Oil shales are source rocks that have not been exposed to heat or pressure long enough to convert their trapped hydrocarbons into crude oil.
Technically speaking, oil shales are not really shales and do not really contain oil, but they are usually relatively hard rocks called marls containing a waxy substance called kerogen. The kerogen trapped in the rock can be converted into crude oil using heat and pressure to simulate natural processes. The method has been known for centuries and was patented in 1694 under British Crown Patent No. 330 covering, "A way to extract and make great quantityes of pitch, tarr, and oyle out of a sort of stone."
Although oil shales are found in many countries, the United States has the world's largest deposits.
The petroleum industry is involved in the global processes of exploration, extraction, refining, transporting (often with oil tankers and pipelines), and marketing petroleum products. The largest volume products of the industry are fuel oil and gasoline (petrol).
Petroleum is also the raw material for many chemical products, including pharmaceuticals, solvents, fertilizers, pesticides, and plastics. The industry is usually divided into three major components: upstream, midstream and downstream.
Midstream operations are usually included in the downstream category.