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Stratiform Breccias in the Mississippian Mission Canyon Formation (Montana) as an Analogue for Understanding Bitumen-Saturated Breccias in the Grosmont-Upper Ireton Formations, Canada


The Devonian Grosmont–Ireton–Nisku succession in Alberta is estimated to contain 508 billion bbl (80.8 billion m3) of bitumen. Pilot testing of the Grosmont is demonstrating the commercial production potential of these reservoirs, which are poised to become some of the largest carbonate fields in the world. A significant amount of the resource in the Grosmont and Upper Ireton formations is contained in stratiform brecciated dolostone interpreted to have formed by the dissolution of interbedded evaporites (Hondo Member). Overprinting the succession are karst-related features associated with the sub-Cretaceous unconformity. Reservoir characterization and modeling of the bitumen-saturated breccias is a critical step in field development but is technically challenging given the inherent limitations of core analysis on breccias and the paucity of analogues to draw upon. Stratiform units of brecciated carbonate in the Mississippian Mission Canyon Formation, exposed at surface in Montana, correlate with carbonate-evaporite successions in the subsurface. These breccias have been interpreted to be the result of evaporite dissolution and mechanical collapse of interbedded carbonate, and published correlations are at similar spatial scales as breccia-evaporite correlations between the Grosmont-Upper Ireton breccias and Hondo evaporites. Karst-related features associated with the Madison unconformity overprint the stratiform breccias. As such, breccias in the Mission Canyon Formation are interpreted to be excellent analogues to the Grosmont-Upper Ireton breccias. Study of outcrop exposures of the Mission Canyon Formation breccias in the summer of 2013 revealed that (1) breccias formed by evaporite dissolution versus karst-related processes could be distinguished by matrix properties, (2) periodic shifting of the breccia caused distortion of cavern and void-filling sediment, leading to a variety of complex, convoluted sedimentary features, and (3) rafting of boulder-sized slabs of wall-rock with negligible rotation is not uncommon in the breccias. Breccia porosity is highly variable. The study of these analogues offers important insight to the heterogeneity in the Grosmont and Upper Ireton breccias and explains a number of sedimentary features that have been difficult to explain. Furthermore, the outcrop analogue highlights challenges that can be expected when attempting to identify and model reservoir flow units in a brecciated reservoir.