THE VIKING (ALBIAN) RESERVOIR SANDSTONES OF CENTRAL AND SOUTH‐CENTRAL ALBERTA, CANADA

Abstract

Bentonite chronostratigraphy indicates that deposition of the Viking Formation in central and south‐central Alberta and SW Saskatchewan occurred within three chrono‐intervals, informally designated Basal, Lower and Upper. Sandstones of the Basal and Lower Viking times were deposited offshore, while most Upper Viking sands formed along the strandline. In general, these Viking sands trend NW‐SE, are multiple, parallel, discrete units surrounded by wales, and are of varying thicknesses.The shoreline sands, which evolved slightly earlier than Basal Viking time, prograded NE towards the offshore for a maximum distance of about 250 kms, in places overlying the offshore sands. The offshore Basal Viking sands, characteristically thin, were deposited around Hamilton Lake, Provost, and up to Viking Kinsella in Alberta, as well as the Dodsland — Hoosier‐Smiley areas of SW Saskatchewan. The development of the Lower Viking sandbodies occurred slightly later, around the Sullivain, Ewing Lake, Joffre and Joarcam areas, SW and west of the basal sands. These offshore sand units migrated imbricatedly shorewards (SW), in opposition to the sense of progradation of the shoreline units. This SW‐direction of offshore sandridge migration is consistent with their NE flanks being relatively thinner, coarser and cleaner, suggesting NE‐SW flow currents along the NE flanks of ridges, and explains why most hydrocarbon reserves occur on the NE flanks of some of the ridges.The opposed sense of Viking sandstone migration resulted in the development of a thick mudstone interval between the strandline and offshore sandbodies, resulting in the development of a topographic sediment gradient that slopes NE. This pattern of Viking deposition explains the regional NE‐thinning pattern of the Viking Formation in Alberta, rather than it being due solely to a regressive shoreline, as previously thought.In general, Viking sandstone deposition appears to have ceased earlier in the NW than SE, suggesting that the SW source area (in the Nelson Batholith region) persisted longer than the western source area (coast Range area, west of Jasper). Missing, thin stratigraphic sections at the top of the Joli Fou, common in places in the area of the Basal sand deposition, suggest that much of the basal sediments are products of sea‐floor scouring by genetically‐related currents, rather than resulting from an unconformity, as thought by some earlier workers.