Devonian (Givetian and Frasnian) reef reservoirs in Alberta and British Columbia contain 60% of the conventional recoverable oil and 20% of the recoverable gas in the Western Canada sedimentary basin. Although the depositional history of these reefs is well understood, it is the diagenetic "overprint" that is often responsible for their reservoir quality.

Frasnian (Woodbend and Beaverhill Lake Group) reefs are characterized by stromatoporoid and coral knoll reef belts deposited near moderately sloping bank edges. Bank margin sediments are composed of skeletal lime grainstones, packstones, rudstones, and rare framestones. In contrast, bank interiors are often extensive (e.g., Redwater, Swan Hills) and characterized by cyclic deposition of lagoonal and tidal flat sediments. Certain Givetian reefs found in evaporate basins (e.g., Rainbow or Zama) usually occur as "pinnacle" reefs with steep (> 20°) margins and only minor bank interior development. Frasnian reef complexes range in size from 1 km² (0.4 mi²) to greater than 600 km² (230 mi²) with thicknesses from 100 to 400 m (330 to 1,300 t). Givetian pinnacle reefs are commonly as much as 300 m (984 ft) thick, but with areal extents of less than 1 km² (0.4 mi²).

Regardless of differences in size, depositional history, and age, most reefs have been subjected to diagenesis in essentially three environments: (1) submarine (marine to hypersaline pore waters), (2) subaerial (fresh to brackish pore waters), and (3) subsurface (below phreatic aquifers, saline to brackish pore waters). Fibrous calcite cements, syndepositional fracturing, displacive calcite cements, micrite cements, and bored hardgrounds are typical submarine diagenetic fabrics, particularly at bank margins in Rainbow reefs and certain Leduc reefs (e.g., Golden Spike, Ricinus). Subaerial disconformities are numerous in most reefs, and associated vadose diagenesis produces localized paleosols, microstalactitic and meniscus cements, and abundant solution porosity. Phreatic or shallow bu ial cements usually include clear, equant calcite or dolomite that vary in Fe⁺⁺ and Mn⁺⁺ concentrations. Subsurface cementation produces nonferroan calcites and dolomites which are often related to stylolite formation (e.g., Kaybob, West Pembina D-2, Strachan, Ricinus). Other diagenesis occurring during burial includes dolomite and anhydrite replacement, sulfide mineralization (e.g., Pine Point, Presqu'ile barrier reef), and bitumen formation (e.g., Clarke Lake, Rainbow).

Primary porosity and permeability are altered by the "overlapping" processes of cementation and solution (vadose and/or phreatic) that occur early in the diagenetic history. In reef interiors these subaerial processes produce stratified reservoirs with impermeable barriers (cemented beds) to vertical flow (e.g., Golden Spike, Swan Hills, Judy Creek). Submarine cementation is rare in most reefs but can be locally pervasive resulting in occlusion

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of bank margin and fore-reef porosity. Absence of significant subsurface cementation in many reefs (e.g., Redwater, Golden Spike) aids in preservation of the reservoirs formed during earlier diagenesis.

In summary, it is the early diagenetic history in many Devonian reefs in the Western Canada basin that is responsible for reservoir distribution and quality. Likewise, the knowledge that reef margins and interiors often have different cementation histories is important in both reef exploration and reservoir management.

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