Petroleum geochemistry

Petroleum is differentiated into types based on its American Petroleum Institute (API) gravity and by how much sulphur it contains. 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. Light crude oil Light crude oils have higher API gravity figures, due to having fewer impurities. It is more commonly used to produce diesel and gasoline than heavier oils are. Due to its lower viscosity, it is easier to extract and to transport. Heavy crude oil Heavy crude oils have lower API gravity figures, and a larger percentage of impurities. It is used in the making of heavier outputs - e.g., asphalt - and has a higher viscosity, making it more difficult to transport and extract. Sulphur content How 'sweet' or 'sour' a crude oil is is based on the amount of sulphur it contains. Sweet crude oil Sweet crude oil has lower sulphur content - lower than 0.5%. It can be refined into kerosene, high-quality diesel, and gasoline. Sour crude oil Sour crude oil has high natural sulphur content (at least 0.5%). Extra treatment is required in the refining process; impurities are removed to refine the crude into gasoline. Due to the greater cost associated, it is more commonly refined into fuel oil and diesel - less valuable outputs than products of sweet crude oil. == Hydrocarbon compounds ==

The three main hydrocarbon compounds in petroleum are paraffins, naphthenes, and aromatics. Paraffins Paraffinic hydrocarbons are part of the alkane series, 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. Correlation Before the use of gas chromatography-mass spectrometry and biomarkers, correlation of locations' geology was used to find how different formations relate to each other and to their environment. Oil-oil correlations (comparing petroleum to other oil found locally or in other areas) and oil-source correlations (comparing petroleum and its source) were performed; infrared spectrometry, refractive indices, solvent extractable organic matter, compound class distribution, and elemental analysis are all methods of doing oil-source correlations. ==References==