--> Abstract: Dolomitization and Reservoir Quality in Upper Devonian Nisku Pinnacle Reefs, West Pembina Area, Alberta, Canada, by A. M. C. Chow; #91012 (1992).

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ABSTRACT: Dolomitization and Reservoir Quality in Upper Devonian Nisku Pinnacle Reefs, West Pembina Area, Alberta, Canada

CHOW, ANDRE M. C., Chevron Canada Resources, Calgary, Alberta, Canada

The Upper Devonian Nisku pinnacle reef trend in the West Pembina area is located in south-central Alberta (T47-52, R8-13W5M) roughly 250 km northwest of Calgary. Since their discovery in the late 1970s, these reefs have been prolific hydrocarbon producers. Approximately 50 pinnacle reefs have been discovered to date in the northeast-southwest-trending West Pembina area, which covers a fairway about 50 km in length and 10 km in width. These pinnacle reefs are small in size (average 1 km in diameter) and grew up to 100 m in height. They are usually circular in plan view and mushroom-shaped in cross section.

The Nisku pinnacle reefs are different from other Upper Devonian reefs of the Western Canada sedimentary basin. (1) They were initiated in deep water (roughly 50 m). (2) Their fauna is dominated by corals rather than stromatoporoids. (3) They exhibit a vertical facies zonation that is different from the typical lateral facies zonation exhibited in the larger Upper Devonian Swan Hills and Leduc reefs of Alberta.

There are several petrographically recognized dolomite types in the Nisku. The three most important types are a fine-grained matrix dolomite, a coarse-grained matrix dolomite, and saddle dolomite. All of the Nisku pinnacle reefs contain fine-grained matrix dolomite in varying amounts. Reefs with only this type of dolomite present are poor reservoirs as this type of dolomite does not enhance porosity or permeability. Reefs that contain the coarse-grained matrix dolomite are generally excellent reservoir rocks. This type of dolomite, along with the associated dissolution of limestone, greatly enhances the porosity and permeability. Saddle dolomite commonly occurs as a pore-filling cement in reefs that contain coarse-grained matrix dolomite. It reduces the porosity but generally does not adversely affect the permeability.

Fine-grained matrix dolomite and coarse-grained matrix dolomite are formed during intermediate burial diagenesis. Saddle dolomite is formed during the late stage diagenesis.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)