Fifty deposits or groups of deposits of oil-impregnated sandstone (tar sand) in Utah contain between 20 and 25 billion bbl of oil, about 95 % of the nation's resource. The Uinta basin, northeast Utah, is ringed by 25 deposits, estimated to contain 10.5 to 11.0 billion bbl of oil in place, with about 95 % in 4 giant

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deposits: Asphalt Ridge, Hill Creek, PR Springs, and Sunnyside. In central-southeast Utah, 21 deposits contain between 10 and 15 billion bbl, a less precise estimate because of the lack of definitive subsurface data. About 90 % of this is contained in 3 giant deposits. Elsewhere in Utah 4 less important deposits are found.

A wide variety of crude oils in varying stages of preservation and alteration has been analyzed. Gravities (API) range from minus values to near 15°; range in deposits considered of commercial interest is 8 to 15°. Uinta basin deposits of Tertiary age contain oil with an average sulfur content around 0.4 %. Permian and Triassic deposits in central-southeast Utah yield oils with between 3.0 and 4.3 sulfur.

Two types of deposits are recognized: in situ (oil fields in their original position breached by erosion) and migrated (oil displaced from a ruptured trap to another position). Most southern Uinta basin and central-southeast Utah deposits are in situ; northern Uinta basin deposits appear to be migrated; actually they are seeps disseminated on the outcrop.

The deposits exist in a wide variety of physical situations, and reservoirs tend to be heterogeneous. Mining appears to be the most likely method of large-scale exploitation. Serious legal, political, environmental, and technologic problems exist. Growing scarcity of energy and petrochemical sources is creating an economic climate in which development is feasible.

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