ABSTRACT

The Colorado/Alberta Group is a predominantly shale unit, which exceeds 1100 m in thickness in the northwestern portion of the Western Canada Sedimentary Basin and thins to less than 200 m in the east. The base of the group immediately overlies the top of the Mannville Formation. In the west, the top of the group is at the top of the Milk River Formation; in the east, it is at the top of the First White Speckled Shale. Coarse clastic units within the group are commonly major hydrocarbon producers. They include the Turonian Cardium, the Cenomanian Dunvegan and the Albian Viking formations. These formations are related to major fluctuations in relative sea level, and are described in detail elsewhere. Finer sandy units include the retrogradational shelf sandbodies of the Doe Creek Member, the shelf to coastal plain sandbodies of the Bad Heart Formation and Chinook Sandstone in northwestern Alberta, the shelf to shoreline sandbodies of the Medicine Hat Formation, the Milk River Formation (commonly placed in the Montana Group), the Chungo Member and the Barons Sand of southern Alberta, and the shelf deposits of the Medicine Hat and Newcastle formations in southern Saskatchewan. Several units are of particular economic importance as producers of nonassociated, sweet, dry, biogenic shallow gas.

During deposition of Colorado Group sediments, warm marine waters extended north from the Gulf of Mexico and intermixed with boreal waters extending south from the Arctic. The First White Speckled Shale, the Second White Speckled Shale and Fish Scale Zone occur as regional markers across most of the basin. They represent stratigraphically condensed marine sections related to maxima in marine transgressions. These condensed sections are characterized by high TOC contents and, where mature, form good potential hydrocarbon source rocks.

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