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. 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.
API gravity
The API gravity of a crude oil is a measurement of purity - i.e., amount of impurities, such as sulphur, nitrogen, or oxygen. Impurities increase the density of the crude. and are the most common hydrocarbon found in crude oil. Paraffins are often a part of gasoline, making them comparatively more valuable.
Naphthenes
Naphthenic hydrocarbons are saturated cyclic hydrocarbons,
Petroleum geochemical techniques
Techniques are used for finding the source rock (the solid material in which the petroleum is found), as well as the type and amount of the petroleum within.
Distillation
While not used as commonly as other techniques today, distillation is used in the process of refining petroleum. It involves the dividation of the crude oil into hydrocarbon categories, and products are recovered from the heated material. A distillation tower is used in separation of the oil, with anywhere between 2 and 300 theoretical plates. Hydrous pyrolysis is performed within water and in high pressures; this method can simulate different depths of burial, demonstrating the possibilities of the fate of the source rock and the associated patroleum.
Biological markers
With credit to the previously listed techniques, biomarkers were found in petroleum and source rock extract. These are fossils from organisms, but are closer in size to molecules than to visible hand samples. They display the same structure as their parent biomolecules and are used in the identification of the organic matter from which the petroleum is derived. Biomarkers are also used in correlating oils and source rocks, finding the oil's maturity, regional differences found between multiple samples, and the history of the basin in which the source rock was located.