Abstract

The Sweetgrass arch of northwestern Montana and southern Alberta is a major, ancient structural feature. Initial anticlinal development occured in early Paleozoic time. Strong uplift followed by peneplanation occurred in the Late Jurassic and Early Cretaceous. During the Cretaceous and early Tertiary the Sweetgrass arch was quiescent but was rejuvenated in mid to late Tertiary, when it was upwarped by a basement flexure to its present structural configuration. The Sweetgrass arch is a 200 mi long, north-plunging anticline showing 10,000 ft of structural relief. Midway down its plunge, the anticline is offset 30 mi by a right-lateral transcurrent fault. A seismic line exhibits the flexure.

During the Late Cretaceous and early Tertiary a number of 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 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, however, failed to destroy many of the stratigraphic traps. As a result, on the flanks of the Sweetgrass arch there are many "frozen" stratigraphic traps including Cut Bank field, the largest single-pay stratigraphic trap in the northern Rockies (164 mm bbl of oil, 0.5 tcf of gas). On the crest are large structural accumulations of remigrated oil at Kevin Sunburst (81 million bbl of oil) and Pondera (26 million bbl of oil). Evidence of remigration is recorded throughout the area by live oil show "tracks" in the reservoirs and remnant gas caps of thieved earlier accumulations. Potential exists for finding new "frozen" traps on the flanks and remigrated oil accumulations on or near the crest of the Sweetgrass arch.