The Origin and Chemistry of Petroleum

4.1 Formation of Crude Oil and Natural Gas

Crude oil is a product of the remains of prehistoric plants and animals, buried in the primeval mud of swamps, lakes, and oceans. Over the centuries, layers of mud and organic debris were subjected to enormous pressure and high temperature, and a petroleum-saturated rock was formed. Current models suggest that the dominant form of organic matter responsible for the formation of petroleum and synthesis of crude oil is derived from microscopic, photosynthetic organisms known as phytoplankton that live at or near the surface of lakes and oceans. Associated with the phytoplankton are their microscopic predators known as zooplankton. These, together with land vegetation washed into lake or near shore marine sediments, accumulate over a period of millions of years.

As more sediment is deposited, the organic matter is buried so that its complete destruction by bacterial activity is prevented. During burial, a number of changes (termed diagenesis) begin quickly under the influence of bacteria. The most notable process is the conversion of major biological building blocks, or biopolymers (proteins, cellulose, and lipids), into their individual components biomonomers (amino acids, sugars, and fatty acids). These accumulate in the sediments, which, as they settle due to overburden, begin to be heated by the earth's geothermal gradient, which averages about 1.2°F per 100 feet of burial. Hence, sediment buried to 10,000 feet would have a temperature increase of 120°F over its ambient temperature at the surface. During this process, the biomonomers begin to react among themselves, growing into a complex two-dimensional refractory organic structure known as kerogen.

Under further thermal stress and over millions of years of burial, slow reactions occur, removing oxygen as carbon dioxide and water and transforming the kerogen to crude oil. When burial is great, resulting in temperature elevations to above about 150 to 200°F, the source rock becomes over-mature and crude oil can be transformed to hydrocarbon gases. At very high temperatures (exceeding 200°F), most of the crude oil and natural gas is converted to methane, known in the industry as dry gas. Following the formation of oil and gas, the fluids are mobilized into reservoirs.

Both time and elevated heating are thus responsible for transforming organic matter derived from decaying organisms to petroleum and gas. The original chemistry of the organic matter, the environment of deposition, and the time and heat imposed on the organic matter dictate the type of crude oil or gas formed. The chemistry of the oil and gas can often help to reconstruct the source of the original organic matter and temperature of hydrocarbon generation.

Crude oil formed during this long and complex process is composed of a mixture of many substances, from which various refined petroleum products (such as gasoline, kerosene, fuel oil, and lubricating oil) are manufactured. These substances are mainly composed of carbon (C) and hydrogen (H), and are therefore called hydrocarbons. Other elements, such as oxygen (O), sulfur (S), and nitrogen (N), may also be present in relatively smaller quantities, together with traces of phosphorus (P) and heavy metals like vanadium (V) and nickel (Ni). Despite wide variations in the chemical composition of crudes (R.J. Hengstebeck, 1959), their elemental compositions generally fall within the following narrow ranges:

Element Composition (%)
Carbon 84-87
Hydrogen 14-Nov
Sulfur 0-3
Nitrogen 0-1
Oxygen 0-2

4.2 Chemical Constituents of Petroleum and Its Refined Products

The simplest hydrocarbon is methane, which consists of one carbon atom and four hydrogen atoms. Its molecular structure can be presented as:

or CH 4

Larger hydrocarbon molecules are composed of two or more carbon atoms joined to one another and also to hydrogen atoms (alkanes, also called paraffins). The carbon atoms may form a straightchain (n-alkanes), a branched-chain (iso-alkanes), or a ring (cycloalkanes, cycloparaffins or naphthenes) structure. Each carbon atom must have four chemical bonds, as shown below:

n-alkane iso-alkane cycloalkane

When two adjacent carbon atoms are linked by two or three bonds instead of only one, the hydrocarbon is said to be unsaturated. Straight- or branched-chain hydrocarbons with one or more double bonds are called alkenes or olefins, and hydrocarbons with a double bond in a ring are cycloalkenes or cycloolefins.

The simplest member of the olefin series is ethylene:

Hydrocarbons containing six-membered ring units with three alternate double bonds form an important group known as aromatic hydrocarbons. The simplest member of this group is benzene, which has only one ring or nucleus. Compounds with multiple condensed benzene rings are called polynuclear aromatic hydrocarbons (PAH). Naphthalene is an example of a two-ring PAH.

benzene naphtalene

The principal compounds in petroleum are paraffins, naphthenes, and aromatic hydrocarbons, with subordinate amounts of asphaltic-type materials. For example, the hydrocarbon type composition of a crude oil from South Louisiana (National Research Council, 1985), is:

Hydrocarbon type Weight %
Paraffins 28
Naphthenes 45
Aromatics 18
Asphaltenes 9

Refining of Crude Oil. Although crude oil may be utilized directly as an energy source, the full benefit of the different properties of the constituent hydrocarbons may be realized only when the constituents are separated. Distillation is the principal method for separating crude oil into useful products. Distillation at atmospheric pressure separates crude oil into fractions of a specific boiling range as schematically shown in Figure A-1.

Fig. A-1 Hydrocarbon composition and boiling ranges for major refined products

Modern refinery practice is much more complex than just distillation. It consists of many interrelated steps designed to manufacture different fuels for specific applications. As most crude oils contain only 10 to 40 percent of their hydrocarbon constituents in the gasoline range, refineries use cracking processes, which convert high molecular weight hydrocarbons into smaller and more volatile compounds. Polymerization converts small gaseous olefins into liquid gasoline-size hydrocarbons. Alkylation processes transform small olefin and iso-paraffin molecules into larger iso paraffins with a high octane number.

Combining cracking, polymerization, and alkylation can result in a gasoline yield representing 70 percent of the starting crude oil. More advanced processes, such as cyclization of paraffins and dehydrogenation of naphthenes to form aromatic hydrocarbons, have also been developed to increase the octane rating of gasoline. Modern refinery operation can be shifted to produce almost any fuel type with specified performance criteria from a single crude feedstock. Figure A-2 summarizes the major fuel manufacturing processes commonly used in petroleum refining.

Fig A-2 Principal refinery process streams

4.3 Characterization and Composition of Refined Products

The properties of refined fuels are a function of the refinery process and the chemistry of the starting crude oil blend. This section briefly describes some physical and chemical bulk properties of different fuels, as well as typical additives.

Gasoline Fuel. Automotive gasoline is a generic term used to describe volatile petroleum fuels used primarily in internal combustion engines. It is a complex mixture of hydrocarbon compounds predominantly in the C3-C12 range, with a boiling-point distribution between 120 to 400° F (77 to 340° F for aviation gasoline) and specific gravity of about 0.74 g/cm3. The specific composition may vary depending on the source of petroleum and refinery method and has changed historically as a function of automotive design and regulatory dictates. Automotive and aviation gasolines include a number of additives, such as antiknock agents, lead scavengers, and antioxidants (Kirk-Othmer, 1977).

Among the additives, lead alkyl antiknock additives and lead scavengers are the most important for fingerprinting purposes, because the composition of lead alkyls and their concentration in gasoline contamination can be important time markers (in free phase products). Leaded gasoline was first marketed in 1923, and until 1960 tetraethyl lead (TEL) was used as the only antiknock agent (L.M Gibbs, 1999). Since 1960, when Chevron (then Standard Oil Co.) introduced another antiknock, tetramethyl lead (TML), different combinations of these two additives as well as redistribution reaction mixtures of TEL and TML were used (L.M Gibbs, 1993). The composition of commercial redistribution reaction mixture resulting from the use of equimolar amounts of TML and TEL is:

Mixture Tetra- methyl
lead (TML)
Trimethyl- ethyl
lead (TMEL)
lead (TMEL)
lead (MTEL)
lead (TEL)
Weight % 3.8 23.4 42.4 25.6 4.8

After about 1980, the most common lead additive was TEL, sometimes containing small amounts of TML. In addition, the total amount of lead added to gasoline as lead alkyls has changed with time. The recommended maximum content of lead at 3.17 grams of lead per US. gallon of gasoline (g Pb/gal) was introduced in 1926 (Public Health Bulletin, 1926). The maximum permitted level peaked in 1959 at 4.23g Pb/gal (Public Health Bulletin, 1959). As a result of government regulations on the use of lead, maximum levels gradually decreased to 0.5 g Pb/gal in 1985 and to 0.1 g Pb/gal in 1988 (L.M. Gibbs, 1990). By the end of 1992, California completely eliminated leaded gasoline, whereas other states had already done so in previous years, but some states still have a waiver on the use of leaded gasoline. A plot of the average historic concentration of lead in U.S. gasolines, based on the information provided by Mr. James W. Caldwell (1994) of the EPA is shown in Figure A-3.

Fig. A-3. Historical lead usage

To reduce adverse effects of lead oxide, which remains in the engine after combustion of the fuel, lead scavengers ethylene dibromide (edb) and ethylene dichloride (edc, also commonly known as 1, 2-DCA) were introduced in 1928 (S.P. Nickerson, 1954). The ratio between the two has changed over the years. A typical motor mix for automotive gasoline additives in the 1980s consisted of about 62 percent TEL, 18 percent edb, 18 percent edc, and 2 percent of other inactive ingredients, such as dye, antioxidants, petroleum solvent, and stability improvers. For aviation piston engines, TEL is still used as an antiknock additive, and the scavenger consists entirely of edb, because one combustion product of edc is the corrosive hydrochloric acid.

With the government-mandated phase-out of lead additives, oxygenate compounds such as ethers and alcohols have been increasingly blended to gasolines to maintain high octane rating and reduce vehicle emissions of carbon monoxide. A common oxygenate, methyl tertiary-butyl ether (MtBE), has been blended with gasoline since its first commercial manufacture in 1979. Its documented use on the East Coast was in 1982 and in California in the late 1980s.

The rapid growth of MtBE usage, which started about 1980, was in response to the implementation of the winter oxygenated gasoline program for 39 areas that did not attain the EPA standard for maximum carbon monoxide atmospheric concentration levels required by the Clean Air Amendments of 1990 (Title I, 1990). Modern reformulated gasoline contains as much as 15 percent by volume of MtBE. Methyl-, ethyl-, and tertiary-butyl alcohol have all been blended with gasoline by different refineries. Some states, such as Alaska and Washington, either exclusively or primarily use alcohols as oxygenate blending agents.

Middle Distillate Products. This group of products generally includes mineral spirits and stoddard solvent, kerosene, most of the jet fuels, diesel, and light fuel oils.

Kerosene (fuel oil No. 1). It is a light-end middle distillate intended for use in vaporizing-type burners, in which liquid fuel is converted to a vapor by contact with a heated surface or by radiation. Kerosene is generally a straight-run distillate with a boiling range of 260 to 570° F and density of approximately 0.81 g/cm3. It is composed of hydrocarbons predominantly in the C9-C16 range. Bulk composition of a typical fuel oil No. 1 is:

Hydrocarbon type Volume %
Paraffins 50.5
Naphthenes 30.9
Aromatics 18.6

Jet Fuels. Many commercial jet fuels have basically the same composition as kerosene, but they are made under more stringent specifications than those for kerosene, mostly in the decrease of sulfur and aromatic hydrocarbons. Other commercial and military jet fuels are referred to as wide-cut fuels and are usually made by blending kerosene with lower boiling streams (such as straight-run naphtha) to include more volatile hydrocarbons. Selected properties and composition of widely used jet fuels are presented in Table A-1.

Table A-1: Selected Properties and Composition of Jet Fuels
Property and Composition Commercial Jet A fuel (kerosene) Military Jet Fuel
JP-4 (USAF) (wide cut) IP-5 (USN) (kerosene)
Gravity, ° API 42.3 54.8 41.0
Boiling range, °C 170-300 48-270 150-290
Saturates content, vol. % 80.7 88.4
Aromatics content, vol. % 15.8 10.8 16.5
Olefins content, vol. % 1.8 0.8 1.4
Sulfur content, vol. % 0.035 0.018 0.020

Diesel fuel No. 2. It is a heavier distillate than diesel fuel No. 1. It is intended for use in atomizing-type burners, which spray the fuel into a combustion chamber where the droplets burn while in suspension. Diesel fuel No. 2, manufactured in the United States, is generally a blend of straight-run and catalytically cracked streams, including straight-run middle distillate, hydrodesulfurized middle distillate, and light catalytically and thermally cracked distillates.

The boiling range of the fuel is approximately 320 to 680° F and density is in the range 0.82-0.86 g/cm3. It consists of hydrocarbons having carbon numbers predominantly in the range C9-C24. It is noteworthy that this fuel has a wide range of PAH hydrocarbons, extending from naphthalenes (dominant) to phenanthrenes. Diesel fuel No. 2 often also includes the sulfur-containing aromatics, such as benzothiophenes and dibenzothiophenes, as well as alkylated benzenes.

A wide range of additives is often blended in the finished product to assure technical performance (IARC Monographs, 1989). The gross composition of a typical diesel fuel No.2 is:

Hydrocarbon type Volume %
Paraffins 55
Naphthenes 12
Olefins 5
Aromatics 24
Residuals 4

Diesel fuel No. 1. It is similar to a blend of kerosene together with a lesser amount of diesel No.2. Diesel No. 1 is manufactured for use in cold climatic conditions because it is less viscous than diesel No.2. However, the long chain hydrocarbons from C16-C24 present in diesel No.2 aid in engine lubrication and are therefore essential for the welfare of motor vehicles. Diesel No.1 is often sold in warm climate states or cities when a local refinery uses its excess kerosene to blend with more expensive diesel No.2.

Heavy Oils. Because of the method employed in their production, heavy fuel oils fall into two broad classes: distillates and residuals. The distillates consist of distilled products, and residual fuel oils are residues that remain after distillation or cracking of crude oil to produce the middle-distillates.

Distillates. Fuel oils No.1 and No.2 are distillate fuels with composition and properties essentially equivalent to diesel fuels No. 1 and No. 2, respectively. Grades No. 4 to 6 fuel oils are residual oils.

Residual Fuels. Fuel oil No.4 is a light residual. It is intended for use in burners equipped with devices that atomize oils of higher viscosity than domestic burners can handle. Fuel oil No.5 is a residual fuel of intermediate viscosity. It may require preheating in some types of equipment for burning under cold climatic conditions. Fuel oil No.6 sometimes referred to as Bunker C, is a high viscosity oil used mostly as a boiler fuel and in commercial and industrial heating. It requires preheating in storage tanks to permit pumping and additional preheating at the burner to permit atomizing. Bunker C spans the hydrocarbon range from C9 to approximately C36 and a boiling range of 340 to 1060° F. The gross composition of a typical Bunker C oil is:

Hydrocarbon type Volume %
Paraffins 14
Naphthenes 7
Olefins -
Aromatics 34
Residuals 45

In addition, residual oils exhibit a wider range and greater concentrations of PAH than distilled products. A comparison of the relative contents of alkylbenzenes and PAH hydrocarbons in gasoline, Diesel No. 2, and Bunker C oil is presented in Table A-2 (Adapted from I.R. Kaplan, et al., 1995).

Table A-2: Relative Concentrations of Alkylbenzenes and PAH Hydrocarbons in Gasoline, Diesel, and Bunker C Reference Fuels
No. m/z Compound Gasoline Diesel Fuel Bunker C Oil
Relative %
1 120 C3-alkylbenzenes 57.6 1.2 0.3
2 134 C4-alkylbenzenes 20.4 2.1 0.8
3 148 C5-alkylbenzenes 4.3 1.6 0.9
4 162 C6-alkylbenzenes 0.6 1.0 0.3
5 128 C0-naphthalene 11.2 0.7 1.6
6 142 C1-naphthalenes 5.5 5.6 5.8
7 156 C2-naphthalenes 0.2 24.6 13.6
8 170 C3-naphthalenes 0.1 26.1 9.4
9 184 C4-naphthalenes 11.8 3.0
10 166 C0-fluorene 0.8 0.5
11 180 C1-fluorenes 2.1 1.2
12 194 C2-fluorenes 3.0 1.8
13 208 C3-fluorenes 0.4 1.4
14 222 C4-fluorenes 0.7

Next: Identifying Hydrocarbons

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