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The Foothills of Western Canada, a Fold and Thrust Belt Natural Gas Play*
By
Andrew Newson1
Search and Discovery Article #10060 (2004)
*Adapted from “extended abstract” for presentation at the AAPG Annual Meeting, Salt Lake City, Utah, May 11-14, 2003.
1Moose Oils Ltd, Calgary, Alberta, Canada (www.mooseoils.com; [email protected])
The Foothills of the Western Canadian Sedimentary Basin (WCSB) has a long history of natural gas production for the Canadian and American markets. As a supplier to the North American gas market, the Foothills will continue to play an important role over the next 10 years due to the steady upward pressure on natural gas prices resulting from increased demand and decreased supply. This firming in price, coupled with the existence of an established infrastructure in the Foothills, will allow the economical development of many of the remaining natural gas prospects.
The current reserves of natural gas for all of North America that are tied into the natural gas market are 250 TCF. Based on the 2000 figures from the National Petroleum Council, the consumption of this product is currently 25 TCF per year for Canada and the United States. This figure is expected to increase to 30 TCF per year by the year 2010. However, the discovery rate for new gas to replace the continual consumption is decreasing each year. Using numbers published annually by the Alberta Energy and Utilities Board (AEUB), the British Columbia Ministry of Economic Development (BCMED), and the Federal National Energy Board (NEB), it appears that there is annual shortfall in Canada of 7% between produced marketable reserves and booked marketable reserves of natural gas. Taking the North American market as a whole, this figure could be as high as 10%. The steady increase in natural gas prices is providing an incentive to explore for new natural gas reserves and efficiently deplete already proven reserves in the Foothills of the WCSB.
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The Foothills is part of the WCSB that lies within the fold and thrust belt of the Canadian Rocky Mountains (Figure 1). The area is situated on the eastern side and directly adjacent to the Canadian Rocky Mountains. It covers 40,000 square miles of land that runs from the northwest to southeast through four of Canada’s territories and provinces. The northwesternmost point lies just north of the Northwest Territories border at the town of Fort Liard. The Foothills is also in part of the adjacent Yukon Territory and then runs southeast through the provinces of British Columbia and Alberta. It terminates at the American/Canadian border at the town of Waterton. The Foothills has already provided 40 TCF of in-place natural gas reserves and has the potential to provide more. It is tied-in via pipeline to the North American gas gathering system that feeds natural gas to eastern Canada, the eastern seaboard, midwest and northwest parts of the United States, and California. The northwestern and southeastern limits of the Canadian Foothills are controlled by political boundaries and the extent of the natural gas gathering system. The width of the belt is defined more on geological grounds (Figure 2). An area called the Triangle Zone defines the eastern side of the Foothills. This is a descriptive term for the subsurface cross-sectional structural style that forms the effective edge of the fold and thrust belt. Beyond this lie the conventional exploration and development plays of the WCSB. The western edge of the Foothills belt is defined generally by the topographic high that is formed by the Front Ranges of the Rocky Mountains. This topographic high provides limitations of access due to its extreme relief. It is also frequently the eastern edge of national or provincial parks, which provide another restriction to access.
One of the reasons for the unique nature of the Foothills belt is the
type of geological structures found at the surface and in the
subsurface. The Foothills is part of the larger fold and thrust belt of
the Rocky Mountains, a geological structural domain where the
sedimentary rock sequence of the WCSB has been deformed by horizontal
compression. The rocks have been effectively shortened by one of two
Many of the Foothills fields have One of the major additions to marketable reserves in the Foothills may well come from an increase in the understanding of naturally fractured reservoirs. If this is true, it may not be unreasonable to add an extra 10 TCF of gas to the marketable reserves of the Foothills belt purely through the more effective development of current in-place reservoirs. The rock formations that produce hydrocarbons in the Foothills are spread throughout the stratigraphic column (Figure 3). The youngest producing formation is the sandstone of the Cardium Formation. The oldest producing formation is the carbonates of the Beaverhill Lake Group. The bulk of the gas produced to date is from the Mississippian-aged rocks, which have produced 26 TCF of the total reserves. The next most prolific producers are the Triassic- and Devonian-aged rocks which have produced 6 TCF each. The Cretaceous has added 2 TCF to the reserves.
The Foothills has been divided into five categories based on structural style, stratigraphic framework, and history of exploration: First Generation, Second Generation, Third Generation, Reef/Stratigraphic, and Triangle Zone play types (Figure 4).
The First Generation plays of the Mississippian-aged
The Second Generation play type dominated exploration efforts from 1960
to 1980. These plays have contributed 27% to the gas-in-place reserves
of the Foothills. They contain both Mississippian- and Devonian-aged
Third Generation play types have become increasingly important in
Foothills exploration since 1970. To date, these plays have contributed
about 20% to the gas-in-place reserve base. Third Generation plays form
structures that are dominated by the detachment fold structural style.
This deformation has the ability to fracture the
Another play type is the Reef/Stratigraphic type. It played a relatively
minor role in Foothills exploration strategy from 1970 to 2000 and has
contributed only 5% of the gas-in-place reserves in the Foothills to
date. This play type is dominantly a stratigraphic play that extends
from the conventional part of the WCSB and can occur either in the
regional autochthone or in thrust sheets in the Foothills. In both cases
the seismic imaging of the play is hampered by the complex geology of
the shallower strata, large variations in topography, and steep dips of
the
The Triangle Zone is a complex interaction of the two structural
styles--it has multiple thrust sheets as well as a detachment folding
component to enhance tight Production from the various play types has changed over time (Figure 5). Early production was dominated by the Reef/Stratigraphic, First and Second generation plays. Since the mid 1980’s the Third generation and Triangle zone plays have contributed an increasing amount to the annual production.
The Foothills has a long history and has proved itself capable of producing large amounts of gas at high rates. Current published data gives the gas-in-place reserves for the Foothills as 40 TCF, of which 19 TCF are considered marketable gas and 13 TCF have already been produced. It currently produces nearly a TCF of raw gas a year. Based on the latest work by the Canadian Gas Potential Committee, using a discovery history process model, it has 27 TCF yet to be discovered in existing plays. The three largest fields left to be discovered in the WCSB will be in the Foothills and each will be greater than 1 TCF in size.
All gas reserve figures are in-place numbers, unless otherwise stated. |