Oil chemistry

Crude oils are fossil organic chemicals that have been transformed by geologic processes into a complex mixture of many different chemical compounds called hydrocarbons. Although the composition of oils varies widely, the most abundant hydrocarbons in most oils are the paraffins, naphthenes, aromatics, and compounds with nitrogen, sulfur, and oxygen attached (NSOs). Less abundant compounds, called biomarkers, are true “geochemical fossils” that retain the original molecular structure and probable biological identity of the organisms from which the oil is derived.

Paraffin Hydrocarbons

The various hydrocarbon (organic matter) compounds found in petroleum differ from one another in two fundamental ways: the number of carbon and hydrogen atoms in the hydrocarbon molecule, and the shape of the molecule. Hydrocarbon molecules are classified according to the number of carbon atoms they contain. A simple numbering system is used in which C₂, for example, refers to a molecule with 2 carbon atoms, and C₃ is a molecule with 3 carbons. In most oils, compounds of very low molecular weight (less than C₅) are dissolved in the oil as natural gas. When the crude oil is pumped out of the ground, these molecules evaporate from the liquid petroleum and are either burned off or collected to be used as fuel.

Carbon atoms can be bonded to one another in straight chains, rings, or combinations of these basic forms. In many hydrocarbon molecules, small molecular fragments called side chains are attached like branches of a tree to the main chain or ring of the molecule. The simplest and most abundant petroleum hydrocarbons in crude oils are straight carbon chain molecules, referred to as the paraffin series. The smallest and simplest paraffin hydrocarbon molecule is methane, the most common component of natural gas. The methane molecule is composed of a central carbon atom bonded to four hydrogen atoms in a three-sided pyramid arrangement, or tetrahedron. The structure of paraffin molecules of higher molecular weight (C₂ to C₃₀) is made when additional carbon atoms are attached to the basic methane tetrahedron, making a carbon chain. As these carbon atoms are successively added, a chainlike arrangement of carbon atoms develops, with hydrogen atoms bonded to all the carbon atoms. This series of chainlike molecules is called the n-alkane series, with “n” denoting the number of carbon atoms in the chain. For example, the compound pentane, an abundant component of natural gas, is a five-carbon alkane chain (C₅H₁₂). Octane, the hydrocarbon compound by which gasoline is graded, is an eight-carbon alkane (C₈H₁₈). The highest molecular weight n-alkane found as a liquid in oil is heptadecane (C₁₇H₃₆), a hydrocarbon with a boiling point of 303 degrees Celsius.

All carbon atoms have four positions, called bonding sites, at which other atoms can attach themselves to form a molecule. In petroleum hydrocarbons, the carbon atoms usually are bonded to either hydrogen atoms or other carbon atoms, although some are bonded to nitrogen, sulfur, oxygen, or other atoms. Hydrocarbon molecules with all bonds joined to four other hydrogen or carbon atoms are called saturated hydrocarbons. For any saturated n-alkane in the paraffin series, the number of hydrogen atoms in the molecule (#H) can be predicted from the number of carbon atoms (n) in the chain by the simple equation #H = 2n + 2. Note that for all n-alkane hydrocarbons the carbon-to-hydrogen ratio is always less than 1:2.

Naphthenic Hydrocarbons

If the ends of a paraffin hydrocarbon chain are linked together to form a ring, the result is the shape of the other abundant group of hydrocarbon compounds in crude oils, the naphthene series. Naphthenic molecules are composed of carbon atoms bonded together in rings; molecules with this molecular geometry are usually referred to as cyclic hydrocarbons or ring compounds. The simplest naphthenic hydrocarbon is cyclopropane (C₃H₆), a three-carbon ring molecule that occurs as a gas dissolved in crude oil. Cyclopropane, like other hydrocarbons of low molecular weight, bubbles out of the oil solution when it reaches the low-pressure conditions of the earth’s surface in the same way that carbon dioxide gas bubbles out of a soft drink when the bottle is opened. In this way, it becomes part of the natural gas that is associated with crude oil production. The liquid naphthene with the lowest molecular weight is cyclopentane (C₅H₁₀); this compound and cyclohexane (C₆H₁₂) are the dominant cyclic hydrocarbons in most oils. Saturated naphthenic hydrocarbon molecules have hydrogen atoms bonded to all the carbon atom bonding sites in a manner similar to that of n-alkanes. In this case, two hydrogen atoms are bonded to each carbon, and the ratio of carbon to hydrogen for these compounds is 1:2.

Aromatic Hydrocarbons

Less abundant in oils than the paraffins and naphthenes are the aromatic hydrocarbons. This group of petroleum hydrocarbons, which constitutes from 1 to 10 percent of most crude oils, is so named because many of the compounds have pleasant, sometimes fruity, odors. The aromatics also have a carbon ring structure, but this structure has a different geometry from that of the naphthenes. Aromatic hydrocarbon molecules are formed of one or more six-carbon rings in which the carbon atoms are bonded to one another with alternating single and double bonds; that is, carbon atoms share two electrons with a neighbor. For this reason, the aromatic hydrocarbons have a carbon-to-hydrogen ratio of 1:1. The petroleum aromatic with the lowest molecular weight is benzene (C₆H₆), a chemical commonly used as an industrial solvent. Toluene, which consists of two rings joined along an edge, is the solvent that produces the familiar odor of correction fluid and many plastic cements.

Many of the aromatic hydrocarbons are cancer-causing (carcinogenic) substances; most potent are the high molecular weight, ringed aromatic molecules referred to as PAHs (poly-aromatic hydrocarbons). The first carcinogen ever discovered was benzopyrene, an aromatic molecule composed of five carbon rings. In the late 1800s, Sir Percival Pott linked this substance with cancer of the scrotum in London chimneysweeps. Their daily exposure to the aromatic hydrocarbons in coal tar and soot, coupled with their poor personal hygiene, was responsible for epidemic proportions of this disease.

NSOs

A small percentage of the hydrocarbons found in oils have distinctive molecular fragments bonded onto basic hydrocarbon structures. These nonhydrocarbon fragments most commonly contain nitrogen, sulfur, and oxygen; for this reason, hydrocarbon compounds with these attached fragments are called NSOs. NSO molecules tend to have much higher molecular weights than the hydrocarbon molecules described earlier. One of the most interesting NSO molecular fragments is the amino group; it contains nitrogen and has the formula NH₂. The amino group is the essential component of the amino acids, the building blocks for the many different proteins of which animal organs, muscles, and other tissues are composed. Amino acids are simple molecules that can form by organic or inorganic processes, but their presence in oil is best explained by inheritance from the organic matter source of the oil.

Biomarkers

The minor, or trace, hydrocarbon components in crude oil generally make up much less than 1 percent of the total oil; they are probably the most interesting of all petroleum compounds. Many of these chemicals have a chemical composition and molecular structure that definitely links their origin to specific organisms; for this reason, they are termed geochemical fossils or biological markers (biomarkers). Biomarker chemicals were synthesized by the organisms from which the oil originated and have been preserved through the long and complex history of sediment deposition and burial, oil formation (catagenesis), and migration of the oil from its source to the reservoir rock. Biomarkers are generally large molecules—molecules with much higher molecular weights than those of the more abundant oil hydrocarbons—that have a carbon-to-hydrogen ratio greater than 1:2.

Some of the long-chain paraffin hydrocarbons are among the best-known biomarkers. One relatively abundant group, the isoprenoid hydrocarbons, have chain-type molecules based on the isoprene group (C₅H₈); isoprene is the primary source of synthetic rubber. Isoprenoids are common in the waxes and chlorophyll of terrestrial green plants and are present in many crude oils and ancient sediments, which indicates that the petroleum isoprenoids are derivatives of chlorophyll, and that kerogen from terrestrial plants is a significant source of isoprenoid-rich oils. The most interesting plant-derived isoprenoid found in petroleum is pristane, a C₁₅ carbon chain with four CH₃ molecular fragments, called methyl groups, attached to the main paraffin chain as side chains. Phytane is an isoprenoid similar to pristane but is composed of a C₁₆ carbon chain with four methyl group side chains. The ratio of pristane to phytane is useful to geologists trying to determine the type of organic matter from which the oil was derived. Oil derived mostly from terrestrial plant tissue, for example, has a high pristane-to-phytane ratio (greater than 4:1), while oils from marine plankton have much lower ratios.

The study of the biomarker composition of oils has given geologists valuable insight into the origin of petroleum, and is used as a valuable tool in oil exploration as well. Every oil has a unique biomarker composition that it inherited from the kerogens that generated the oil and the conditions of catagenesis. Certain biomarker compounds, even when present in exceedingly minute quantities, can be detected with the mass spectrometer. This “geochemical fingerprint” allows petroleum geologists to recognize distinctive chemical similarities between oils and their source sediments. The geochemical fingerprinting technique is routinely used to distinguish different oils from one another, to correlate similar oils from different areas, and to demonstrate a similarity between kerogen-bearing source sediments and the oil that was generated from them. It is also used to increase oil-exploration efficiency by serving as a critical clue to the presence of undiscovered petroleum. The technique also has been used at the sites of oil spills and polluted groundwater, and in other instances of oil pollution to identify the source pollutant; it has been successfully used as evidence in courts of law to determine the guilty party and to calculate damages for episodes of oil pollution.

To determine the chemical composition of an oil, petroleum geochemists employ many different techniques, most of which are modifications of standard techniques of organic chemistry. Crude oils are composed of a vast number of individual chemical compounds that differ from one another in their molecular weight and shape, and in the distribution of electric charges on the outer portions of the molecules. These individual compounds are separated from one another by a technique called chromatography. Portions of the oil are slowly passed through a long glass or metal column packed with a chemical substance (usually an organic chemical) that attracts hydrocarbon molecules having certain size or charge characteristics. The attractive chemical inside this “chromatographic column” has a greater affinity for the heaviest or most highly charged molecules, so hydrocarbon molecules of different sizes and charges move through the column at different rates and emerge from the end of the column at different times. For example, the n-paraffins are separated from one another by their molecular weight, so the lightest of these molecules emerges first from the column, and the others come out in the order of their carbon number. It should be noted that no column exists that separates all petroleum hydrocarbons, and several different chromatic columns are needed to separate an oil into its constituent chemicals. At the end of the column, a detector that is sensitive to molecular weight, charge, or other characteristics measures the amounts of each hydrocarbon compound as it emerges from the column and graphs the results. As with chromatographic columns, no one detector is able to separate all petroleum hydrocarbon compounds, and several different types are used. One of the most valuable and interesting of these detectors is the mass spectrometer. This detector breaks a hydrocarbon molecule into fragments as it leaves the chromatographic column and measures the molecular weight of these fragments. Laboratory studies have confirmed the way in which various known hydrocarbon compounds are fragmented in a mass spectrometer; this information is stored in the memory of a computer connected to the detector. During the analysis of an unknown hydrocarbon compound in an oil sample, its fragments are recognized and “reassembled” by the computer to determine the composition of the original hydrocarbon.

Petroleum Products

Oil is a basic raw material, from which many thousands of products are made. The types and quantity of petroleum products obtained by refining crude oil are determined by its bulk chemical composition. For example, paraffin-rich oils with low aromatic content usually yield the highest quantity of hydrocarbon fuels, while highly aromatic oils refine into the best lubricating oils. Oils containing a large percentage of n-alkanes of low molecular weight (C₅ to C₁₃) yield high quantities of gasoline, kerosene, jet fuel, and expensive hydrocarbon solvents used in many manufacturing processes; oils rich in n-alkanes of higher molecular weight (C₁₄ to C₄₀) yield high quantities of diesel fuel and lubricating oils. Oils dominated by hydrocarbons of high molecular weight (heavy oils) yield asphalt and the hydrocarbons used to make plastics and synthetics.

Oils from different areas of the world can be divided into various types based on their bulk chemistry. Oils composed primarily of paraffins, termed paraffin-based crudes, are the most sought-after of all oil types by the refining industry but represent only a small percentage of the oil being produced worldwide. Most paraffin-based crude oils in North America are of Paleozoic age (about 600 to 250 million years ago) and are produced from oil fields in the midcontinent region. One of these is the famous Pennsylvania crude, which has historically been the standard against which all oils are compared. Similar oils on other continents are usually much younger. Paraffin-based oils of Mesozoic age (about 250 to 65 million years ago) are produced in Chile, Brazil, and the Caucasus region. Paraffin-based oils of Cenozoic age (the last 65 million years) are found in Africa, Borneo, and China.

Crude oils dominated by naphthenic hydrocarbons, sometimes called asphalt-based oils, are relatively rare. Significant production of naphthenic oils occurs in the Los Angeles-Ventura area of Southern California and some oil provinces of the US Gulf Coast, the North Sea, and South America. Highly aromatic oils are generally the heaviest and most viscous of all oil types; some are actually solids at surface temperatures, although they are generally liquid in the subsurface. Important deposits of this unusual oil type include the famous black oils of Venezuela, the very large Athabasca tar sand deposits of western Canada, and certain oils from West Africa. Oils of intermediate composition, called mixed-base oils, make up the bulk of worldwide petroleum production.

Principal Terms

aromatic hydrocarbons: ring-shaped molecules composed of six carbon atoms per ring; the carbon atoms are bonded to one another with alternating single and double bonds

biomarkers: chemicals found in oil with a chemical structure that definitely links their origin with specific organisms; also called geochemical fossils

hydrocarbons: natural chemical compounds composed of carbon and hydrogen, usually of organic origin; they make up the bulk of both petroleum and the tissues of organisms (plants and animals)

molecular weight: a measure of the mass of the molecule of a chemical compound, as determined by both the total number and the size of atoms in the molecule

naphthenic hydrocarbons: hydrocarbon molecules with a ring-shaped structure, in which any number of carbon atoms are all bonded to one another with single bonds

oxidation: a very common chemical reaction in which elements are combined with oxygen—for example, the burning of petroleum, wood, and coal; the rusting of metallic iron; and the metabolic respiration of organisms

paraffin hydrocarbons: hydrocarbon compounds composed of carbon atoms connected with single bonds into straight chains; also known as n-alkanes

saturated hydrocarbons: hydrocarbon compounds whose molecules are chemically stable, with carbon atoms fully bonded to other atoms

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