The Devonian Grosmont: Unconventional Oil in a Fractured and Karstified Dolostone-Evaporite Platform, Alberta, Canada
The Upper Devonian Grosmont platform in Alberta, Canada is the world’s largest unconventional oil reservoir hosted in carbonates, with at least 400 Bbbl bitumen in place depths of about 250 - 500 m. Since ~2006 there has been a flurry of activity to bring this reservoir to production.
The reservoir characteristics are due to five major factors and/or processes: sedimentary stratigraphy, dolomitization, fracturing, karstification, and biodegradation. Exploitation of the reservoir must take into account all these aspects.
The sedimentary stratigraphy of the reservoir consists of 6 stacked carbonate units: LGM, UGM1, UGM2, UGM3 (Grosmont Formation), and the overlying Upper Ireton and Nisku Formations. These units are shallowing-upward cycles, interbedded with marls (called ‘shale breaks’). In addition, the reservoir contains an evaporite sub-unit, the Hondo Formation, in parts of the platform. The marls and evaporites acted as aquitards during diagenesis and oil migration but are breached or missing in parts of the area today. Dolomitization by density-driven reflux was the first pervasive diagenetic process. Most dolostones are fine-crystalline and tight, however, and the only notable porosity related to dolomitization is scattered molds and vugs. Subsequent fracturing and karstification combined to generate most of the present porosity and permeability. A dense facture network was created probably by Laramide tectonics to the west accompanied and/or followed by local isostatic adjustment of the basement. The Laramide tectonics formed a foreland bulge near the Grosmont platform, which flexed the carbonates regionally, whereas isostatic adjustment of the basement created subvertical stresses. The resulting fracture network was invaded by corrosive water mostly from the top, as shown by countless sinkholes and other epikarstic features (vugs, solution-enhanced factures, collapse-breccias, wholesale evaporite removal, Cretaceous cave infills) that are widely distributed across the Grosmont platform. In addition, circumstantial evidence suggests that the downdip part of the Grosmont was affected by hypogene dissolution from basinal fluids that probably migrated updip along with the oil, followed by extensive biodegradation that may have further acidified the formation waters. Present bitumen in-situ viscosities are > 1 million cP, API gravities range from 5-9 dgs.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California