Conduit Controlled Burial Dolomitization in Devonian Reservoirs of Western Canada Sedimentary Basin

Eric Mountjoy

More than half of the carbonate buildups consist of secondary dolomites, whereas adjacent ones are only slightly dolomitized. Examples occur at seven different stratigraphic levels: Keg River, Pine Point, Slave Point, Swan Hills, Leduc, Nisku, and Wabamun (ranging from Givetian to late Famennian). The diagenesis, especially the dolomitization, is poorly known.

These secondary dolomites are syn- to post-stylolitization and probably formed at intermediate burial depths below about 700 to 1,000 m. Most are not related to unconformities or evaporites and were not subjected to freshwater diagenesis or refluxing fluids from an evaporite basin. These dolomites are medium to coarse crystalline, sucrosic, or saddle types. Porosities remain similar to the precursor limestones (6 to 10%) whereas permeabilities generally increased, depending on the type of dolomite. Enormous amounts of fluids are required during dolomitization.

Three main factors appear to govern the occurrence of these secondary dolomites: (1) platform margins or buildups adjacent to shale basins; (2) underlying permeable carbonate platforms; and (3) fracture and fault systems. The best examples of platforms and buildups associated with basin shales are the Slave Point platform in British Columbia and the Miette buildup in the Rocky Mountains. Beneath some Leduc buildups a permeable platform (Cooking Lake) probably acted as a conduit for transporting dolomitizing fluids and later hydrocarbons updip. They are only dolomitized where the underlying platform is also dolomitized.

Some buildups are related to fracture and fault systems (trending north-northeast, northeast, and northwest) that appear to have formed vertical conduits in conjunction with porous and permeable units. The best examples are Pine Point, Nisku (West Pembina), and Wabamun (Normanville). Carbonates associated with these conduits were extensively to completely dolomitized whereas adjacent carbonates and buildups are not. It is inferred that during differential subsidence, burial fluids utilized some of these conduits causing extensive solution and dolomitization. These dolomites are commonly preceded by fine crystalline matrix dolomites that formed during early burial. Extensive solution, probably due to generation of CO₂ from organic matter, followed creating moldic and vuggy p rosity. In addition to the chemistry, increased temperatures and long periods of time promoted dolomitization along the main conduit system.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.