What is Oil Shale?
Oil Shale refers to the sedimentary shale rock that contains oil-prone kerogen (partially converted fossil organic matter) which has not been submitted to enough pressure and temperature over millions of years to release oil1. Through advanced processing methods, the oil shale can be converted into shale oil and other hydrocarbons.
The kinds of hydrocarbons produced depends largely on the type of kerogen within the shale rock. For example, heating coal, which has kerogen mostly derived from plant matter, produces hydrocarbon gases. Oil shale kerogen, on the other hand, is largely derived from algae. After heating and refining, shale oil is chemically identical to conventional petroleum2.
Unlike conventional oil, shale oil cannot be recovered by drilling; it requires heating for oil to be released3. There are two methods of recovering shale oil – ex-situ and in-situ processing. In ex-situ processing, shale oil is mined with conventional mining methods, like open pit, strip or underground mining. The shale is then transported to processing units that will retort, or heat up the shale in a process known as pyrolysis. Pyrolysis of shale rocks takes place in the absence of oxygen in temperatures between 900 and 1000 °F (450 to 500 °C) 4. At these high temperatures, the kerogen will decompose relatively quickly. Pyrolysis, in short, is a substitute to the natural mechanism that decomposes kerogen in earth over millions of years in high pressure and temperature conditions.
In-situ processing allows for extraction of the shale oil by heating the shale directly underground on much longer timescales and lower temperatures24. There are various methods with which in-situ processing and extraction can be achieved. However, all of them are still largely in their experimental stages and have yet to achieve commercial levels of production5.
Shale oil is a hugely untapped resource that has been mined and processed since the 1800s. Relative to conventional oil, however, the production quantities have been low. According to the International Energy Agency, there are about 1 trillion barrels of economically recoverable shale oil resources, compared to 1.3 trilion of recoverable conventional oil reserves6. However, with ex-situ technology being the only economically viable method of recovering shale oil, even with government subsidies, shale oil is not expected to account for more than 1 million barrels of oil per day (mb/d) by the year 2035 (with current oil production at 93 mb/d)6. In a 2005 report by the European Commission7, Estonia accounted for 70% of world shale oil production.
As with most unconventional resources, there are environmental impacts associated with shale oil excavation, especially with the ex-situ method where surface mining can damage the land2. Taking into account all of the energy going into excavation, the emission levels of direct combustion of shale oil is on par with the lowest grade of coal8.
Furthermore, many experimental in-situ methods require large amounts of water (with solvents) to hydraulically fracture the shale rock formation. This, as well as the spent (retorted) shale, poses viable risks of contaminating the ground water supplies8. However, the in-situ method could also serve as a potential CO2 sequestration method that could greatly lower its green house gas emission impact2.