Approximately 45 per cent of Wyoming's oil production has come from fields in the Powder River basin which is a large asymmetric intermontane syncline lying in the northeastern part of the state. Every post-Proterozoic system except Silurian is represented in the basin, but the important oil-producing formations are of Cretaceous, Jurassic, Permian, and Pennsylvanian age. The structure of the basin is mainly a result of late-Cretaceous-early-Tertiary orogeny. However, major tectonic features appear to have an inherited orientation.

The crude oils of Cretaceous age are strikingly different in composition from those of the Permian and Pennsylvanian. Jurassic oils have characteristics typical both of Mesozoic and Paleozoic crudes.

Certain patterns of oil occurrence in the Powder River basin can be related to the structural history. Accumulations in structural features formed by the Cretaceous-Tertiary orogeny predominate, and oil fields are most numerous near the areas of most intense deformation. Isopach studies of pre-Tertiary formations indicate that early structural patterns had some influence on oil accumulation.

The basin also displays oil-occurrence patterns dependent on stratigraphic history. The distribution of reservoir and source beds played an important part in localizing oil accumulations. Composition of the various crude oils is closely related to their depositional environments, although the characteristics of many oils have later been altered by other means.

A pattern of increase in A.P.I. gravity with greater depth of burial can be distinguished in oils from formations of similar age.

The geologic history of the Powder River basin is favorable to discovery of many more oil fields.