ABSTRACT: Diagenesis of Middle-Upper Devonian Kee Scarp Reefs, Norman Wells, N.W.T.: Chemical and Isotopic Evidence

AL-AASM, IHSAN, and KAREM AZMY, University of Windsor, Windsor, Ontario, Canada

The Middle to Upper Devonian Kee Scarp reef complexes of Norman Wells, N.W.T., are oil-producing, stromatoporid-dominated carbonates. These complexes, composed of invariably altered limestone, have original interskeletal, intraskeletal, and intergranular porosity, often occluded by nonferroan dull luminescent cements. Secondary porosity is represented mainly by micropores of various modes. The micropores represent the main reservoir porosity in Kee Scarp limestone. 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 level. Later diagenetic events, represented by microfracturing, and cementation with two generations of equant calcite and stylolitization, were responsible for the second generation of microporosity. Very minor silicification, dolomitization, and vertical fracturing occurred at variable burial depths. Portions of well-preserved marine cements, stromatoporids, and rare crinoids of postulated hig Mg-calcite precursor mineralogy have escaped diagenetic alteration and preserve the original marine Oxygen 18 and Carbon 13 signatures (-4.70 +/- 0.3o/oo PDB for oxygen; 1.0 +/- 0.45o/oo PDP for carbon). Minor and trace element data, however, show less preservation of the postulated original composition. Neomorphic stabilization of skeletal components caused further depletion in Oxygen 18 but very little change in Carbon 13, an argument for modification of the original marine fluids with increasing burial depth. Variations in magnitude of water/rock interaction with depth, facies changes, and porosity change probably exerted some control of fractionation and distribution of stable isotopes and trace elements in reef components.

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