Petroleum and the equivalent term crude oil cover a wide assortment of materials consisting of mixtures of hydrocarbons and other compounds containing variable amounts of sulfur, nitrogen, and oxygen, which may vary widely in volatility, specific gravity, and viscosity. Metal-containing constituents, notably those compounds that contain vanadium and nickel, usually occur in the more viscous crude oils in amounts up to several thousand parts per million and can have serious consequences during processing of these feedstocks (Gruse and Stevens, 1960; Speight, 1984). Because petroleum is a mixture of widely varying constituents and proportions, its physical properties also vary widely and the color from colorless to black.
Petroleum occurs underground, at various pressures depending on the depth. Because of the pressure, it contains considerable natural gas in solution. Petroleum underground is much more fluid than it is on the surface and is generally mobile under reservoir conditions because the elevated temperatures (the geothermal gradient) in subterranean formations decrease the viscosity. Although the geothermal gradient varies from place to place, it is generally on the order of 25°C–30°C/km (15°F/1000 ft or 120°C/1000 ft, i.e., 0.015°C/ft of depth or 0.012°C/foot of depth).
Petroleum is derived from aquatic plants and animals that lived and died hundreds of millions of years ago. Their remains mixed with mud and sand in layered deposits that, over the millennia, were geologically transformed into sedimentary rock. Gradually the organic matter decomposed and eventually formed petroleum (or a related precursor), which migrated from the original source beds to more porous and permeable rocks, such as sandstone and siltstone, where it finally became entrapped. Such entrapped accumulations of petroleum are called reservoirs. A series of reservoirs within a common rock structure or a series of reservoirs in separate but neighboring formations is commonly referred to as an oil field. A group of fields is often found in a single geologic environment known as a sedimentary basin or province.
The major components of petroleum are hydrocarbons, compounds of hydrogen and carbon that display great variation in their molecular structure. The simplest hydrocarbons are a large group of chain-shaped molecules known as the paraffins. This broad series extends from methane, which forms natural gas, through liquids that are refined into gasoline, to crystalline waxes.
A series of ring-shaped hydrocarbons, known as the naphthenes, range from volatile liquids such as naphtha to high-molecular-weight substances isolated as the asphaltene fraction. Another group of ring-shaped hydrocarbons is known as the aromatics; the chief compound in this series is benzene, a popular raw material for making petrochemicals.
Nonhydrocarbon constituents of petroleum include organic derivatives of nitrogen, oxygen, sulfur, and the metals nickel and vanadium. Most of these impurities are removed during refining.
Geologic techniques can determine only the existence of rock formations that are favorable for oil deposits, not whether oil is actually there. Drilling is the only sure way to ascertain the presence of oil. With modern rotary equipment, wells can be drilled to depths of more than 30,000 ft (9,000 m). Once oil is found, it may be recovered (brought to the surface) by the pressure created by natural gas or water within the reservoir. Crude oil can also be brought to the surface by injecting water or steam into the reservoir to raise the pressure artificially, or by injecting such substances as carbon dioxide, polymers, and solvents to reduce crude oil viscosity. Thermal recovery methods are frequently used to enhance the production of heavy crude oils, whose extraction is impeded by viscous resistance to flow at reservoir temperatures.
Crude oil is transported to refineries by pipelines, which can often carry more than 500,000 barrels/day, or by ocean-going tankers. The basic refinery process is distillation, which separates the crude oil into fractions of differing volatility. After the distillation, other physical methods are employed to separate the mixtures, including absorption, adsorption, solvent extraction, and crystallization. After physical separation into such constituents as light and heavy naphtha, kerosene, and light and heavy gas oils, selected petroleum fractions may be subjected to conversion processes, such as thermal cracking (i.e., coking) and catalytic cracking. In the most general terms, cracking breaks the large molecules of heavier gas oils into the smaller molecules that form the lighter, more valuable naphtha fractions.
Reforming changes the structure of straight-chain paraffin molecules into branchedchain iso-paraffins and ring-shaped aromatics. The process is widely used to