ABSTRACT

In central Alberta, the Cooking Lake limestone or foundation unit for reef growth is divisible into four widespread lithologic units, in ascending order: basal dolomitic unit, lower calcarenite, middle argillaceous unit, and upper calcarenite. Carbonate buildup in the Cooking Lake in reef areas is marked generally by coalescence of the lower and upper calcarenites at the expense of the middle argillaceous unit, and by thickening of the upper calcarenite at the expense of the overlying Duvernay (brown to black shales and limestones).

Limestones ranging from calcarenites to calcilutites and exhibiting negligible to complete dolomitization are present in the Cooking Lake and Leduc formations. The calcarenites comprise bioclastic and pseudo-oölitic (precipitated) types, and likewise the calcilutites include both bioclastic material and precipitates.

Shallow- and agitated-water calcarenites and local patch reefs commenced to form in latest Beaverhill Lake time along a restricted north-south axis passing through the North Woodbend and South Acheson fields in the Leduc area. Reef sedimentation, initially confined to a narrow 2–3-mile belt along the western part of the shoal, onlapped eastward from a few to 10 miles by late Cooking Lake time. The ancient shoal is now marked by completely dolomitized limestones which extend farthest eastward in the area directly southwest of Edmonton (“Whitemud lobe”). On this relatively broad foundation, complete reef buildup was largely localized in the western part of the foundation. The final reef configuration, conforming closely to the productive Acheson, Leduc, Glen Park, and Wizard Lake fields, is marked by intervening channels or saddles. Formation of the northeast and southeast reef extensions at the Leduc field is believed to have been controlled in part by ocean currents in later Leduc time.

In the Stettler area, late Cooking Lake sedimentation produced shallow-water limestones, now dolomitized, along three main trends termed Fenn-Big Valley, Stettler-Erskine, and Gadsby shoals. The Gadsby shoal was characterized by widespread biostromal carbonate deposition in the northern part of the Southern Alberta shelf. The south-southwest-trending Fenn-Big Valley shoal, apparently extending several miles southwest of Big Valley field, marked the site of earliest reefoid deposition (latest Cooking Lake time), followed somewhat later (early Duvernay time) by incipient reef foundation development in the Stettler and Erskine field areas. Later, mainly vertical carbonate buildup occurred along the central parts of the Fenn-Big Valley and Stettler-Erskine shoals. However, lateral growth and onlap occurred west of the Fenn field and between the Stettler and Fenn-Big Valley fields, and caused the final Leduc configuration to extend south continuously from the Stettler to the Big Valley field, as well as west of these fields. Although a common carbonate foundation underlay the Stettler and Erskine fields, localization into separate reefs occurred.

In the Redwater area, a prominent shoal exhibited deposition of relatively uniform limestone types and organic content throughout, except at the eastern margin. In the lower and upper Cooking Lake relatively narrow algal-stromatoporoid biofacies belts are developed at 6 to 10 and 4 to 5 miles, respectively, east of, and subparallel with, the Redwater oil field. Essential coalescence of the lower and upper calcarenites and carbonate buildup in the Cooking Lake at the expense of the overlying Duvernay characterizes the biostromal limestone foundation as far as its eastern margin at the algal concentration. It is interpreted that the biostromal margins in the Cooking Lake east of Redwater represent incipient reef margins or fronts, which migrated westward between Cooking Lake and late Leduc time. In the uppermost Leduc reef algal structures are concentrated in a narrow easternmost belt parallel with the reef front in the eastern part of the Redwater field. Stromatoporoids and associated organisms formed part of the “organic lattice” in a belt adjoining at the west. In turn, a non-stromatoporoidal biofacies, including precipitated and poorly fossiliferous limestones, was deposited west of the organic lattice.

Calcarenite and biofacies patterns indicate that the organic lattice at Redwater represented an actively growing reef front subjected to ocean currents and possibly prevailing winds from the northeast. West of the organic lattice some relatively quiet, locally restricted lagoon-type sedimentation occurred. The upper Leduc deposits in the central part of the Redwater reef mass are believed to have been subject to solution-compaction effects, resulting in a slightly thinner Leduc section as compared with the margins. The highest level of occurrence of significant stromatoporoids, a biostratigraphic boundary, rises in the Leduc limestone section at least 500 feet from the interior to marginal parts of the Redwater reef mass.

Lithofacies and biofacies studies of the Cooking Lake foundation may be applied in delineation of favorable areas for reef exploration. Patterns on facies maps showing early shoals or incipient algal-stromatoporoid belts may be used in determining possible reef extensions as well as outlining reef prospects in wildcat areas.

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