Crude oil contains organic compounds, heteroatom compounds (S,N,O), hydrocarbons (C, H), metals and organic (Ni, V, Fe) and inorganic (Na+, Ca++, Cl-) compounds as listed in Figure 1.7. Compounds that contain only elements of carbon and hydrogen are called hydrocarbons and constitute the largest group of organic compounds found in petroleum. There might be as many as several thousand different hydrocarbon compounds in crude oil. Hydrocarbon compounds have a general formula of CxHy, where x and y are integer numbers.

Figure 1.7. Constituents of crude oil.

Hydrocarbons are generally divided into four groups: (1) paraffins, (2) olefins, (3) naphthenes, and (4) aromatics (Figure 1.8). Among these groups, paraffins, olefins, and naphthenes are sometimes called aliphatic compounds, as different from aromatic compounds. The lightest hydrocarbon found as a dissolved gas is methane (CH4), the main component of natural gas. Olefins are not usually found in crude oils, but produced in a number of refining processes.

Figure 1.8. Chemical bonding in hydrocarbons.