Geology of a Carbonate Breccia Reservoir Rock: Grosmont Formation, NE Alberta, Canada

Abstract

The Upper Devonian Grosmont Formation contains over 400 billion barrels of bitumen resource. The main reservoir rock is a breccia consisting of angular monomictic dolomite clasts (> 1 cm) in a matrix of unconsolidated, predominantly silt size (20-200 microns) dolomite crystals, crystal fragments and crystal aggregates termed disaggregated dolomite (Barrett, In Press). When bitumen is extracted, the matrix is reduced to a white powder. The disaggregated dolomite has approximately 40% intergranular porosity. Due to the small pore throat size, permeability is measured in only tens to hundreds of millidarcies. These breccias are stratigraphically controlled. The main reservoir unit, the Lower D breccia, is 11-15 m thick and has an areal extent of greater than 3,000 sq km. SEM imaging, CT scan images, photomicrographs, wet sieve analysis, core analysis data and geological cross-sections and maps are the tools used during this presentation. It is interpreted that the brecciation and formation of disaggregated dolomite is a bi-product of early Cretaceous karsting. Breccias occur mainly in slightly argillaceous dolomite intervals. Disaggregation of primary rock texture commonly forms along fractures leading to speculation that the breccias are a bi-product of dolomite disaggregation. Breccia clasts are remnants of thin beds that escaped disaggregation but were fragmented by compaction of the matrix. Migration of hydrocarbons into the Grosmont Formation lead to the preservation of the unconsolidated, powdery nature of the disaggregated dolomite. A phenomenon termed inside-out dolomite (Jones, 2007), where the interiors of dolomite crystals have been preferentially dissolved, is common within the Grosmont breccias. Widespread development of this phenomenon could be responsible for the partial disintegration of rock fabric observed within the breccias. Disaggregated dolomite is found in other intervals in northeast Alberta including the Upper Ireton and Blueridge Upper Devonian Formations. A similar rock type, commonly termed pulverulite, occurs in south central Texas in the Upper Cretaceous Edwards Group. Field examination suggests that although much more local in occurrence, the Texas pulverulite may be an analogue to the Alberta breccias. These will be discussed in this paper. Barrett, K.R., In Press, Geology of the Saleski Grosmont Steam Pilot, Bull. Canadian Soc. of Petrol. Geol. Jones, B., 2007, In-side out dolomite. Jour. Sed. Research., p. 539-551.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016