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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. The Paleozoic and Mesozoic strata extend westward below the leading edge of the Belt, which is at the east edge of the anticline. 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 nearshore 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. Other important Mesozoic reservoirs may be the Morrison Formation and the Ellis Group.
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 (Upper Mississippian, 0 to 400 ft [120 m] thick) consists of marine sandstone, limestone, and black shale; many outcrops are notably petroliferous. The Mission Canyon Limestone (Lower Mississippian, 700 to 1,500 ft [210 to 460 m] thick) has abundant porosity and overlies the Lodgepole Limestone (Lower Mississippian, 300 to 750 ft [90 to 230 m] thick), which is thin bedded and dark and may be a hydrocarbon source for the Mission Canyon and other reservoirs. The Jefferson Dolomite (Upper Devonian, 400 to 700 ft [120 to 210 m] thick) is dark colored, marine, fetid, and has outcrops with oil seeps. Other important Paleozoic reservoirs may be the Phosphoria and Ams en Formations.
This geologic setting is similar to the giant Waterton field to the north just across the Canadian border where the Belt has been thrusted eastward over Mesozoic clastics and Paleozoic carbonates. The latter are the major reservoirs at Waterton and may be productive in the study area. Some drill holes at Waterton spud in the Belt.
In the study area, the Belt so much resembles younger sedimentary rocks that it has been misidentified as Cretaceous shale on sample logs available from a reputable commercial firm. It should present no great hindrance in geophysical or drilling programs.
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