Oil and natural gas chemistry

The dominant chemical components of crude oil, or petroleum, are carbon and hydrogen; it also contains smaller quantities of nitrogen, oxygen, and sulfur. Oils consist of hundreds of individual chemical compounds. The dominant component of natural gas is methane, with smaller quantities of ethane, propane, and butane. Some natural gas deposits contain inorganic impurities, such as carbon dioxide, nitrogen, or hydrogen sulfide.

Background

Oil (petroleum) and are two of the most important sources of energy in the world. They can be classed as hydrocarbon fuels, since the dominant chemical compounds in each contain only hydrogen and carbon. They are also classed as fossil fuels, since they derive from once-living organisms. Crude oil, or petroleum, is a liquid of variable characteristics, usually having color ranging from light amber to black, of moderate to high viscosity, and less dense than water. Natural gas is a colorless gas, usually odorless unless contaminated with sulfur compounds.

Kerogen Formation

Most oil and natural gas deposits derive ultimately from plankton and algae. When these aquatic organisms die, their remains can be kept from complete decomposition if they accumulate in an anaerobic environment (an environment without oxygen). For example, they may accumulate on the bottom of a lake or lagoon and be covered by silt or mud. The remains are partially degraded by anaerobic bacteria, which rapidly decompose proteins and less slowly attack fats and oils (lipids). Other components of the organisms may resist bacterial attack. The partially altered remains of these organisms collect and are compacted into materials called kerogens. The principal kerogen precursors to oil and natural gas are algal (type I) kerogen, which are derived primarily from algae, and liptinitic (type II) kerogen, which are derived from plankton and algae. These kerogens consist primarily of carbon and hydrogen, with small amounts of oxygen, nitrogen, and sulfur. The conversion of remains of organisms to kerogens is called diagenesis, or the biochemical phase of fuel formation.

When the kerogen is buried more deeply in the Earth, its temperature may rise to a point at which thermal reactions begin to take place. These reactions involve the heat-induced breakdown of the kerogen; as they proceed, the large, complex hydrocarbon molecules of the kerogen eventually reach a point at which some of the molecules appear as a liquid. This process represents the onset of oil generation. The process of actual formation of oil and gas from kerogen is called catagenesis, maturation, or the geochemical phase.

Catagenesis

Early in catagenesis, some of the oxygen-containing molecules, such as alcohols, fats and oils, and organic acids, may be partially broken down to form carbon dioxide or water. The carbon dioxide and water escape, thereby reducing the oxygen content of the organic material remaining behind. As molecules are broken apart, their fragments are stabilized by hydrogen atoms that are picked up from other molecules in the system. This internal transfer of hydrogen generates a family of compounds that are generally hydrogen-rich and that dominate the composition of the products as well as a second family of compounds that are low in hydrogen. The hydrogen-rich compounds are the paraffins (or alkanes) and naphthenes (cycloalkanes), while the hydrogen-poor compounds are olefins (alkenes) and aromatic compounds. At this stage of maturation, many of the sulfur-containing compounds have not yet broken down. The oils formed in the early stages of maturation could therefore contain dissolved aromatic compounds and potentially have a high sulfur content. Because the paraffin molecules are still fairly large, the oils may be waxy and of high viscosity.

As maturation continues, the size of paraffin molecules continues to decrease. The viscosity of the oil drops. The sulfur compounds may begin to break apart, though the hydrogen sulfide that forms from the breakdown of sulfur compounds might remain dissolved in the oil. The continuing stabilization of molecular fragments requires more and more internal shuttling of hydrogen, and as a result larger molecules of aromatic compounds form. A point may be reached at which these big aromatic molecules are no longer soluble in the oil, and they precipitate as a separate material. The precipitated materials are called asphaltenes or asphaltites, and may be solids or highly viscous semisolid materials.

Further breakdown of paraffin molecules may reach a point at which some of the molecules are small enough to be in a vapor phase. Depending on the temperature, these molecules might contain up to about eight carbon atoms (those with eight carbon atoms are “octane” molecules). The formation of the vapor phase represents the onset of gas formation. As maturation continues, the relative proportions of gas and oil change, favoring gas. At high temperatures or extensive maturation, only gas will form. At these conditions, the gas contains small paraffin molecules: methane, ethane, propane, and butane. The gas may also contain various inorganic components, including carbon dioxide, water vapor, nitrogen, helium, and hydrogen sulfide. Extensive catagenesis could produce a gas that is almost pure methane.

Classification Systems

Several classification systems are used for oils. One is based on the three major classes of hydrocarbon components: paraffins, naphthenes, and aromatics. Depending on the proportions of each, oils are classified as paraffinic, paraffinic-naphthenic, naphthenic, aromatic-intermediate, aromatic-naphthenic, or aromatic-asphaltic. Paraffinic crudes are usually the most desirable for refinery feedstocks, and aromatic-asphaltic are the least desirable. Oils are also classified in terms of their geological age and depth of burial of the kerogen. Young-shallow oils have had little time to mature and have not been exposed to high temperatures. These oils can be viscous and contain relatively high contents of aromatics and sulfur. Old-deep oils have seen high temperatures and had long burial times; thus, they have experienced the greatest extent of maturation. Old-deep crudes are likely to be paraffinic, rich in relatively low-boiling compounds, and low in sulfur content. They are ideal refinery feeds. Young-deep and old-shallow crudes are intermediate classifications. Some of the best quality old-deep oils in the United States were first found in Pennsylvania. These oils are low-viscosity, low-sulfur, paraffinic oils. The term “Pennsylvania crude” is used as a classification term for oils of such quality.

Nitrogen, sulfur, and oxygen compounds in oils are sometimes lumped together and abbreviated NSOs. The major concern regarding NSOs is their impact on the environment if they are not removed from the oil during refining. Combustion of nitrogen- and sulfur-containing compounds produces the oxides of these elements, which, if emitted to the air, can result in serious air pollution. Oils that are high in NSOs will require more extensive refining for the products to comply with environmental regulations. Oils that contain sulfur compounds or dissolved hydrogen sulfide are said to be “sour.” In contrast, low-sulfur oils are “sweet.”

Depending on the temperature at which gas is confined underground, it may contain vapors of some compounds that would be liquids at ordinary temperatures (these compounds include pentane, hexane, heptane, and octane). When the gas is brought to the surface, where temperatures are lower, these vapors condense to a product called natural gasoline. In addition, the gas may contain appreciable amounts of butane and propane, which are relatively easy to condense if the gas is cooled further. Butane and propane may be separated and sold as separate fuel gases or combined as liquefied petroleum gas (LPG); they may also be sold as chemical feedstocks. A gas that contains more than 0.04 liter of condensable products per cubic meter of gas is said to be “wet.” If the condensable liquids are less than 0.013 liter/cubic meter, the gas is “dry.” Gases that contain hydrogen sulfide are sour, whereas sweet gases do not have this component. A sour gas is undesirable for several reasons: Hydrogen sulfide has a dreadful odor, it is a mild acid and can be corrosive to fuel-handling systems, and it produces sulfur oxides when the gas is burned. Unless a company can derive benefit from selling natural gasoline or LPG, the ideal gas would be a sweet, dry gas.

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