The first oil production from the Upper Devonian Nisku Formation in northeastern Montana came with the discovery of the Tule Creek field in late 1960. Six additional Nisku fields have been discovered within a 10-mi radius of Tule Creek. Even-textured, saccharoidal dolomite, with a pay thickness ranging up to 50 ft, produces 40°-47° API oil in these seven fields.

Data from closely spaced wells amply demonstrate that structural closure is important as the basic trapping mechanism for Nisku oil. Various interpretations have been given regarding the influence which stratigraphic factors have had in creating these accumulations. Current information indicates that primary stratigraphic differences do impose a semi-regional control on Nisku oil traps, but are less important in individual traps.

Steep-sided, flat-topped structures, which typically cover less than 1 sq mi, control the limits of Nisku fields. The writer believes that the structures are not the products of normal tectonic development. Instead, it can be demonstrated both theoretically and by deep well data that they are "sedimentary structures" which result from multiple-stage solution of the Lower Devonian Prairie salt beds.

The present solutional zero edge of the Prairie salt beds is east and downdip from the complex of Nisku fields, but it is believed 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.