How does it form?

Colorado’s oil shales are found in the Eocene-age Green River Formation. The formation is widespread throughout western Colorado, southwestern Wyoming, and eastern Utah.A wide variety of exquisitely preservedfossils are abundant in the strata.In the Green River formation, at least, there seem to be fundamentally two pathways that lead to formation of oil shale beds: 1) accumulation of abundant planktonic organic matter in the bottom sediments, or2) build up of organic material through the growth of microbial mats. The former process led to formation of homogeneous oil shale beds, and the latter resulted in finely laminated oil shales.(Schieber and others, 2007)

The GreenRiver Formation was deposited in two large lakes that periodically dried up. This caused evaporite (salt, nahcolite, and dawsonite) deposits to be laid down with the oil shale.Thenahcolite and dawsonite also comprise important resources of soda and aluminum.

Ancient lakes in which the Green River Formation were deposited.
Extent of Nahcolite resources in the Piceance Basin.