How does oil form?
From dinosaurs? NOT! The old popular myth about oil coming from dead dinosaurs, has long been disproven. Actually, most oil's origin is from tiny microscopic organisms.
Oil forms from kerogen, a mixture of organic compounds in sedimentary rocks. It is most abundant in shales. Shales are analyzed and characterized as potential "source rocks" for oil based largely on their TOC (total organic content).
For kerogen to be transformed into oil, it must be buried to a depth where the temperature and pressure are sufficiently high to convert the kerogen into oil. The place where the depth is sufficient to achieve this, is called the "oil window". Oil shale which contains only kerogen was never buried deeply enough to generate oil. Therefore, humans must artificially convert the kerogen in oil shale into oil at high temperatures.
Oil shale is different from shale oil. People are increasingly developing shale oils such as the Bakken shale oil in North Dakota and Niobrara shale oil in Colorado. These strata were buried deeply enough to convert the kerogen into oil. However, substantial quantities of the generated oil remained in the source rock, rather than migrating out of the source rock into a more conventional reservoir rock. So, oil companies are successfully using fracking technology and horizontal drilling to extract the oil from these units of shale oil.