Nevada and western Utah, essentially the Great Basin, can be divided into four subprovinces with different geologic settings for petroleum potential: (1) eastern Nevada and western Utah, containing up to 40,000 ft (12,190 m) of predominantly carbonate source and reservoir rocks deposited in the Paleozoic miogeosyncline; (2) the Sevier orogenic belt, that part of western Utah that was uplifted and eroded in Cretaceous time; (3) the Paleozoic black shale--limestone transitional facies in western Nevada that was folded and overridden by thrust sheets during the Mississippian Antler orogeny; and (4) the Paleozoic eugeosynclinal siliceous shale, chert, and volcanic rocks of westernmost Nevada that were deposited in a tectonically unstable environment.

The youngest marine strata in eastern Nevada are Early Triassic and those in western Nevada are Early Jurassic. In Late Jurassic and Cretaceous time, western Nevada was intruded by large volumes of granitic material. The rest of Nevada and western Utah were deformed intensely. Resulting uplift and erosion may have destroyed previous accumulations of oil. Irregularities on the surface were filled by lacustrine and fluvial sediments during Cretaceous and Tertiary time. Oligocene volcanism spread a thick blanket of flows and pyroclastic material across most of the area. Beginning in Pliocene time, the present mountains were uplifted by tilting of fault blocks, and the intervening valleys were filled with alluvium. Volcanism and mountain building have continued to the present.

In the 140,000 sq mi (362,600 sq km) of Nevada and western Utah, 286 wells have been drilled. One oil field (Eagle Springs), containing on the order of 10 million bbl of recoverable oil with a high pour point, has been found. The primary reservoir in the field is Eocene lacustrine limestone. Similar "high pour-point" oil is produced from fractures in Oligocene ignimbrite and Pennsylvanian limestone. The Eagle Springs oil field is the result of a complex structural, stratigraphic, and migration history. Miocene silt and clay cap the accumulation.

Occurrences of hydrocarbons in the Great Basin are few, consisting of three seeps, scattered shows in cuttings and cores, and three areas of low-volume gas reservoirs in Pleistocene strata. Intense fracturing caused by several periods of deformation apparently has allowed fresh water to flush most of the potential reservoir rocks. Future oil fields probably will be located in places where this flushing has not occurred, but exploration will be difficult because of concealment by alluvium and volcanic rocks in 75 percent of the area.

Extrapolation of the known production to the vast untested area would have no significance. It is doubtful that the Great Basin will contribute a significant part of the future petroleum resources of the United States.