Abstract

The Sweetgrass Arch is a positive structural feature extending from central Montana into southeastern Alberta. Rock units ranging in age from Pre-Cambrian Beltian to Upper Cretaceous Montanan are exposed along the 350 mile axis. Three major elements can be highlighted: the South Arch culminates in the Beltian exposures on the south end, the Kevin-Sunburst Dome and the Sweetgrass Hills dominate the center along the International boundary, and the Bow Island Arch plunges northward into southern Alberta.

Early exploration for hydrocarbons was naturally focused on the search for structural traps because such a large positive trend would be expected to have many faulted and domal anomalies. Active development work over the years proved that the stratigraphic trap predominated as the accurately accounts for the accumulations found to date.

The largest oil reserves on the Arch are found in a group of channel sandstones in the Lower Cretaceous Basal Mannville Group, extending from the Cutbank field of Montana northward through the Taber, Hayes, and Bantry fields of Alberta. The Middle Mannville Glauconitic-Moulton Sandstones produce from a string of sandbars trending northward from the Darling area on the border through the Taber area and scattered trends in the Jenner, Countess, and Hussar fields of Alberta.

Large gas reserves are found in thin blanket sandstones of Upper Cretaceous age, principally in the Medicine Hat and Second White Specks zones. Minor gas deposits are also found in the Lower Cretaceous Bow Island Formation in long, narrow sand bars. A three hundred foot thick interval of silty and sandy shale in the Milk River Formation has been known to contain gas for some time, but only recently has this enormous deposit been developed. This deposit has been ignored because of its low productive rate, but since 1970 a small drilling boom has been undertaken to put this huge reserve on the market. The Milk River pool may be the largest single gas field in areal extent in western Canada.