Diagenesis and Evolution of Microporosity of Middle–Upper Devonian Kee Scarp Reefs, Norman Wells, Northwest Territories, Canada: Petrographic and Chemical Evidence

Azmy, Karem and Al-Asam, Ihsan S. (1996) Diagenesis and Evolution of Microporosity of Middle–Upper Devonian Kee Scarp Reefs, Norman Wells, Northwest Territories, Canada: Petrographic and Chemical Evidence. AAPG Bulletin, 80 (1). pp. 82-100. ISSN 0149-1423

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Abstract

The Middle–Upper Devonian Kee Scarp reef complexes of Norman Wells, Northwest Territories, Canada, are oil-producing, stromatoporoiddominated 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 diagenesis 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 their shapes: (1) stepwise rhombic, about 1 mm to 2 mm in diameter, developed mainly in stromatoporoids; (2) intercrystalline rhombic, about 1 mm in diameter, developed mainly in algal aggregates; (3) microvugs, 4 mm to 10 mm in diameter, developed mainly in algal aggregates; and (4) microchannels, about 12 mm in length and 0.5 mm 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 microporosity via dissolution on a micron scale. Later diagenesis, represented by microfracturing and cementation, with two generations of equant calcite and stylolitization, was responsible for the second generation of microporosity. Very minor silicification, dolomitization, and vertical fracturing occurred at variable depths. Portions of well-preserved marine cements, stromatoporoids, and rare crinoids of postulated high-Mg calcite precursor mineralogy have escaped diagenetic alteration and preserve the original marine d18O and d13C signatures (–4.7 ± 0.3‰ PDB for oxygen; 1.0 ± 0.4‰ 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 d18O but very little change in d13C, 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 reef components.

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