ABSTRACT

The subsurface clastic deposits of the Borden Island gas zone constitute the major reservoir in the two largest gas fields yet discovered in the Sverdrup Basin. These gas fields (Drake Point and Hecla) lie on and adfacent to the Sabine Peninsula, Melville Island. The interval has been previously assigned in part to the Lower Jurassic Borden Island Formation and in part to the Wilkie Point Formation. The relationship of the rocks in the study area to those of the type locality of the Borden Island Formation is, however, uncertain.

On the basis of colour, lithology, mechanical-log character and position in sequence, three units are recognized in the sediments of the gas zone and are correlated through 15 wells on northern Melville Island. Each unit appears as a basinward thickening wedge of sediment, the combined thickness of which ranges from 20 to 108 m (65 to 354 ft). Cores from 5 wells, when combined, provide an almost complete lithological record of the interval in the subsurface. The three units are informally labelled Units A, B and C in ascending order. Unit A is an oolitic ironstone lithofacies; Unit B, the major reservoir unit, is a cyclic sandstone-siltstone lithofacies, and Unit C is a mud-rich, glauconitic sandstone, siltstone and oolitic ironstone lithofacies.

Based on sedimentary structures and textures, granulometric analyses, unit geometry, lithology, mineralogy and palaeontology, the over-all environmental model suggested is that of an interdistributary beach/offshore transition developed in a series of semirestricted, shallow-water coastal embayments. Units A and C represent deposition in a transition subenvironment between sand-silt and clay deposition. The glauconitic sandstones and siltstones of these units are interpreted as "normal" sediments of the transition zone and the oolitic ironstones are interpreted as high-energy storm deposits. Unit B cyclic sandstones and siltstones represent deposition in the dune, beach and shoreface subenvironments.

Maximum porosity and permeability in the Borden Island gas zone is developed in sandstones attributed to deposition in the dune and beach/upper shoreface subenvironments. Porosity and permeability reduction is ascribed mainly to the presence of detrital and authigenic clay minerals.