Organic geochemistry is the study of the impacts and processes that organisms have had on the Earth. It is an interdisciplinary field that incorporates the sub-disciplines of biogeochemistry, petroleum geochemistry, isotope biogeochemistry, geomicrobiology, and environmental geochemistry.
One of the most commonly studied features of the features in organic geochemistry are biomarkers, which are organic molecules that can be detected in the geosphere, which have enough structural information that they can be used to indicate the existence of present and past forms of life. Below are two examples of hydrocarbon biomarkers. Cholestane (left) and hopane (right) that diagnostic of eukaryotes and bacteria, respectively.
The movie below provides an example of how we can visualize the mix of hydrocarbon biomarkers extracted from a rock. Each peak in this multidimensional gas chromatogram is a unique organic compound.
There are two fundamental steps to the formation of oil and natural gas
Step 1: Diagenesis and the formation of kerogen
Diagenesis is a process of organic matter compaction and chemical alteration under mild conditions of temperature and pressure. When organic-rich sediments that contain proteins, lipids, carbohydrates are deposited, they are very saturated with water and rich in minerals and oxidants. A complex series of chemical reaction that are highly mediated by microbial activity results in the near complete breakdown of proteins and carbohydrates and the partial degradation of lipids to form new structures that comprise a waxy material known as “kerogen” and a lipid-rich, black tar like substance called “inherited bitumen”. This often occurs within the first several hundred meters of burial.
Step 2: Catagenesis (or “cracking”) breaks down kerogen to form bitumen that can further be cracked to release petroleum and natural gas
After kerogen forms catagenesis begins with further deeper burial that results in increased temperatures and pressure. When temperatures reach >70⁰C the intermolecular bonds of kerogen become unstable crack to release bitumen, oil and natural gas. Catagenesis is not only caused by elevated temperature and pressure. It is also partially catalyzed by minerals that are deposited and persist through marine diagenesis.
The type of organic matter buried and the conditions during diagensis and catagenesis determines type of hydrocarbons that are generated. With further burial and elevated temperatures lighter hydrocarbons are generated to the extent that only natural gas is produced. Subsequently both petroleum and natural gas have “window” of conditions where they can be formed.
This is not the only pathway of carbon sequestration in the geosphere. Terrestrial organic matter from peat mires and forests have the potential to form coal. In this case, the cellulose and lignin of plants and trees, which contains much higher abundances of oxygen, and the lower mineral contents condenses to form peat. With burial the elevated temperatures and pressures results in the progressive transformation of peat into bituminous coal. As this process continues higher ranks of lignite and anthracite coals are eventually produced.