Hydrocarbon:
Chemically, hydrocarbons are organic compounds composed entirely of hydrogen and carbon.
What are the forms of hydrocarbon occurrence?
There are many forms of occurrence of hydrocarbon. Physically, hydrocarbon grades from gases, via liquids and plastic substances to solids. The hydrocarbon gases include dry gas (methane) and the wet gases (ethane, propane, butane, etc).
Condensates are hydrocarbons that are gaseous in the subsurface, but condensate is liquid when they are cooled at the surface. Liquid hydrocarbon is formed oil, crude oil, or just crude. The plastic hydrocarbons include asphalt and related substances.
Solid hydrocarbon includes kergen. Gas hydrates are ice crystals with peculiarly structured cosmic lattices that contain molecules of methane and other gases.
Classification of Hydrocarbon:
Major groups of hydrocarbon are,
- Paraffins/ Alkaine series
- Napthenes/ Cycloalkanes series
- Aromatics/ Benzene series
- Heterocompounds
Paraffins:
The paraffins, often called alkanes, are saturated hydrocarbons with a general formula CnH2ntz for values of n< 5. The paraffins are gaseous at normal temperatures and pressures. These gases are methane, ethane, propane, and butane.
For values of n=5(pentane) to n=15 (pentadecane C15H32), the paraffins are liquid at normal temperatures and pressures. The values of n > 15 grade from viscous liquids to solid waxes.
Two types of paraffin molecules are present within the series, both having similar atomic composition, which increase in molecular weight along the series by the addition of CH2 molecules. One series consists of straight-chain molecules, the other of branched-chain molecules.
Paraffin occurs abundantly in crude oil. The normal straight-chain varieties dominate over the branched-chain structures. Individual members of the series have been recorded up to C78 H158
Napthenes:
The second major group of hydrocarbons found in petroleum are the napthenes or cycloalkanes. This group has a general formula (CnH2n). They occur in a homologous series consisting of five and six-membered carbon rings termed the cyclopentanes and cyclohexanes.
All the napthenes are liquid at normal temperatures and pressure. They make up about 40% of both light and heavy crude oil.
Aromatics:
Aromatics are the third major group of hydrocarbons commonly found in crude oil. This molecular structure is based on a ring of six carbon atoms. The simplest members of the family is benzene (C6H6).
One major series of aromatic compounds is formed by substituting hydrogen atoms with alkane molecules. Another series is formed by straight or branched chain carbon rings. The aromatic hydrocarbons include asphaltic compounds.
The aromatic hydrocarbons are liquid at normal temperatures and pressures. They are present in relatively minor amounts in light oils but increase in quantity with decreasing API gravity to more than 30% in heavy oils.
Hetero compounds:
Crude oil contains many different compounds that contain elements other than hydrogen and carbon. The principal ones are oxygen, nitrogen, and sulfur.
