Abstract

The Mannville Group in southwestern Saskatchewan is characterized by a complex association of sands, shales and coals which are riddled with unconformities. The Group has been historically divided into several formations and members resulting in a complex nomenclature. The oldest of these units is the Success Formation which is composed of the Jurassic S1 and the Cretaceous S2 members. The S1 is bounded by unconformities and characterized by laminated and bioturbated siltstone, shales and minor mudstones deposited within marginal-marine environments. The sand-stone prone S2 overlies the S1, the contact being the Sub-Cretaceous Unconformity. Sandstones are quartzose, cherty, kaolinitic and cross-bedded and formed on a widespread braidplain. Like the S1, the S2 is capped by a well-developed paleosol and an unconformity formed on a long-term stable surface.

Uplift and erosion occurred after deposition of the S2, forming deep valleys with relief greater than 70 m. Between adjacent valleys, the Success, Roseray and Rush Lake formations formed cuestas. The occurrence of these valleys and cuestas provided the main trapping configuration for the oil-producing trend.

The valleys were progressively infilled by onlapping sediments of the McCloud, Glauconitic, Dimmock Creek, Atlas and Chokecherry Creek (newly defined) members of the Cantuar Formation. Distribution of the McCloud Member is restricted to the thalweg of the valleys and the member is comprised of shales, silts and quartzose sand-stones deposited within high-sinuosity meandering channels and floodplain environments. Sandstones of the Glauconitic Member, were identified overlying the McCloud member in a few wells. Correlation of the Glauconitic Member from Alberta into southern Saskatchewan is considered significant although the distribution of the member remains unknown. A significant change in mineralogy occurs at the base of the Dimmock Creek and extends throughout most of the overlying Atlas Member. Sediments from both members include green mudstone and siltstone, highly lithic and feldspathic cross-bedded fining-upward sandstones and coal. These sediments were deposited within meanderbelt and floodplain environments. In several wells, quartzose and porous sandstones, occurring stratigraphically higher than the Glauconitic Member were interpreted from wireline logs and confirmed from cuttings samples. Like the Glauconitic Member, the distribution of these potentially economic sandstones remains unclear.

A new stratigraphic unit, the Chokecherry Creek Member, occurring at the top of the Atlas Member is comprised of lithic and quartzose sandstones with tidal mud drapes, black shales and a limited suite of trace fossils. These sediments were deposited within an estuary setting.

Depending on the location, the marine shales and sandstones of the Pense Formation overlies either Success S2 (cuestas), the Chokecherry Creek Member or the non-marine sediments of the Atlas Member (valleys). The base of the Pense represents a transgressive surface of erosion, but may also be a sequence boundary. The Pense is comprised of series of stacked coarsening-upwards, wave and hummocky rippled bioturbated sandstones and shales of lower shoreface and shelfal origin. The IHACM (Index Horizon Above the Cantuar Marker) represents a condensed section, signifying the maximum flooding surface of the Pense transgression. Several sandstones thought to represent incised valley fill were also recognised in core and wireline logs within the Pense Formation.

The most important reservoirs and potential reservoirs, excluding the prolific Jurassic Roseray and Upper Shaunavon formations are the Success S2, quartzose sandstones in the Cantuar (including those found in the McCloud, Glauconite, Dimmock Creek/Atlas and Chokecherry Creek members) and the Pense.