Within the intermontane basins of the Intermountain region, the Great Basin, and the Columbia Plateau are enormous volumes of Tertiary continental sediments. These sediments may be classified on the basis of depositional loci into two general categories: (1) suites of sediments deposited in large lakes which in their later histories became saline, and (2) basin fills of fluvial, paludal, and fresh-water lacustrine beds associated with varying amounts of extrusive volcanics.
To sediments of the first type are attributed many of the properties of marine strata. They commonly contain pyrobituminous shales and dolomites, truly petroliferous shales and dense limestones, tar sands, and various semi-solid or solid bitumens. Oil and gas production has been established from such beds and closely associated strata. Though some of this hydrocarbon content in local areas may have migrated into place from subjacent marine strata, most of such occurrences are considered indigenous.
Sediments of the second type commonly are either lacking in hydrocarbon content or contain methane gas which can be shown to be entrapped emanations from intercalated lignitic beds or other swamp deposits. In the Columbia Plateau where beds of Miocene age are intercalated with a thick series of basalt flows, tarry residues from the distillation of organic matter have been mistaken for shows of oil.