The process of extracting liquid crude oil from the ground is comparatively simple to extracting oil shale. Pressure from gases trapped in the chamber where oil is present force the crude oil to the surface. After this pressure is alleviated, the more difficult secondary and tertiary phases of oil drilling begin. In some cases, water may be pumped in to loosen compressed oil. Sometimes gasses are introduced to repressurize the oil chamber. And in many cases, the remaining oil is simply left for future drilling with more advanced equipment.
Getting crude oil from rock represents perhaps the most difficult process of extraction. Oil shale must be mined using either underground- or surface-mining methods. After excavation, the oil shale must undergo retorting. This is when the mined rock is exposed to the process of pyrolysis -- applying extreme heat without the presence of oxygen to a substance, and producing a chemical change. Between 650 and 700 degrees Fahrenheit, the kerogen -- the fossil fuel trapped within -- begins to liquefy and separate from the rock [source: Argonne National Laboratory]. The oil-like substance that emerges can be further refined into a synthetic crude oil. When oil shale is mined and retorted above ground, the process is called surface retorting.
Source: Oil shale