The Upper Devonian Nisku Formation became a productive unit for the first time in late 1960, when the Tule Creek field was discovered in northeastern Montana. Since then 6 additional Nisku fields have been discovered within a 10-mi. radius of Tule Creek. Even-textured, saccharoidal dolomite, which has 50 ft. of net pay, is productive of 40°-47° API oil at these 7 fields.

Data from closely spaced wells amply demonstrate that structural closure is the basic trapping mechanism for oil in the Nisku. Various interpretations have been advanced regarding the type and degree of influence of stratigraphic factors in these accumulations. Current information indicates that primary stratigraphic variations do impose a semi-regional control on the entrapment of oil in the Nisku, but are less important in individual traps.

Steep-sided, flat-topped structures, which typically are less than 1 sq. mi. in area, control the limits of Nisku fields. The writer suggests that the structures are not the products of normal tectonics. Instead, it can be demonstrated, both theoretically and by deep-well data, that the structures are "sedimentary structures" resulting from multiple-stage solution of the Lower Devonian Elk Point salt beds.

The present-day solutional zero edge of the Elk Point salt beds is east of and downdip from the Nisku fields. The writer suggests that an original thickness of 50-100 ft. of salt was present in the Tule Creek area, and that the original depositional edge of the salt was farther west.

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