ABSTRACT: Subsurface Dolomite--Solution Breccias in Upper Devonian Leduc Buildups, Central Alberta, Western Canada Sedimentary Basin

AMTHOR, JOACHIM E., and ERIC. W. MOUNTJOY, McGill University, Montreal, Quebec, 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. Collapse breccia and associated fractures are observed in some buildups along the Rimbey-Meadowbrook reef trend. Examples from three buildups are illustrated: Morinville (5-13-54-26 W 40, Leduc (8-17-50-26 W4), and Medicine River (10-23-38-4 W5), along with examples of nonporous and vuggy replacement dolomites. The types and macroscopic characteristics for the dolomite breccias including angular cl sts in point and sutured contacts, rotation and displacement of clasts, and open breccia porosity are shown. Thin-section

and cathodoluminescence photomicrographs highlight the petrographic relationships such as truncation of stylolites and fractures at the edges of clasts, and inclined stylolites within individual dolomite clasts, indicating clast rotation and displacement. Dolomite clasts in breccias are petrographically identical to replacement dolomites in nonbrecciated host rocks and were discontinuously overgrown by compositionally zoned dolomite cements.

These dolomite collapse breccias differ from debris flow and buildup-flank deposits at Medicine River (1-30-39-3 W5) in terms of: (1) angular clasts of replacement dolomite, (2) lack of limestone clasts, (3) clast support texture, (4) open framework breccia porosity and lack of mud matrix, and (5) coarse-crystalline dolomite cement lining clasts. All these features are consistent with a second dissolution event and collapse of earlier replacement dolomites. The breccias clearly postdate massive replacement dolomitization, which occurred during shallow burial (500-1000 m). They probably formed during intermediate burial, and possibly deeper, prior to oil generation.

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