Abstract

Saddle dolomite, anhydrite and coarsely crystalline, late mosaic calcite cements in the Devonian Swan Hills Formation at the Rosevear Field in Alberta, Canada are interpreted from petrographic and geochemical evidence as the products of burial diagenesis. These inferred burial cements are commonly found as a partial to complete filling of moldic and vuggy porosity in massive replacement dolostones. Fluid inclusions (T h = 125-159 degrees C; T m = -18 to -22 degrees C), 87 Sr/ 86 Sr ratios (0.7101-0.7115), enrichment in Ba and Zn, and light delta 18 O values (- 6 to - 11.2 per thousand , PDB) suggest that saddle dolomite precipitated from hot, radiogenic brines with salinities six times greater than seawater. Ions for saddle dolomite precipitation may have been sourced from off-reef shales, the Precambrian basement, Middle Devonian halite, Mesozoic clastics and Swan Hills replacement dolomite undergoing pressure solution. In the presence of a "normal" paleogeothermal gradient ( nearly equal 30 degrees C/km), saddle dolomite would have formed in the Late Cretaceous-Paleocene, at depths of 3-5 km. This time corresponds to the emplacement of gas at the Rosevear Field. Dolomite-hosted anhydrite cement with nonradiogenic 87 Sr/ 86 Sr (0.7083) and low delta 34 S compositions (15 to 23 per thousand CDT) has been quantitatively modeled using a two-component mixing equation to constrain fluid and ion sources. The results indicate that ions for cementation were derived from fluids that passed through and dissolved Middle Devonian anhydrite. This produced a Sr-rich brine with nonradiogenic 87 Sr 86 Sr and low delta 34 S. These fluids mixed with radiogenic formation water already in the reservoir to precipitate anhydrite cement with the observed isotopic composition. Fluid inclusions (T h = 90 to 170 degrees C; T m = -6 to -17 degrees C) and elevated 87 Sr/ 86 Sr compositions (0.7105-0.7173) in coarsely crystalline, late mosaic calcite support precipitation during the hydrological transformation of the Western Canada Basin into a topographically-controlled flow regime. The reduced precipitation temperatures of some fluid inclusions in late mosaic calcite compared to saddle dolomite may be attributed to the decrease in western basin paleogeothermal gradients following the Laramide Orogeny. The presence of calcitized anhydrite, coarsely crystalline calcite with light delta 13 C values (-5.4 to -13.2 per thousand PDB), 34 S-enriched pyrite, and 8% H 2 S in reservoir gases provide evidence for thermochemical sulfate reduction during burial diagenesis at Rosevear.

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