Petroleum Geology of Sweetgrass Arch, Alberta

Abstract

The Sweetgrass arch is a positive structural feature extending from central Montana into southeastern Alberta. Rock units ranging in age from Precambrian Beltian to Late Cretaceous Montanan are exposed along the 350-mi axis. Three major elements can be highlighted: the South arch, which culminates in the Beltian exposures on the south end; the Kevin-Sunburst dome and the Sweetgrass Hills which dominate the center along the International Boundary; and the Bow Island arch which plunges northward into southern Alberta. Early exploration for hydrocarbons naturally was focused on the search for structural traps; 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 setting for hydrocarbon accumulation. Even the large closed structural anomaly of the Kevin-Sunburst dome does not entirely cause the entrapment of oil and gas there, but irregular porosity development in Mississippian carbonate rocks, and lensing and pinchout of Cretaceous sandstones more accurately account for the accumulations. The largest oil reserves on the arch are 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 International Boundary through the Taber area and in scattered trends in the Jenner, Countess, and Hussar fields of Alberta. Large gas reserves are found in thin blanket sandstones of Late Cretaceous age, principally in the Medicine Hat and Second White Specks zones in the Alderson, Medicine Hat, Bantry, and Princess areas. Minor gas deposits are also in the Lower Cretaceous Bow Island Formation in long, narrow sandbars in the Pakowki Lake and Bindloss areas. A 300-ft-thick interval of silty and sandy shale in the Milk River Formation has been known to contain gas for some time, but, because of its low productive rate, only recently has this enormous deposit been developed. Since 1970 a small drilling boom has been under way 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.