The Tertiary Fort Union Formation of Paleocene age has yielded oil and gas fields of major significance in the northern Rocky Mountain area during the past few years. Notably, the LaBarge platform area on the west flank of the Green River basin in Wyoming is the "classic" example of productivity and an excellent subject area for stratigraphic and environmental study of the Paleocene. As of January 1, 1967, the fields on the platform had a cumulative production in excess of 43 million bbls of oil and 770 Bcf of gas, produced primarily from sandstones in the Fort Union Formation.

After the uplift and erosion of the Late Cretaceous Lance Formation in the platform area, Laramide orogenic movements began with gentle uplift and truncation which cut into the Cretaceous Hilliard Formation and established an unconformity surface which subsequently was folded in response to Laramide adjustments. Following these events, general Tertiary deposition began with the formation of coaly swamps. In the steeper synclinal parts of the Green River and other intermontane basins, large bodies of water developed and thick sequences of source-bed shales filled these troughs which approached marine conditions as inland seas. The significant Fort Union production that has been found to date in the northern Rockies is associated with a beach-type environment developed along the shoreli es of these inland seas. Toward the end of the time of deposition of the Fort Union, these seas were filled with sediment and only the coaly sequence remained until additional orogenic movement folded and eroded these beds--in some places as deep as the older Fort Union-sea units. The Wasatch Formation of Eocene age was deposited on this unconformity surface.

The depositional and producing environment of the Fort Union in the LaBarge platform area is not unique to the Green River basin of Wyoming. Similar conditions are evident in many other basins in the northern Rocky Mountains where little or no exploratory effort has been made to locate lucrative production similar to that found and continuing to be developed in the Green River basin.

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