Abstract

The Nisku or Arcs Formation is part of the Upper Devonian Winterburn Group. Near Enchant it is overlain by the anhydritic Stettler Formation and overlies the argillaceous Ireton/Grotto Formation. It was deposited on the interior of the southern Alberta shelf in a restricted peritidal to supratidal environment.

The Nisku Formation can be divided into three informal members each of which is composed of several restrictive upward cycles. The dominant lithology is dolomite. Diagenetic effects are pervasive and anhydrite is common as a cement and void filling. Repeated subaerial exposure has produced abundant dissolution and collapse features.

Twelve different facies were recognized within the Nisku Formation. Some of these are recurrent in the three members. The main reservoir facies are:

1) Planar laminated and current bedded calcisiltites and grainstones, which represent deposition on the tidal flat and within tidal channel-ways on a very broad restricted shelf.

2) Stromatolitic mudstones and boundstones, which were deposited dominantly as bioherms that were located on preexisting highs. Subordinate thin biostromes are also developed.

3) Peloidal packstones and grainstones, which represent deposition of a detrital wedge shed off of the stromatolitic bioherms.

Typically, these reservoirs have porosity ranging from 10% to greater than 20% and average 15%. Permeabilities range from 10 to several hundred millidarcies and average between 30 and 50 millidarcies. Other lithofacies include nodular bedded argillaceous mudstone, altered mudstone, inter-laminated shale and mudstone, shale, synsedimentary breccia, crackle breccia, laminated anhydrite and wispy mudstone.

Although the reservoir facies are in vertical and horizontal communication, anomalous oil-water contacts in the Arcs “F” pool demonstrate reservoir heterogeneity.