The Sweetgrass arch of northwestern Montana and southern Alberta is a major ancient structural feature. Initial anticlinal emplacement occurred during the early Paleozoic and was parallel with the cratonic margin. Strong uplift followed by peneplanation occurred during the Late Jurassic and basal Cretaceous during the westward drifting of the North American plate following the breakup of Pangea. During Cretaceous and early Tertiary times, the Sweetgrass arch was quiescent, but was rejuvenated in mid to late Tertiary, upwarped by a basement flexure to its present structural configuration: a 200 mi (322 km) long, north-plunging anticline showing 10,000 ft (305 m) of structural relief. Midway down its plunge, the anticline is offset 30 mi (48 km) by a right-lateral transcurr nt fault.

During Late Cretaceous and early Tertiary, plutonic uplifts were emplaced on the east flank, forming traps for oil then migrating updip from the Williston and Alberta basins. Oil and gas accumulated in Mississippian, Jurassic, and basal Cretaceous reservoirs in structural and stratigraphic traps around these plutonic uplifts. Subsequent late Tertiary doming of the Sweetgrass arch tilted the earlier structural traps and drained them, resulting in remigration of much of the oil and gas to the crest of the arch. The tilting failed to destroy many of the stratigraphic traps. As a result, down the flanks of the Sweetgrass arch are many "frozen" stratigraphic traps including Cut Bank field, the largest single-pay stratigraphic trap in the northern Rockies (164,000 bbl of oil, 0.5 tcf gas). n the crest are large structural accumulations of remigrated oil at Kevin Sunburst (81,000 bbl of oil) and Pondera (26,000 bbl of oil). Evidence of remigration is recorded by live oil show "tracks" in the reservoirs and remnant gas caps throughout the area of earlier accumulations. A potential exists for finding new "frozen" traps on the flanks and remigrated oil accumulations on or near the crest of the Sweetgrass arch.

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