Abstract

The Queen Charlotte Basin lies on the continental shelf off British Columbia, Canada. The basin is 100 km wide between the Queen Charlotte Islands and the mainland, and 400 km long between southeast Alaska and northern Vancouver Island. Aeromagnetic and seismic data, plus geological information from six onshore and eight offshore exploratory wells, have enabled Shell Canada Ltd. to prepare an isopach map of the 2 to 3 km of Mio-Pliocene sediments which overlie the Eocene Masset volcanics or late Mesozoic volcanics and sediments. Gravity measurements over the continental shelf and adjacent land areas have been reduced to Airy-Heiskanen isostatic anomalies for T = 30 km and Δp = 0.4 g. cm^-3. Interpretation of the gravity data in terms of a single density contrast between the Mio-Pliocene sediments and the underlying volcanic sequences leads to a basin model up to 2 km shallower than that shown in Shell Canada Ltd.’s isopach map for the northern half of the basin, and approximately 1 km deeper below the southern part of the basin. These discrepancies can be explained if the Pal-Eocene and late Mesozoic volcanics which generally floor the basin are underlain by dense, volcanic sequences, such as the Karmutsen Formation, in the north, and by a predominantly sedimentary sequence, such as the Nanaimo Group in the south.

Drilling to date has tested the petroleum potential of the Cenozoic sediments and nothing of economic importance has been reported. The knowledge gained from Shell Canada Ltd.’s program and the additional information derived from the gravity data, suggest future exploration should be concentrated in the southern part of the Queen Charlotte Basin where late Mesozoic sediments may have hydrocarbon potential.