Crude oil and shale samples from the Denver basin of Colorado, Wyoming, and Nebraska were analyzed using organic geochemical techniques to determine oil-source bed relationships. In general, oils in Cretaceous reservoirs are compositionally similar throughout the basin and are geochemically dissimilar to the oil produced from Permian Lyons Sandstone. The Cretaceous oils are compared with hydrocarbon extracts from Cretaceous shales to determine the stratigraphic occurrence and regional distribution of petroleum source beds. The results show that oils produced from the Cretaceous Terry and Hygiene Sandstone Members of the Pierre Shale, "D", and "J" sandstone reservoirs have probably been derived from the Carlile, Greenhorn, Graneros, and Mowry Formations. The source bed for oil from the Lyons has not yet been identified. Samples of the Carlile-Greenhorn-Graneros-Mowry interval have been analyzed throughout the basin. Only samples from the basin-axis area contained hydrocarbon distributions similar to those Cretaceous oils. The occurrence of petroleum on the east flank of the basin, in light of the limited geographic distribution of effective source beds, indicates that extensive (as much as about 150 km) lateral migration has occurred. This suggests that an understanding of lateral migration pathways is important for petroleum exploration in the Denver basin.