DIAGENESIS AND EVOLUTION OF MICROPOROSITY OF MIDDLE-UPPER DEVONIAN KEE-SCARP REEFS, NORMAN-WELLS, NORTHWEST-TERRITORIES, CANADA - PETROGRAPHIC AND CHEMICAL EVIDENCE

The Middle-Upper Devonian Kee Scarp reef complexes of Norman Wells, No rthwest Territories, Canada, are oil-producing, stromatoporoid-dominat ed carbonates. Episodic increases in the rate of sea level rise produc ed multiple cycles of reef growth that exhibit backstepping characteri stics. These carbonates, composed of invariably altered limestones, ha ve original interskeletal, intraskeletal, and intergranular porosity m ostly occluded by nonferroan, dull luminescent cements, Secondary poro sity, represented by micropores of various types, developed during dia genesis by aggrading neomorphism and dissolution. The micropores repre sent the main reservoir porosity in the Kee Scarp limestone. Micropore s in the Kee Scarp limestone can be classified into four categories ba sed on their shapes: (1) stepwise rhombic, about 1 mu m to 2 mu m in d iameter, developed mainly in stromatoporoids; (2) intercrystalline rho mbic, about 1 mu m in diameter, developed mainly in algal aggregates, (3) microvugs, 4 mu m to 10 mu m in diameter, developed mainly in alga l aggregates; and (4) microchannels about 12 mu m in length and 0.5 mu m in width, developed in algal aggregates and stromatoporoids. The st epwise rhombic and microvugs are developed mainly at the Kee Scarp ree f margin and constitute the best type of reservoir porosity. Petrograp hic, chemical, and isotopic studies of Kee Scarp reef components revea l a complex diagenetic history involving marine fluids modified by inc reasing water/rock interaction and burial. Early diagenetic processes include marine cementation and micritization followed by neomorphic re placement of high-Mg calcite and aragonite reef components, resulting in the creation of the first generation of microporosity via dissoluti on on a micron scale. Later diagenesis, represented by microfracturing and cementation, with two generations of equant calcite and styloliti zation, was responsible for the second generation of microporosity. Ve ry minor silicification, dolomitization, and vertical fracturing occur red at variable depths, Portions of well-preserved marine cements, str omatoporoids, and rare crinoids of postulated high-Mg calcite precurso r mineralogy have escaped diagenetic alteration and preserve the origi nal marine delta(18)O and delta(13)C signatures (-4.7 +/- 0.3 parts pe r thousand PDB for oxygen; 1.0 +/- 0.4% PDB for carbon), Minor and tra ce element data show less preservation of the postulated original mari ne composition. Neomorphic stabilization of skeletal components caused further depletion in delta(18)O but very little change in delta(13)C, an argument for modification of the original marine fluids with incre asing burial, Variations in magnitude of water/rock interaction with d epth, facies changes, and porosity modifications probably exerted some control on fractionation and distribution of stable isotopes and trac e elements in reef components.