Biomarkers (molecular fossils) in 26 Beaufort Sea Mackenzie delta oils reveal three genetic oil groups and suggest descriptions of their probable sources. Group 1 (21 Tertiary-reservoired oils) was soured from Tertiary deltaic sediments. Group 2 (three Cretaceous- and one Devonian-reservoired oil) and group 3 (one Lower Cretaceous-reservoired oil) derive from two high-quality sources deposited in open-marine environments. Geographical variations in the geochemistry of the 21 group 1 oils suggest that their deltaic source was substantially more oil-prone in the distal deltaic portions than in the region closer to the paleoshoreline. Although these data do not address source rock volumes, they do indicate that poor source quality is not a cause of underfilled traps in the o fshore. The map pable lateral variations in group 1 oil compositions illustrate how, in basins with vertically drained sources, lateral source rock facies changes can bc inferred from regional variations in the geochemistry of the overlying oils. Furthermore, the data suggest that geochemical studies attempting to deduce genetic relationships between geographically separated oils should carefully consider the potential effects of source rock facies changes on the oil compositions.