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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 80 (1996)

Issue: 1. (January)

First Page: 82

Last Page: 99

Title: Previous HitDiagenesisNext Hit and Previous HitEvolutionNext Hit of Previous HitMicroporosityNext Hit of Middle-Upper Devonian Kee Scarp Reefs, Norman Wells, Northwest Territories, Canada: Petrographic and Chemical Evidence

Author(s): Ihsan S. Al-Aasm (2), Karem K. Azmy (3)


The Middle-Upper Devonian Kee Scarp reef complexes of Norman Wells, Northwest Territories, Canada, are oil-producing, stromatoporoid-dominated carbonates. Episodic increases in the rate of sea level rise produced multiple cycles of reef growth that exhibit backstepping characteristics. These carbonates, composed of invariably altered limestones, have original interskeletal, intraskeletal, and intergranular porosity, mostly occluded by nonferroan, dull luminescent cements. Secondary porosity, represented by micropores of various types, developed during Previous HitdiagenesisNext Hit by aggrading neomorphism and dissolution. The micropores represent the main reservoir porosity in the Kee Scarp limestone. Micropores in the Kee Scarp limestone can be classified into four categories based on thei shapes: (1) stepwise rhombic, about 1 µm to 2 µm in diameter, developed mainly in stromatoporoids; (2) intercrystalline rhombic, about 1 µm in diameter, developed mainly in algal aggregates; (3) microvugs, 4 µm to 10 µm in diameter, developed mainly in algal aggregates; and (4) microchannels, about 12 µm in length and 0.5 µm in width, developed in algal aggregates and stromatoporoids. The stepwise rhombic and microvugs are developed mainly at the Kee Scarp reef margin and constitute the best type of reservoir porosity.

Petrographic, chemical, and isotopic studies of Kee Scarp reef components reveal a complex diagenetic history involving marine fluids modified by increasing water/rock interaction and burial. Early diagenetic processes include marine cementation and micritization followed by neomorphic replacement of high-Mg calcite and aragonite reef components, resulting in the creation of the first generation of Previous HitmicroporosityNext Hit via dissolution on a micron scale. Later Previous HitdiagenesisNext Hit, represented by microfracturing and cementation, with two generations of equant calcite and stylolitization, was responsible for the second generation of Previous HitmicroporosityTop. Very minor silicification, dolomitization, and vertical fracturing occurred at variable depths. Portions of well-preserved marine cements, stromatoporoids, an rare crinoids of postulated high-Mg calcite precursor mineralogy have escaped diagenetic alteration and preserve the original marine ^dgr18O and ^dgr13C signatures (-4.7 ± 0.3^pmil PDB for oxygen; 1.0 ± 0.4^pmil PDB for carbon). Minor and trace element data show less preservation of the postulated original marine composition. Neomorphic stabilization of skeletal components caused further depletion in ^dgr18O but very little change in ^dgr13C, an argument for modification of the original marine fluids with increasing burial. Variations in magnitude of water/rock interaction with depth, facies changes, and porosity modifications probably exerted some control on fractionation and distribution of stable isotopes and trace elements in ree components.

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