Oil Shale (not to be confused with Shale/Tight Oil) is shale rock containing an immature precursor to oil called kerogen. It is in may ways similar to tar sands in that it isn't oil but can be turned into oil with sufficient effort. In the case of Oil Shale it needs to heated to between 450 and 500 degrees Celsius to convert the kerogen to oil. Like tar sands there are two possible extraction methods, one involving mining followed by processing and an in-situ process where the shale rock is heated underground and the produced oil pumped to the surface. Oil Shale has been exploited on a small scale in a limited number of places where the shale is close to the surface and of high quality. In particular in Estonia has been mining oil shale for the last 80 years.
However turning rocks into oil is not simple or easy and has many environmental implications. It takes a lot of energy and water to run the process and large amounts of toxic waste is produced. Air pollution has also proved to be a major issue. Attempts to commercialise oil shale production, in the US and elsewhere, have failed up until now due to its high costs and in Estonia the environmental impact of Oil Shale production over has been high. Huge mountains of toxic ash that occasionally catch fire are just one issue. As energy prices rise Oil Shale exploitation may be considered in; Estonia, Australia, China, Israel, Jordan, Morocco, Russia and the US. As with tar sands these low grade resources are extremely carbon intensive and any large scale exploitation would massively accelerate climate change.