Petroleum source beds in the Great Basin region include marine Paleozoic rocks and nonmarine upper Mesozoic and lower Cenozoic rocks. Potential source beds have been identified in continental-rise deposits of the Ordovician Vinini and Devonian Woodruff formations in the eastern part of the Roberts Mountains allochthon (Antler orogene), in central and north-central Nevada; in flysch-trough and prodelta-basin deposits of the Mississippian Chainman Shale and equivalent rocks of the Antler foreland basin, in Nevada and western Utah; and in lake-basin deposits of the Cretaceous Neward Canyon Formation and the Paleogene Sheep Pass and Elko formations and equivalent rocks, in central and eastern Nevada. Oil fields in the Great Basin are located within Neogene-Quaternary basins that formed during Neogene basin-range block faulting. Most of the oil shows and crude oils analyzed can be correlated with Mississippian or Paleogene source rocks. The Mississippian Chainman Shale is confirmed as the major petroleum source rock, having generated the oil in the Trap Spring, Bacon Flat, and Grant Canyon fields in Railroad Valley and the Blackburn field in Pine Valley The Paleogene Sheep Pass Formation is the source of the oil in the Eagle Springs field and probably the Current field in Railroad Valley Oil occurrences in the northern Great Basin are believed to be derived from two or more other Tertiary lacustrine sequences.