OIL SHALE IN THE GREEN RIVER FORMATION The largest resources of shale oil in the United Formation in Utah signaled the end of the single large States occur in the Green River Formation of Eocene extensive lake and again deposition took place in two age in Colorado, Utah and Wyoming. The calculated basins. In Colorado, white bands, consisting of marlresources underlie an area of about 8,000 square miles stone and low-grade oil shale, interbedded with the (Fig. i); the remainder of the area outlined on Fig. 1 sandstones indicate the presence of short-lived lakes is underlain by Green River oil-shale beds that are of limited extent during a time of dominantly fluviatile thin, or lean, or where data are inadequate to permit deposition. The lake in Utah retreated westward and evaluation of the oil shale. finally gave way to a fluviatile environment.
The oil shale in the Green River Formation was Oil and gas are produced from the shore facies of formed mainly in two large lakes-Lake Uinta in the Green River Formation at several fields in the Colorado and Utah, and Lake Gosiute in Wyoming. northern part of the Uinta Basin. Production is princi-The oil shale in the lower part of the formation in Utah pally from stratigraphic traps that lie between the and Colorado was deposited in two lakes restricted to present structural axis and the Eocene depositional parts of the areas now occupied by the Uinta and axis of the basin. The shore facies also contains Piceance Creek Basins. It is estimated that the average asphaltic sandstones in several parts of the basin. area of the Utah lake was about 2,500 square miles Gilsonite veins, which are essentially vertical, are as and that of the Colorado lake 500 square miles. In much as 18 feet wide in massive sandstone beds of the Colorado, saline minerals, notably nahcolite and Uinta Formation and of the shore facies of the Green dawsonite, were deposited with the oil shale. The River Formation. lateral and vertical extent of the saline deposits is not In Wyoming, the first widespread oil-shale unit, the known; however, they extend through as much as Tipton Shale Member of the Green River Formation, 500 feet of strata (Fig. 2). In Utah, nahcolite and was deposited in late early Eocene time in Lake dawsonite have not been reported in the lower part of Gosiute which inundated an area of about 12,500 the Green River Formation. square miles. The upper 50 feet yields as much as 30 After the deposition of the saline minerals in the gallons of oil per ton in the area west of locality 2 Piceance Creek Basin the two lakes expanded and (Fig. 4). joined, forming one body of water-Lake Uinta. During the deposition of the overlying Wilkins Peak The sequence of rich, thick, and persistent oil shale, Member, Lake Gosiute became a closed fluctuating known as the Mahogany ledge or zone, in the upper saline lake that was confined largely to the Green part of the Parachute Creek Member of the Green River Basin (Fig. 1). About 100