The existence of oil and gas accumulations in the Rocky Mountain region which are due to stratigraphic rather than structural traps, leads to an analysis of the conditions under which these accumulations occur and to a consideration of methods by which other similar deposits may be found. Examples of stratigraphic traps are not numerous in the Rocky Mountains but it is reasoned that such fields may have escaped discovery because wildcatting is almost invariably confined to favorable structure.

The problem is approached from the standpoint of source rocks as well as reservoir rocks. Localities in which there are apparent relationships between petroliferous provinces and ancient geography are described by reference to generalized paleogeographic maps of the Rocky Mountain region from Pennsylvanian to Cretaceous time. The nature of the sediments and the conditions under which they were deposited were profoundly affected by ancient positive elements which are called the "Ancestral Rockies."

Numerous instances of lenticular and wedging sand bodies are pointed out as having possibilities for the formation of stratigraphic traps on the flanks of major uplifts or in structural basins. A great deal of detailed stratigraphic work must supplement the present general knowledge.