More than forty per cent of Canada's known natural gas resources, or 18.5 trillion cu ft of indicated gas reserves (Oil and Gas Conservation Board, Alberta, 1965), are contained in Paleozoic rocks of the Alberta Plains. The constant growth of Alberta's reserves since 1947 has encouraged the gas industry to seek extensive markets in Canada and the United States.

The Paleozoic rocks are predominantly shelf carbonates, limy marine shale, and evaporites. These strata are on the eastern flank of the Alberta syncline and dip southwestward at an average of 40 ft/mi. They form a wedge which is more than 5,000 ft thick near the Rocky Mountain Foothills and thins to the outcrop in the northeastern corner of the province. Emergence of the Alberta Plains between the end of Cambrian time and Middle Devonian time, plus periodic emergence and erosion between Mississippian time and Cretaceous time, resulted in the removal of considerable parts of the Paleozoic systems.

The major Paleozoic reservoirs are organic and clastic carbonates of the Devonian and Mississippian Systems. They form three types of stratigraphic traps--reefs, lithological pinchouts and unconformity traps. "Draped" anticlinal structure also aided in localizing accumulations in Nisku reefal reservoirs. which overlie Leduc bioherms.

The Devonian reefs of the Swan Hills, Leduc, and Nisku units mark the areas where shoaling occurred as the later Devonian seas transgressed the evaporite-filled Elk Point basin. The reef fields contain gas reserves of 8 trillion cu ft.

In the lithologic pinchout fields, gas accumulations of the Wabamun Group commonly include hydrogen sulfide gas up to 35 per cent by volume. The indicated reserves in these fields are 1.8 trillion cu ft of nonassociated residue gas and 35 million long tons of sulfur.

Gas accumulations associated with the post-Paleozoic unconformity have reserves of 8.2 trillion cu ft in Mississippian rock.

Production from Paleozoic gas reservoirs has been from both nonassociated-gas fields and dissolved gas from oil-producing areas. Paleozoic gas reserves are expected to increase considerably as exploration continues in the deeper drilling areas and in the more northerly areas of the Alberta Plains. An estimate of the ultimate potential gas resources in the Paleozoic rocks of the Alberta Plains, based on criteria similar to those used in recent forecasts for Canada and the United States, is 45 trillion cu ft.