ABSTRACT: Origin of Zebra Dolomites and Associated Secondary Porosity in Devonian Sequences of Western Canada
DRAVIS, JEFFREY J., Rice University, Houston, TX, and IAIN D. MUIR, Esso Resources Canada Ltd., Calgary, Alberta, Canada
Zebra dolomites reservoir hydrocarbons and lead-zinc deposits in the Devonian Keg River, Muskeg, and Sulfur Point formations. Examination of cores (10,000 ft), wireline logs (145 wells), and thin sections
(750) from the Upper Elk Point Group along the northern margin of Rainbow basin reveal that the Zebra dolomite formation was controlled by depositional facies and stratigraphic position, but only when certain diagenetic and
structural
requirements were met.
Zebra dolomites consist of darker brown, fine to medium crystalline matrix dolomites alternating with white, often baroque, coarse dolomites. They developed in highly stylolitic, fractured burial dolomites that replaced restricted subtidal Amphipora-peloidal packstones and wackestones. As a result, most Zebra dolomites were confined to the bases of platform-interior, upward-shoaling cycles. Vuggy porosity resulted from burial dissolution of darker matrix dolomites along pressure solution seams cross
-cut by fractures. This explains the horizontal to subhorizontal orientation of most Zebra dolomite "banding" and associated vuggy porosity.
A deep-burial origin for Zebra dolomite is confirmed by the replacement of stylolitic matrix dolomite by white, coarse, often baroque, dolomites and by stylolitic seams and fractures (including tectonic ones) terminating into vuggy porosity, or solution-enlarged into such porosity. White, coarse dolomites also precipitated as cement although many were subsequently dissolved to varying degrees.
Close spatial association of Zebra dolomites with burial anhydrites suggests that acidic calcium-rich fluids, moved upward along faults and fractures, promoted the observed dolomite dissolution. Most of the white, coarse dolomites, therefore, were derived from dissolution of precursor matrix dolomites.
AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)