This paper outlines and depicts geologic conditions in representative fields in the Rocky Mountain region that produce oils of highly divergent character from variable zones under relatively exceptional conditions. It is shown that oil ranging in gravity from 11° to 76° is produced from strata ranging in age from lower Mississippian to Oligocene, and that oil has been found in traps of almost all known types. The fields described include (1) domes that produce oil from continental Eocene beds, (2) stratigraphic traps, (3) highly folded anticlines that yield heavy oil from Permian and Pennsylvanian strata under conditions suggesting that the quality of the oil is not related to tectonic compression, (4) faulted domes and anticlines, and (5) largely non-faulted an iclines and domes. As has long been known, some of the conditions of oil accumulation in the region defy satisfactory explanation.