Oil accumulations of the Tule Creek area, that are found in the Devonian Nisku Formation, are a result of structural and stratigraphic conditions. Present-day productive structures have about 50 ft. of closure. These structures occur in an area of favorable Nisku patch-reef environment that was subjected to secondary dolomitization.

The Nisku of the Tule Creek field consists of anhydrite, limestone, and dolomite and averages 81 ft. in thickness. The upper one-third of the formation is primarily anhydrite and the lower two-thirds are dolomite. Some limestone always is present in the middle of the anhydrite unit. In the lower two-thirds, where found off structure, the unit is non-productive and(or) out of the favorable facies.

Oil production is from vuggy and intercrystalline porosity and permeability developed by secondary dolomitization. Occasional high-angle fractures contribute only slightly to the over-all reservoir productivity. Abundant relict Amphipora and lesser amounts of algae are apparent in the dolomite and appear to be the primary host for the dolomitization process. Only minor amounts of Amphipora are found in the non-dolomitized carbonate strata.

On structure, oil-productive dolomite consists of large-sized rhombohedral crystals oriented randomly or in point-to-point contact, improving the primary porosity and creating excellent permeability. The lesser dolomitized, non-productive, off-structure Nisku exhibits good apparent log porosity but very low actual effective permeability. These off-structure dolomites are finer-grained, slightly to moderately calcareous, and have a face-to-face crystal orientation.

The favorable patch-reef facies of the Nisku Formation is controlled by environment and trends locally in a southwest-northeast direction.

Structural closures that occur in the favorable facies area contain reservoir rocks with excellent porosity and permeability.

With a 24-ft.-thick anhydrite bed above the "pay" zone, a 10-ft.-thick dense anhydritic shale (Ireton Member) below, and a lack of extensive fracturing, the Nisku is postulated to be its own source rock.

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