Abstract

The Wembley Field, located in Twp 72 and 73 Rge 8W6 about twenty kilometers northwest of Grande Prairie, produces oil and gas from the Halfway Formation of Triassic age.

The Triassic sequence in northwest Alberta consists of a rhythmic section of shallow marine siltstones, sandstone, dolomites, coquinas, evaporites, carbonate muds and red beds. In the Wembley area, the Halfway is underlain by the Doig and overlain by the Charlie Lake. The Doig and Halfway Formations of the Middle Triassic were deposited in shallow seas in the east, and in deeper marine waters in the west. In the field area, this sequence consists of shallow marine sands, coquina banks, dolomitic mudstones and shales.

The contact between Charlie Lake and Halfway Formations on logs is usually picked at the top of a thinly laminated bed of dolomite and anhydrite. An unconformity marked by chert grains occurs immediately below this previously described unit and the Halfway sand. The Halfway and Doig are usually separated by a transition zone in which several disconformities are concentrated above the Doig sand. Other apparent unconformities are occasionally found within the Halfway Formation itself.

The Halfway is comprised of quartz sandstones and coquina beds which grade from dolomitic sandstone to sandy dolomite. Anhydrite cement is common within the sandstone and coquinas. In the sandstone it occurs as disseminated cement, scattered poikilitic nodules, and concentrated in horizontal beds of one to several centimeters in thickness. When the anhydrite cement is concentrated in horizontal beds, it greatly reduces vertical permeability. Phosphate is common in very small pelletoids, as collophane–coated quartz grains and in phosphatic shell fragments. In places, a concentration of phosphatic Lingula brachiopods in the sand is marked by a corresponding strong Gamma response on logs mistakenly indicative of a shale interval.

Three facies of the Halfway are the well sorted quartz sand, shell banks which form the coquina facies, and carbonate mud. These three facies types can be seen in 7-2-73-8W6, 7-1-73-8W6 and 6-3-73-7W6 cores. Clasts of dolomitic mudstone are occasionally found in the clean sandstones and coquina beds. Most of the porosity is secondary. In the sand, the cement is leached as well as some of the grains. In the coquina facies, the skeletal fragments are leached to produce mouldic porosity. The permeability is variable.

The Halfway Formation was deposited in near-shore shallow marine environment where frequent minor changes in water depth resulted in many facies variations.