Most of the reef rock is unbedded, coarsely crystalline dolomite. This dolomite was not secreted by the reef-building organisms but has replaced the original limestone structures produced by stromataporoids and corals. In some of the reefs original textures and fossils have escaped pervasive dolomotization.
The porosity of the reef that makes it a good reservoir for oil and gas is formed partly by the haphazard growth of the framebuilding organisms and partly by the enlargement of voids during dolomotization. The dolomotization probably took place as sea water, incorporated in adjacent shales, was forced through the porous limestone, replacing some of the calcium ions with magnesium.
When clays are deposited, the pore space between the grains is occupied by the sea water and may make up 50 percent of the volume of the sediment. When the clay is compacted under the weight of sediment accumulating above and becomes shale, its porosity is reduced to about 10 percent and great quantities of water are squeezed out. The reefs within the shales act as rigid porous bodies into which the water is squeezed and through which it moves upward to escape eventually through overlying beds to the surface.
The flow of water from the shales carried with it not only magnesium and other salts leached from the shales, but also hydrocarbons derived from the decay of micro-organisms that were entombed in the shales as they accumulated. The reefs acted as traps for these hydrocarbons and became the reservoirs for many of the oilfields that now underlie Alberta.
Nearly all the patched reef and barrier reef tracts mentioned in the preceeding paragraphs have produced oil and gas except those in the Middle Devonian of Saskatchewan, where interreef areas are occupied by evaporites and not the organic-rich, dark shales that are the source beds of petroleum [ Devonian reefs of Western Canada, Peace River Dome, Swan Hills, Miette Reef, Fenn oilfield of Alberta, Tathlina Arch, West Alberta Arch ].
Under the Northern Great Plains Devonian rocks record the complex interplay between basin subsidence, the growth of reefs, and the accumulation of evaporites. Because these Devonian reefs localize Canada's major oilfields and contain much of her petroleum reserves, they have been extensively drilled from Montana to the Arctic Circle. Secondary changes in Middle Mississippian fossil-fragment limestones have produced reservoirs in which oil and gas are trapped in the Canadian Rocky Mountains, the western states, and the Prairie provinces.
As basinal shales compact, the "flushing" of waters through the "plumbing" provided by reef bodies may also produce concentrations of metals in the reefs. One of the largest lead-zinc deposits in the world is located in the Presqu' ile Barrier on the south side of Great Slave Lake at Pine Point. Metals leached as chloride complexes from the shales in the basin are believed to have been carried northward up the tip of the basin flank as the shales compacted, and precipitated as sulfides in the reef bodies where sulfide ions were available through the action of sulfate-reducing bacteria on adjacent anhydrite deposits.
This association of lead (galena) and zonc (sphalite) sulfides with reefs is not confined to the Middle Devonian of western Canada but is common throughout the stratigraphic record.