Petroleum geochemistry is a branch of geochemistry (the application of chemical concepts to understand geological systems) which deals specifically with petroleum and its origin, generation, and accumulation, as well as its extraction, refinement, and use. Petroleum, also known as crude oil, is a solid, liquid, and/or gaesous mix of hydrocarbons. These hydrocarbons are from the burial and metamorphosis of organic matter from millions of years ago; the organic matter is from marine animals, plants, and algae. Petroleum is extracted from the Earth (above or below its surface, depending on the geology of the formation), refined, and used as an energy source.
Crude oil is most commonly organised into four types - light, heavy, sweet, and sour. Petroleum is a non-renewable energy source (also known as a "fossil fuel"), so the efficacy of extraction and refining is important for its continued use; multiple techniques are used to detect and to extract crude oil, based on the source rock it is found in and the type of oil itself.