Undrilled giant anticline in overthrust belt, Broadwater and Gallatin counties, Montana
Southeast of Helena, Montana, in the Missouri River valley, the Precambrian Belt Supergroup has been thrusted eastward over Paleozoic and Mesozoic reservoirs and source beds, and forms an undrilled anticline about 16 mi (26 km) long, 7 mi (11 km) wide, with about 1 mi (1.6 km) of structural closure. Although the anticline is clearly evident on published cross sections, its axis does not appear on published maps. Cross sections constructed from surface data indicate that Paleozoic and Mesozoic strata may be present below the Belt in the anticline, but confirmation by geophysical work is needed. Drill depths to Cretaceous targets should be about 8,000 to 10,000 ft (2,400 to 3,000 m). The Eagle Sandstone (Upper Cretaceous, 100 to 300 ft (30 to 90 m) thick) probably is a regressive shoreline and near shore deposit. It is overlain and underlain by thick marine Cretaceous shale from which oil and gas were tested in nearby drill holes, and which could be a hydrocarbon source for the Eagle and other reservoirs. The Kootenai Formation (Lower Cretaceous, 400 to 1,000 ft (120 to 300 m) thick) is a coal-bearing deposit with fluvial sandstones which may form reservoirs having the coal as a gas source. Drill depths to Devonian and Mississippian targets should be about 15,000 to 18,000 ft (4,500 to 5,500 m). The Big Snowy Group consists of marine sandstone, limestone, and black shale; many outcrops are notably petroliferous. The Mission Canyon Limestone has abundant porosity and overlies the Lodgepole Limestone, which is thin bedded and dark and may be a hydrocarbon source for the Mission Canyon and other reservoirs. The Jefferson Dolomite is dark colored, marine, fetid, and has outcrops with oil seeps. Other important Paleozoic reservoirs may be the Phosphoria and Amsden Formations.
- Research Organization:
- McMahon-Bullington, Englewood, CO
- OSTI ID:
- 6526646
- Report Number(s):
- CONF-8309274-
- Journal Information:
- Am. Assoc. Pet. Geol., Bull.; (United States), Journal Name: Am. Assoc. Pet. Geol., Bull.; (United States) Vol. 67:8; ISSN AAPGB
- Country of Publication:
- United States
- Language:
- English
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020200* -- Petroleum-- Reserves
Geology
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03 NATURAL GAS
030200 -- Natural Gas-- Reserves
Geology
& Exploration
ANTICLINES
DRILLING
FEDERAL REGION VIII
GEOLOGIC DEPOSITS
GEOLOGIC STRUCTURES
MINERAL RESOURCES
MONTANA
NORTH AMERICA
PETROLEUM DEPOSITS
RESERVOIR ROCK
RESOURCE POTENTIAL
RESOURCES
SOURCE ROCKS
USA
WELL DRILLING
WESTERN US OVERTHRUST BELT