ABSTRACT: Burial Dissolution of Dolomites in Upper Devonian Leduc Buildups, Western Canada Sedimentary Basin

AMTHOR, JOACHIM E., and ERIC W. MOUNTJOY, McGill University, Montreal, Quebec, Canada, and HANS G. MACHEL, University of Alberta, Edmonton, Alberta, Canada

Dolomitized Leduc buildups (Upper Devonian) of the Rimbey-Meadowbrook reef trend in Central Alberta produce hydrocarbons from secondary pore systems, which are predominantly vuggy and moldic. Dissolution of remnant calcitic components such as fossils and matrix during and after pervasive replacement dolomitization has been invoked in the past to explain the generation of secondary porosity.

Evidence for a second, extensive dolomite-dissolution event in addition to the earlier vuggy and intercrystalline porosity is evidenced in several buildups (e.g., Leduc, Bonnie Glen, Westerose) by breccias of massive replacement dolomites. Collapse breccia and associated fracture systems are most commonly observed in buildup interiors. In the Leduc buildup (8-14-50-26 W4), a 50 ft thick breccia occurs at the base of the Leduc above Cooking Lake limestones. Breccias are characterized by centimeter-sized angular clasts with point or sutured contacts, forming open framework breccia porosity. The dolomite clasts are identical to nonbrecciated dolomite intervals and display truncation of stylolites and fractures at the edges of clasts. Inclined stylolites within individual dolomite clasts ndicate rotation and displacement. Clasts were sharply overgrown by compositionally zoned dolomite cements.

Paragenesis, petrographic, and geochemical data suggest a shallow burial origin (500-1000 m) for the massive replacement dolomites. Dolomite dissolution of these replacement dolomites had to occur in the subsurface, postdating stylolitization. Thus, porosity enhancement occurred during intermediate burial, and possibly deeper. Vertical to subvertical fractures associated with the dissolution and collapse of reef interior may locally improve permeability by connecting the breccia porosity with the earlier vuggy and moldic pore systems.

The fact that dolomite dissolution took place along the Rimbey-Meadowbrook reef trend, and that brecciated and nonbrecciated rocks are of the same dolomite type, suggests that dissolution is a function of pore-fluid chemistry rather than of a specific dolomite type. The sources of fluids causing dolomite dissolution are problematic. Calcium and possibly CO2-rich burial fluids moving along the reef trend are thought to be responsible for dolomite dissolution, causing collapse and porosity enhancement in these buildups.

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