The habitat of oil in the northern Great Plains and adjacent Rocky Mountain region is related to the sedimentational, erosional, tectonic, and igneous history of the area.
The sedimentational history is that of a western geosynclinal sedimentary facies changing eastward into a shelf and marginal facies as the seas repeatedly overlapped across a "continental platform," which extended continuously eastward to merge with the more stable Canadian shield. Through the Cambrian and Ordovician there were progressive eastward encroachments of the seas across the Precambrian basement complex. Variations in the land and sea pattern and in sedimentary types were the result of differing sea connections and areas of warping. There was a general withdrawal of the seas in the region during the Silurian, followed by the re-establishment of marginal and epeiric seas during the Devonian and Mississippian. Late Mississippian, Pensylvanian, and Permian seas gradually shrank. At the beginning of the Mesozoic there was again a progressive eastward overlap until by middle Upper Cretaceous, marine sediments were deposited over all the upper Great Plains, much of Minnesota, and farther east.
During the latest Cretaceous and earliest Tertiary the marine basins were drained and gradually filled so that later Tertiary deposition consisted of continental sediments in the form of alluvial, lacustrine, and volcanic accumulations. The middle and late Cenozoic history of the region is essentially one of extensive erosion, except for the fluviatile, glacial, and lacustrine deposits which were formed in certain areas in Pleistocene time.
In evaluating the stratigraphic oil possibilities of an area, the relationship of overlying beds to underlying beds is important. Formations, normally considered nonproductive, may become productive if they contain reservoir beds that are brought into juxtaposition with source beds or other reservoirs. An analysis of the age relationships of beds in contact with each other, as well as a study of their lithological characteristics, is necessary for an evaluation.
A fair evaluation of the oil possibilities of a particular area depends upon the analysis and interpretation of the 1. nature of the traps present, 2. local stratigraphic variations, 3. sedimentational environments, 4. regional and local tectonic histories, and 5. correlation of a structure with the period in which it originated and its later development.