Geologic Nature and Characterization of "Tight" Gas Reservoirs in the Western Canada Sedimentary Basin and Comparison to U.S. Rocky Mountain Basins

Thomas F. Moslow
Midnight Oil and Gas Ltd, Calgary, AB

While successful exploitation of tight gas sand reservoirs has received much notoriety in United States Rocky Mountain Basins, there has been very limited economic success in low permeability clastic reservoirs in the Western Canada Sedimentary Basin. The reasons for this contrast are multifaceted but principally geological in nature.

Most U. S. Rocky Mountain Basin “tight gas” plays (< 0.1md matrix permeability) occur in thick (10's to 100's of meters), laterally extensive sandstones of generally continental to fluvial origin with extremely high sand/shale ratios, relatively low porosity (8-12%) with permeability and deliverability provided by extensive networks of open fractures. A tectonic history including multiple phases of compression and extension since mid-late Mesozoic has provided for complex stress fields in both basin margin and basin center settings.

In contrast, the Western Canada Sedimentary Basin has undergone only tectonic compression since the late Jurassic restricting fracture permeability in clastic reservoirs to the Foothills and Front Ranges. One exception is the Cadomin Fm., a micro-fractured alluvial conglomerate of regional extent. Other “Basin Center” gas reservoirs occur in the Foredeep, or “Deep Basin”, of Alberta and northeast British Columbia. Conventional reservoirs, or “sweet spots”, consist predominantly of Lower Cretaceous, low-porosity conglomerates (8-10%) varying by orders of magnitude in permeability, deposited as marine shorelines flanked by “tight” gas-saturated sandstones. With time, the conglomerate reservoirs serve as mega-scale horizontal wells in a basin center setting. Economic gas production from wells completed in the low permeability “tight” sandstones alone is rare. Historically, and currently, the most common drilling strategy in the Deep Basin is to target multi-zone stacking of higher permeability conglomerates.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005