The Green River Formation consists of shales, siltstones and carbonates deposited in three ancient lakes within intermontane basins that formed as the Rockies were rising: its average thickness is 600 m (3000 m at its thickest point). Sediments were deposited in thin layers, yearly “varves,” each on the order of ~0.20mm average thickness.
The fish were not actually “squished” to death, but by an environmentally complex good fortune (for fossil hunters anyway), their bodies settled onto anoxic bottom layers rather than floating to the top of the water as dead fish are wont to do. Preservation in anaerobic conditions and fossilization in such fine-grained sediments has preserved impressions of the soft parts of their bodies in the best samples. Other creatures, including crocodiles, birds and even a bat (possibly the oldest bat fossil in the world) have been recovered in Green River shales.
In addition to fish, the Green River Formation is the world’s largest known deposit of kerogen-rich rocks: kerogen is a primitive type of crude that hasn’t yet converted to actual oil, but will on heating. According to the USGS, these oil shales contain an estimated potential of 4.285 trillion barrels of oil. If even a third of this amount could be recovered, it would equal the entire world's proven oil reserves.
Dat’s a lot of oil, and a dat means a lot of potential conflicting economic interests.
Exploiting the shale for petroleum is difficult; it must to be heated to nearly 3000 degrees centigrade before it will give up its oil. Conventional drilling or fracking are not applicable. Digging up the shale and heating it in surface facilities is an established methodology for recovering the crude from oil shale, but it is expensive and, well, digging up much of Utah may have an environmental impact or two. The other proposed method involves underground heating, which melts off the oil from within the shale, flushing it out with steam, and then drilling for the newly liberated petroleum. The heat and drill solution requires immense water reserves – so much water would be needed that possibly Denver would become quite thirsty. Technology has not yet progressed in these recovery methods to allow economic exploitation of the Green River reserves: it still costs more to get oil from the shale than the price of the oil produced.
For better or worse, environmentally speaking, when people want oil and when so much money can be made, it’s only a matter of time before an economic method is developed for the practical exploitation of this immense resource.
Photo: A fish I took from the Green River formation that smells a lot like petroleum.
I recommend these references: