Devonian oil in Mississippian and Mesozoic reservoirs-unconformity controls on migration and accumulation, Sweetgrass Arch, Montana
- Amoco Production Co., Houston, TX (United States)
- Amoco Production Co., Denver, CO (United States)
- Amoco Production Co., Tulsa, OK (United States)
Lower Cretaceous and Mississippian strata of the Sweetgrass Arch of western Montana have produced over 300 MMBO, primarily from three large combination traps. Gas chromatography-mass spectrometry (GC-MS) data suggest a Devonian/Mississippian Bakken Formation oil source. Most thermally mature Bakken strata are located at least 60 mi (90 km) to the west in the footwall to the thrust belt. Hydrocarbons have migrated vertically through fractures in the Mississippian Madison Group to regional seals in Jurassic shales. Lateral migration occurs predominately within the Jurassic subcrop of the Mississippian Sun River Dolomite. Permeability barriers, paleohills, subtle structures and possible hydrodynamic modification along the migration pathway account for most of the Sun River production. A lack of effective bottom seals generally prevents these subtle traps from developing large accumulations in areas of steep structural dip. The pre-Cretaceous unconformity, which underlies the Lower Cretaceous Cutbank sandstone, bevels across this Mississippian migration route downplunge in Canada, diverting oils southward to the giant Cutbank field accumulation. Alluvial plain and fan sandstones west of the field have sheet-like geometries and appear to have poor lateral seals. Most of the Cutbank accumulation occurs where valley incisement of a north-south trending paleodrainage system juxtaposes these sheet sandstones updip against Jurassic Rierdon and Sawtooth shales, forming a valley wall trap. Additional minor Jurassic and Lower Cretaceous production occurs updip from leak points created by poor lateral seals adjacent the pre-Cretaceous unconformity. These interpretations provide new insight into field distributions throughout the Sweetgrass Arch. 37 refs., 26 figs., 1 tab.
- OSTI ID:
- 5453240
- Journal Information:
- Mountain Geologist; (United States), Vol. 30:4; ISSN 0027-254X
- Country of Publication:
- United States
- Language:
- English
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