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Deposits of both oil shale and tar sand are classified as unconventional sources of petroleum. A number of dissimilarities exist in these two potential synfuel resources. From a geologic standpoint, major differences are found in depositional environments including both lacustrine and marine; in physical characteristics such as grain size, cementation, and fracture trends; and in chemical characteristics, mineralogy, and methods of migration. Other comparisons are made in kinds of organic matter, derived oils, and methods of extraction. Oil shale and tar sand deposits are found in both the western and eastern parts of the country, but usually not in the same localities. One exception is in the Uinta basin of Utah where tar sand beds are stratigraphically adjacent to and s metimes interbedded with oil shale. In Utah, Wyoming, and Colorado, oil shale is found in thicker and richer concentrations of smaller area extent compared to eastern oil shale deposits of Devonian age, which are thinner and less rich but which exist over a much larger area. Ninety-three percent of the known U.S. tar sand resources are found in Utah.
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