ABSTRACT: Example of Burial Dolomite from the Devonian Swan Hills Formation, Rosevear Field, Central Alberta

KAUFMAN, JONATHAN, Exxon Production Research Company, Houston, TX

The Swan Hills Formation provides an excellent opportunity to observe and interpret the origin of burial dolomite from the Western Canada basin. Unlike dolomitized Leduc reefs, Swan Hills depositional fabrics are preserved despite massive dolomitization. In this display, representative Swan Hills burial dolomite from the Rosevear field is exhibited. Limestone to dolomite transitions are well displayed in the core examples provided. A direct relation between increasing extent of dolomitization and creation of moldic porosity is illustrated.

Massive dolostone at Rosevear is primarily restricted to opposing margins of a marine channel that penetrates into the bank margin. Branching and bulbous stromatoporoid floatstones and rudstones along the channel margin are selectively dolomitized, resulting in excellent moldic and vuggy porosity. This porosity was partially occluded by, in order of decreasing age: (1) saddle dolomite; (2) anhydrite; (3) bitumen; and (4) coarse calcite cement.

Petrographically, replacement dolomite is coarsely crystalline, inclusion-rich, and exhibits euhedral through anhedral textures. Geochemically, dolomite is characterized by light Oxygen 18 values (-4.5 to -8.7o/oo PDB), nearly stoichiometric composition, and variable 87Sr/86Sr ratios (0.7082-0.7104). Evidence for a burial origin is provided by the following observations: (1) petrographic relations show that dolomite is younger than or coeval with nonsuture seams, intergranular pressure solution, and early fractures; (2) dolomite replaces marine and meteoric calcite cements; (3) dolomite is depleted in 18O and enriched in radiogenic Sr compared to a hypothetical Devonian marine dolomite; and (4) massive dolostones are limited to the deeper buried Swan Hills bank; updip Swan Hills atoll reefs are unaffected by replacement dolomitization. It is hypothesized that Swan Hills dolomite formed in a burial environment at depths greater than 300 m and temperatures between 35 and 75 degrees C. The hydrologic regime responsible for dolomitization is poorly constrained at this time.

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