The Cut Bank oil and gas field is in Glacier County, Montana, 50 miles east of Glacier National Park, and extends to within 3 miles of the Canadian border. It is 31 miles long and up to 10 miles wide. The field is on the west flank of the Sweetgrass arch. The structure is monoclinal and the regional dip is about 75 feet per mile slightly south of west. The trend of the producing area is parallel with the regional strike. It is still in an active stage of development and as of July 1, 1940, 506 oil wells, 78 gas wells, and 86 dry holes had been drilled, proving 40,000 acres for oil and 55,000 acres for gas production. Average daily oil production was about 11,000 barrels, which also represented the potential for the field, and daily gas withdrawal about 21 million cubic fe t. Total oil production to July, 1940, was 18,645,500 barrels and total gas withdrawal 62,890 million cubic feet. Average well depth in the oil area is 2,950 feet.

The discovery well, completed in 1926, sought a west extension of the Kevin-Sunburst field when it found a stray sand containing commercial amounts of gas above the objective horizon. Another well found oil lower structurally in the same sandstone in 1929, but active development of the oil field did not take place until 1932. The stray sand was first named the Darling, later renamed the Cut Bank, and is the main reservoir of oil and gas.

The Upper Cretaceous Two Medicine, Eagle sandstone, and Colorado shale formations; the Lower Cretaceous Kootenai (lower Blairmore) formation; and the Upper Jurassic Ellis formation are penetrated. The Kootenai lies unconformably on the Ellis.

The three producing zones lie in the lower 200 feet (lower third) of the Kootenai and are: the Moulton sand zone at the top, the Sunburst sand zone in the middle, and the Cut Bank sand at the base. The lower Cut Bank or principal producing sand rests unconformably on the Ellis shale.

The Kootenai is a continental formation and the source of its sediments and that of the producing sands is believed to have been from the west and northwest from the erosion of a Jurassic landmass occupying the approximate position of the Purcell and Selkirk ranges in southern British Columbia and adjacent parts of Montana and Idaho, 160-300 miles from Cut Bank.

Production in the Sunburst and Moulton is from lenticular sandstone beds and in

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Fig. 1. Map of Montana showing location and regional structural setting of Cut Bank field and its relation to other fields of Sweetgrass arch. July 1, 1940.

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the Cut Bank from a porous black chert sand developed locally in a rather impervious "blanket"-type sandstone.

In general, the stratigraphic trap in the Cut Bank sand is formed by a permeable conglomeratic black chert sand, underlain by shale, overlain by tight sand, and bounded laterally on three sides by impermeable chert-free sandstone and siltstone and on the fourth side by water. Porosity of the black chert sand (lower Cut Bank) ordinarily ranges from 12 to 19 per cent and permeability from 20 to 300 millidarcys. The average thickness of saturated sand is 13 feet. Porosity of the Sunburst oil sand ranges from 14 to 24 per cent and permeability from 100 to 3,000 millidarcys. The average thickness of saturated sand is 12 feet. Ninety-one per cent of the oil produced to July, 1940, was from the Cut Bank sand and 9 per cent from the Sunburst and Moulton sands.

Fluids in the reservoir have not reached equilibrium and the water line in the lower Cut Bank (producing) sand is tilted so that the oil-water contact rises from 600 feet above sea-level on the west or downdip side to 1,300 feet above sea-level on the northeast. Regardless of the water line the field produces under volumetric control because of low permeability of the sandstone in the periphery of the oil and gas reservoir.

Because of the great extent and slow withdrawal from the gas cap, reservoir pressures decline slowly.

The average gas-oil ratio in commercial wells is about 900 cubic feet per barrel. Average initial from the Cut Bank sand is 56 barrels per day and average decline about 17 per cent per year. The oil is 38.4° A.P.I. gravity and of intermediate base.

The edge water of the field is relatively "fresh" with 10,000 parts per million of total dissolved solids.