ABSTRACT

An economic deposit, whether oil, gas, saline, metallic, non-metallic or otherwise, will occur where there has been the right combination, for the given type of deposit, in space and time of physical and chemical environments and of tectonic activities. Therefore, the location of a given type of economic deposit necessitates first knowing the "habitats" or the environments in which the given type of deposit can occur, and then the location of areas which have had the proper sequence of geological events to give that environment.

Certain of the factors controlling the accumulation of petroleum are known. This paper presents some of the results of regional and local geological investigations covering some strati graphic environments which may be of importance in controlling the occurrence of oil and gas in the Northern Great Plains, and in particular along the eastern side of the Williston basin.

The region under discussion lies upon a shallow marine shelf located between the relatively stable Canadian shield on the east and the active Cordilleran geo-syncline on the west. It is a region which was subjected to repeated warpings. These warpings produced local basins of deposition filled with varying types of sediments which may be uniform over wide areas or which may have different interfingering facies. Some of the areas of deposition were separated by shoal areas or by regions of uplift and erosion. At various times the entire area was regionally uplifted and subjected to widespread beveling.

Within the area petroleum has been discovered in both structural and stratigraphic traps. Although a wide variety of stratigraphic types of traps are present in the region, the present paper will confine itself to a discussion of the pinch-out and unconformable relationships of various portions of the stratigraphic section. However, since in many cases localization of petroleum accumulations within a stratigraphic trap may be controlled by structure, the correlation of structure with the geological period in which it originated, and through its various stages of development in the potential stratigraphic trap area, is significant and must be determined.

A knowledge of the relationship of overlying to underlying strata, the lithology, physical properties and the past physical history of the beds which are in contact with each other may be controlling factors in determining the economic potentialities of any stratigraphic trap. Therefore, within the area the relationship of each group of beds immediately overlying and immediately underlying each geological group of beds being analyzed is considered.