oil shale - teaching hand specimen of oil shale from Parachute Canyon, Colo.
The oil shale of the Green River Formation underlies 1,500 square miles of the Piceance Creek Basin in northwestern Colorado, see map. It contains a rubbery hydrocarbon called kerogen which becomes liquid when heated to 900˚F. In the 1970s, Occidental Petroleum planned to mine oil shale 1,500 feet underground, blast it to rubble in situ, fire it underground and siphon off the liquid hydrocarbon. Unocal planned to mine oil shale and retort it in a plant on the surface. Neither method ever became economical. There are huge reserves, with estimates in the early 1980s of 1.2 trillion barrels.
Kerogen has the same origin as petroleum and natural gas. The organic material in zooplankton rains to the sea floor in vast quantities. Buried by other sediments, it is then converted to kerogen by pressure and heating. Further heating produces petroleum, and even more heating natural gas. The oil shale in the Green River Formation, classified as lamosite, originated as lake sediments deposited 50 million years ago. Its chief organic constituent is lamalginite derived from lacustrine planktonic algae.
"Oil shale"is used for a kerogen-saturated sedimentary rock, not necessarily a shale. In Parachute Canyon it is a cliff former, weathers to a whitish tan. Broken open, it is a dark brown, almost black. These specimens are from the "mahogany layer," actually a kerogen-saturated dolomicrite, a lime mud with more dolomite than calcite, exposed in the walls of Parachute Canyon. It has a brittle fracture, almost conchoidal. Freshly broken specimens have the faint odor of kerogen, and yes, it does burn. Purchase a display or hand specimen, and if you want to experiment, we can include a small piece to experiment with. Tap it with a hammer and sniff. Ignite it over a bunsen burner or with a propane torch.
In 1882, pioneer Mike Callahan built a cabin in Parachute Canyon with an oil shale fireplace. He ignored advice from local Utes, who said that the rock would burn. During the housewarming, the shale of the fireplace ignited and the cabin burned to the ground.
Oil shale occurs worldwide, though in thinner deposits than in Colorado. It actually can be burned as a fuel in power plants. A plant near Narva, Estonia uses this fuel. Russia, Israel and Romania have used it in power plants, but have since converted to natural gas.
Slight weathering produces the gray and white striped specimens, emphasizing the sedimentary nature of this rock. It eventually weathers to a creamy white. On a freshly broken surface, it is almost black. Both sides of the last example are shown to illustrate how the weathering varies.
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