Some general information on oil shale

The largest deposit of oil shale in the world is found in the Green River basin of Colorado, Utah, and Wyoming. It contains the equivalent of about 1.5 trillion barrels of shale oil.

What is oil shale?

Oil shale is a fine grained sedimentary rock formed from the compaction and heating of organic rich sediments and containing significant amounts of kerogen. Its name is, in fact, a bit misleading, as the rock itself is not necessarily shale, and the organic matter it contains is not oil. However, kerogen, as we have seen before, is a precursor to the formation of petroleum, and so the name does hold some truth: oil shale yields substantial amounts of oil and combustible gas upon distillation.

Formation of oil shale has occurred in a number of environments, from fresh to saline lakes, marine basins, and in some swamps usually in association with coal deposits. In some ways, oil shale may seem similar to coal, but in fact differs greatly in composition. Oil shale may contain between 60 and 90% mineral matter (non-organic), while coal will contain, by definition, less than 40%. The kerogen within oil shale is also of different organic composition than coal, which enjoys a more matured organic makeup that is lower in hydrogen and higher in oxygen than oil shale kerogen.

See this factsheet on oil shale from the U.S. DOE: DOE FACTSHEET  

And this comprehensive document on its history, processing, and economics from the Congressional Research Service: CRL DOCUMENT

Formation of oil shale

We have now covered the process that leads to the formation of petroleum and petroleum-like resources. The process, in general, begins with the building of organic matter through the action of PHOTOSYNTHESIS. In the case of marine deposits, this begins with phytoplankton, which then enter the food chain that leads to the sedimentation of zooplankton rich in proteins, carbohydrates, and lipids. In terrestrial burial, the process begins in higher order organic life such as trees and shrubs, which then deposit primarily carbohydrates and lignin. The fraction of minerals to organic matter and the composition of the organics themselves lead to differences in the way that organic chemical reactions proceed and are catalyzed.

As organic matter is buried, the process of diagenesis compacts it into organic rich sedimentary rocks. Further degrees of diagenesis then lead to the generation of kerogen which is then, through deeper burial and catagenesis, transformed through various means into crude oil and natural gas, or coal, depending on the precise conditions. In the case of oil shale and, according to some geologists, bitumen sands, the process of catagenesis is never allowed to be completed due to insufficient burial (temperatures and pressures too low).

A comprehensive report on oil shale resources from the USGS can be found here: http://pubs.usgs.gov/sir/2005/5294/pdf/sir5294_508.pdf

What is diagenesis?

Diagenesis is a process of compaction under mild conditions of temperature and pressure. When organic aquatic sediments (proteins, lipids, carbohydrates) are deposited, they are very saturated with water and rich in minerals. Through chemical reaction, compaction, and microbial action during burial, water is forced out and proteins and carbohydrates break down to form new structures that comprise a waxy material known as “kerogen” and a black tar like substance called “bitumen”.  All of this occurs within the first several hundred meters of burial.

Refining the oil shale

In the same manner that natural mineral catalysts help to transform kerogen to crude oil through the process of catagenesis, metal catalysts can help transform large hydrocarbons into smaller ones. The modern form of “catalytic cracking” utilizes hydrogen as catalyst, and is thus termed “hydrocracking”. This is a primary process used in modern petroleum refining to form more valuable lighter fuels from heavier ones.

Oil that is produced from the refining of oil shale is typically referred to as synthetic crude oil, but the process is more closely allied with traditional crude oil refining than syn-fuel processes such as “gas to liquids”. The primary process is referred to as “retorting”, and involves the “cracking” (or destructive distillation or pyrolysis) of larger carbon chains into smaller ones in the absence of oxygen. Syn-fuel processes (such as Fisher-Tropsch) actually build up larger hydrocarbons from smaller ones, which is the opposite of cracking. Retorting is the cracking process used in shale oil refining, and first breaks down the kerogen to release hydrocarbons, and then further cracks the hydrocarbons into lower weight products.

Retorting can occur in a traditional refining capacity, or may be conducted in-situ. In-situ processes require that the oil shale be heated, to release the petroleum liquids, prior to extraction from the ground.

Oil shale distillates (products of retorting) typically favor the production of middle-distillates (diesel and kerosene), and have higher concentrations of nitrogen than crude oil. To produce light-distillates (such as gasoline) additional processing, such as hydrocracking, is required to break down the larger hydrocarbons. Also, the nitrogen must be removed through some hydrotreating process, comparable to hydrogen desulfurization to remove sulfur from crude oil, such as hydrodenitrogenation.

Because hydrogen will be required in the refining process (bitumen is carbon rich and hydrogen poor), an additional source of hydrogen (such as methane) is needed. Therefore, some other fuel in addition to bitumen is needed to produce synthetic crude oil, which adds to the cost and energy intensity (and carbon dioxide emissions) of the process (2 fuels  1 fuel).