Abstract

The Nisku Formation in the subsurface of central Alberta, Canada, contains pervasive dolomites that occur mainly as a replacement of reefal limestones. The degree of pervasive dolomitization increases downdip along a structural homocline from about 25 to 100 vol-% and associated replacive anhydrites increase from about 5 to 20 vol-%. Pervasive dolomites occur as two types. One type consists of almost non-porous and impermeable mosaics of cloudy crystals that have diameters of 5-300 microns. The other type has significant intercrystalline porosity and permeability and consists of cloudy and clear crystals ranging in size from 100-500 microns. Cathodoluminescence zonation is rare, but modulated structures observed with TEM are common. Both types of dolomite have strong XRD ordering reflections and are nearly stoichiometric. Sr-concentrations (25-125 ppm) increase and Mn-concentrations (20-400 ppm) decrease upwards in several cores. The boron contents of both dolomite types (20-50 ppm) are enriched relative to other diagenetic phases (< 18 ppm). The Sr-isotope ratios vary from 0.7089-0.7107 and are enriched relative to Devonian and younger seawater. Oxygen and carbon isotope ratios cluster around -5.8 permil PDB and +2.0 permil PDB, respectively, and are similar to those of remnant calcites. From the combination of stratigraphic, petrographic, petrophysical, and geochemical data, it is inferred that both types of matrix dolomite formed contemporaneously, from the same fluids, in a flow regime with predominantly upward fluid movement, at temperatures between about 40-50 degrees C, and at burial depths of about 300-1,000 m. Most anhydrites formed originally as gypsum during the advanced stages and after matrix dolomitization, and later recrystallized to the anhydrites now present. Based on all accumulated data and on mass balance calculations, at least two types of subsurface flow could have caused pervasive dolomitization and gypsification/anhydritization: 1) expulsion of burial compaction water, i.e., chemically modified seawater, with or without additional contributions of Mg-bearing formation waters from deeper parts of the basin (e.g., Middle Devonian brines); and 2) thermal convection of formation fluids originally underlying and/or overlying the Nisku.

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