Western Canada Tight Gas

Comparative Evaluation Of Tight Gas Play Opportunities, Western Canada Sedimentary Basin

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Summary:
Location: Large portions of BC and Alberta
Strata: Jean Marie to Belly River
Year of Study: 2005

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Introduction

Sustained high demand has driven prices and long-range price forecasts for natural gas upward, while advances in drilling, evaluation, and completion technologies have made many unconventional gas reservoirs more accessible. Infrastructure growth and improved regulatory regimes have opened up new exploration areas. Companies thus have a variety of new options to augment their gas production – low-deliverability but areally-extensive shallow gas plays, coalbed gas, gas from shales, and “tight” or low-permeability gas from sandstone and carbonate reservoirs.

Petrel Robertson Consulting Ltd. (PRCL) has conducted a comparative evaluation of tight sandstone/carbonate gas play opportunities in the Western Canada Sedimentary Basin (WCSB), to provide guidance and specific recommendations for these choices. We have chosen a tight gas focus, as many of these plays offer large reserves over comparatively small areas. In addition, tight gas plays are generally not heavily burdened by the surface access, infrastructure, and water disposal problems associated with shallow gas and coalbed methane. Gas from shale is still in its infancy in Canada, and there is not sufficient information available to support its comparison with other unconventional play types.

In this report, PRCL undertakes a comprehensive comparison of the major tight gas opportunities in the WCSB today. Our analysis addresses play geography, geological characteristics, resource density, and engineering (drilling, evaluation, and completion) aspects of each tight gas reservoir unit, illustrated with selected case studies.

Background to the Area

Assessment of tight gas reservoir production and potential in Western Canada is a huge topic, which will require enormous amounts of work over years and decades to come. PRCL’s approach in this study is to assemble current geological and reservoir engineering knowledge about each major tight gas target on a regional scale, augmented by selected local case study analyses, in order to provide a first-cut comparison of each. More detailed analyses, focused on specific reservoirs and/or play areas, may be undertaken in future studies with the guidance of this scoping review.

Geologic Setting

The Western Canada Sedimentary Basin comprises the eastern Cordillera and two major sedimentary basins. The Alberta Basin is a northwest-southeast trough in front of the foreland fold and thrust belt, extending eastward to the Canadian Shield. The intracratonic Williston Basin is centred in North Dakota, and extends into southern Saskatchewan and Manitoba. The two basins are separated by the broad, low-relief Bow Island Arch. Most of the discussion in this report will deal with the Alberta Basin component of the WCSB.

The western margin of the WCSB was deformed during Middle Jurassic to Eocene time, as the result of docking of allochthonous terranes to the west. The foreland fold and thrust belt (Rocky Mountains and Foothills) was created as Middle Proterozoic through Eocene strata were compressively deformed, resulting in up to 170 km of structural shortening. Loading of the North American craton (causing foreland subsidence) and creation of western sediment source areas were profound influences upon Jurassic and younger sedimentation.

Although the western U.S. Rocky Mountain region was deformed by the same regional tectonic events, the structural history of the two areas diverged in Late Cretaceous time. In the U.S. Rockies, Laramide uplifts segmented the central part of the foreland basin into smaller discrete basins. In the WCSB, which we can recognize southward to central Montana, local uplift and segmentation did not occur. Instead, the regional basin structure persisted, with relatively limited subsidence in the western foredeep, bounding the fold and thrust belt. Laramide compressive stresses were transmitted throughout the basin with relatively little opportunity to generate extensional fracturing, and thus fractured sweet spots in tight gas reservoirs are generally less common.

Uppermost Cretaceous and lowermost Tertiary strata are generally not as thick in the WCSB, as subsidence was less extreme, and sediments were derived only from the regional western highlands (not the multiple, sharply-bounded uplifts flanking the U.S. basins). Also as a result of less extreme subsidence, much of the youngest section has been eroded, or has not been buried sufficiently to form regional tight gas reservoirs.

Methodology

PRCL has assembled current geological and reservoir engineering knowledge about each major tight gas target on a regional scale, augmented by selected local case study analyses, in order to provide a first-cut comparison of each. More detailed analyses, focused on specific reservoirs and/or play areas, are recommended for future studies with the guidance of this scoping review. The report consists of five major sections:

1. Tight Gas Overview and Background – A historical perspective on the evolution of the tight gas reservoir concept. In particular, we highlight the “classical” picture of tight gas development, focused largely on the Rocky Mountain Basins of the western United States. An overview of WCSB tight gas characteristics highlights key similarities and differences, and emphasizes the adoption of a Canadian definition of tight gas.

2. Geological Review / Comparison of WCSB Tight Gas Reservoirs – A systematic review of individual tight gas reservoir targets. We highlight regional geology, reservoir characteristics, current production distribution and strategies, and overall resource/reserves potential. Selected case studies provide local detail and insights into more regional potential.

3. Structural Assessment of WCSB Applied to Tight Gas Prospectivity – A review of the structural framework of the WCSB, and its influence upon tight gas reservoir development.

4. Engineering Analysis of WCSB Tight Gas Reservoirs – Several case studies highlight reservoir engineering characteristics of selected tight gas units, providing a basis for better understanding current economic realities and future economic potential.

5. Summary of Tight Gas Potential in the WCSB – Includes a ranking of tight gas reservoir targets derived from our geological and engineering assessments. However, there are many ways to undertake such rankings, and individual companies must apply knowledge of their own corporate land/production holdings and areas of operational expertise in order to apply these rankings.