The MonDak Mississippian limestone oil field contains 13.7 million to 68.6 million MT (100 million to 500 million bbl) of recoverable oil. It produces from irregular intervals, both vertically and horizontally, in a 168 m section of fractured Mississippian limestone. Dip in the field is eastward into the Williston basin at 3 to 11 m/km, with some structural flattenings and irregularities. The trap is not structural. The reservoir is limestone with a depositional texture range from mudstone to grainstone, but most of the reservoir rock is wackestone. Normal matrix porosity is 2 to 4% and matrix permeability is less than 0.01 md. Some lentils of fossil

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fragment grainstone have porosities in the 6 to 15% range and permeabilities of 0.01 to 1.0 md. Production is primarily from vertical microfractures and macrofractures. The reservoir is slightly ovepressured. Oil gravity is 32 to 39° API, gas-oil ratios range from 250 to 650 cu ft/bbl, and produced water range from 10 to 90%. The field is located in the west-central part of the Williston basin in the states of Montana and North Dakota. The discovery well was completed in 1958 as a workover of a test plugged and abandoned in 1954. A diagonal offset to the discovery was completed in 1960. No other completions were made in the Mississippian until 1976. As of June 1, 1980, there will be 182 completed Mississippian wells on a 65-ha. spacing pattern and 10 drilling rigs operating.

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