Geochemical and geological data were used to identify effective source rocks and oil-types, and to determine stratigraphic sequences and areas that are prospective for crude oil and thermal hydrocarbon gas. The source rock volumes and generation-expulsion performance data for each effective source sequence provided the basis for calculating quantities of expelled oil and gas. These quantities readily account for discovered in-place reservoir oil of more than 7 billion barrels and relatively minor amounts of gas, mainly associated.

Lower and Upper Cretaceous source beds expelled most of the indigenous oil. These oils are chemically similar, regardless of their source. Lower Cretaceous Mowry Shale and Upper Cretaceous Niobrara and Carlile formations expelled most of the discovered oil. Oil expulsion from Cretaceous source rocks began during the early Tertiary and continued through much of Miocene time as the expulsion fronts moved up section and updip. Laramide structure controlled directions of migration of Cretaceous oil.

The second major type of oil is nonindigenous to the Powder River Basin and is correlated to the remote Upper Permian Phosphoria Formation source area centered in southeastern Idaho. This oil entered northeastern Wyoming during Late Jurassic time, before the Powder River Basin formed, through carrier beds of Pennsylvanian and Permian age. Phosphoria-type oil is preserved in four separate parts of the basin, primarily in sandstone reservoirs of Early Permian age in the Minnelusa and Tensleep formations.

A minor oil-type found in the southeastern part of the Powder River Basin was expelled from relatively thin, local shales of Pennsylvanian age.

Several giants fields with more than 100 million barrels of recoverable oil and major oil fields of at least 50 million barrels are located on structural positives around the periphery of the Powder River Basin. These salients served as gathering areas to concentrate migrating oil. Other large fields are in stratigraphic traps oriented parallel with structural strike on the eastern flank, this orientation permitting large accumulations to form from a big drainage area in downdip source rocks.

Meteoric water, aerobic bacteria, distillation, and thermal cracking are affecting the quality of preserved oil. Two types of bacterial alteration are common. Much of the gas generated with oil has escaped or dispersed. Oxygenated recharge waters appear to be degrading organic matter in Cretaceous source rocks around the basin perimeter. Both chemical and physical properties of rocks and fluids proved to be useful in defining prospective areas for the various types of oil.