Abstract

One-third of Canada's known natural gas resources, or 13.5 trillion cubic feet of indicated gas reserves, are contained in Paleozoic rocks of the Alberta Plains. The constant growth of Alberta's proved reserves since 1947 has encouraged the gas industry to seek extensive markets on the North American continent.

The Paleozoic rocks are predominantly shelf carbonates, limy marine shales and evaporates. These sediments are located on the eastern flank of the Alberta syncline and dip southwestward at an average of 40 feet per mile. They form a wedge which is over 5000 feet thick near the Rocky Mountain foothills and thin to the outcrop in the northeastern corner of the province. Emergence of the Alberta Plains between the end of Cambrian and Middle Devonian times, plus periodic emergence between Mississippian and Cretaceous times, interrupted sedimentation causing the erosion of considerable portions of each 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, lithologic pinch-outs and unconformity traps. "Draped" anticlinal structure also aids in localizing accumulations in Nisku reefal reservoirs which overlie Leduc bioherms.

The Devonian reefs of the Swan Hills, Leduc and Nisku units indicate the existence of shoaling conditions when the Devonian seas transgressed over the Elk Point basin. The reef fields contain gas reserves of eight trillion cubic feet, 50 percent of which are essentially non-associated.

In the lithologic pinch-out fields gas occurrences of the Wabamun group frequently include hydrogen sulphide gas up to 35 percent by volume. The indicated reserves in these fields are 1.9 trillion cubic feet of non-associated residue gas and 19 million tons of sulphur.

Gas accumulations associated with the post-Paleozoic unconformity have reserves of 3.4 trillion cubic feet in Mississippian rocks, 60 percent being non-associated.

Production to date from Paleozoic gas fields has been mainly dissolved gas from oil producing areas. Non-associated gas production awaits the development of adequate markets. Paleozoic gas reserves are expected to increase considerably as exploration continues in the deeper drilling areas of the Alberta Plains. An estimate of the ultimate potential gas resources in the Paleozoic rocks of the Alberta Plains, based on similar criteria used in recent forecasts for Canada and the United States, is 50 trillion cubic feet.