red Head Rapids Formation in the Hudson Bay offshore exploration wells: 1) three prominent positive gamma ray kicks; 2) possible organic-rich fragments found in well cuttings; 3) some reasonably high TOC values (2.29–5.73%) are obtained from several samples within the interval covering the three positive gamma ray kicks by using preferentially picked organic-rich fragments from the well cuttings.