Abstract

Shaly sandstones of the dominantly argillaceous Colorado and Montana Groups (Middle Albian to Campanian), laid down in nearshore and shelf settings of an epeiric sea, are prospective for hydrocarbons at depths of less than 1,000 m in southern Saskatchewan. Current production is restricted to the vicinity of the Fourth Meridian, where: (1) largely conventional reservoirs in the Viking Formation yield high-gravity crude oil and natural gas; and (2) unconventional (low-permeability) reservoirs of the Medicine Hat Sandstone and Milk River Formation are exploited for major accumulations of non-associated natural gas. Gas pools in the Spinney Hill Sandstone, St. Walburg Sandstone, Phillips Sandstone (Second Specks Sandstone) and Belly River Formation are also under development.

The main shallow hydrocarbon prospects of the area are: (1) conventional and unconventional hydrocarbon reservoirs in the Bow Island-Viking sequence of the southwest and in the Newcastle-Viking sequence of the southeast; and (2) extensive low-permeability gas reservoirs occurring throughout the succession in western Saskatchewan. Hydrocarbon accumulation has been localized by basinward attenuation of reservoir strata and concomitant diminution in grain size in conjunction with: (1) regional, dominantly northeast-trending basement structures, including major arch forms; (2) smaller-scale northeast- and northwest-trending linear features, delineating juxtaposed basement blocks which have undergone differential movement; (3) solution-generated collapse features, associated with the Middle Devonian Prairie Evaporite; and (4) drape folds mimicking the topographic relief of the sub-Cretaceous unconformity along erosional scarps developed in Jurassic, Mississippian, and Devonian units.

Examples of drainage of natural gas at productive locales in the area demonstrate that poor reservoir quality does not preclude the possibility of gas migration through low-permeability strata.