Depositional Control of Lateral Reservoir Variability within an Extensive Shoreline Complex: Examples from the Lower Cretaceous Cadotte Member of Western Canada

Curtis D. Lettley, S. George Pemberton, Murray K. Gingras, and Tom Saunders
University of Alberta, Edmonton, AB

The Cadotte Member (Albian Peace River Formation) forms an extensive shallow-marine complex in the subsurface and Rocky Mountain foothills of western Canada. It spans approximately 300 km along depositional strike, and 150 km along dip. Significant deep basin type gas accumulations are present in the subsurface. Permeability and gas deliverability in the Cadotte Member are strongly tied to primary grain size and mineralogy (i.e. the ratio of quartz and chert). Coarser, chert-rich sediments have generally preserved a greater proportion of initial porosity and permeability due to a combination of larger original pore size and inhibited secondary quartz overgrowth.

A sharp, but conformable facies contact is typically seen in the Cadotte Member between fine-grained quartz sandstone of the lower shoreface and coarser, chert-bearing sandstone and conglomerate of the upper shoreface. While this defines a consistent vertical trend observed ubiquitously throughout the Cadotte Member, great lateral variation is encountered in the texture and mineralogy of the upper shoreface sediment. This upper package varies from medium-grained sandstone, through interbedded sandstone and chert pebble conglomerate, to well-sorted, matrix-free chert pebble conglomerate.

The spatial variations in shoreline texture are interpreted to reflect the influence of local deltaic sources on grain size and rate of sediment input. Evidence of deltaic influence includes an abundance of wood and carbonaceous debris within the Cadotte Member (and correlative offshore facies), low abundance and diversity of biogenic structures, hyperpycnal mud beds within the offshore facies, and other features suggestive of high fluvial input subsequent to peak offshore storm conditions.