Click to see the molecular structures of methane, ethane, benzene, and butane.

Natural gas is petroleum gas as distinguished from manufactured gas. Natural gas consists of lighter paraffin hydrocarbons (hydrocarbons of the methane series), the most abundant being methane gas (CH4).

North Sea: An example of
a lighter crude oil.

Semi-, and solid forms are called heavy hydrocarbons and bitumens. They comprise materials such as asphalt, tar, pitch, albertite, or any number of names depending on their individual characteristics and local usage.

NATURE OF PETROLEUM

The basic components of petroleum and natural gases are mixtures of hydrocarbons, carbon and hydrogen being the only elements essential to their composition. A great number of hydrocarbons occur in nature while even more organic and synthetic hydrocarbons are routinely prepared in the laboratory and in chemical plants. The components of crude oil can be classified into two hydrocarbon series: methane series and the naphthene series.

Crude oil belonging to the methane series is also referred to as paraffin. Paraffins are straight-chain hydrocarbons having a general formula of CnH2n+2. The four principle types of paraffins are methane (CH4), ethane (C₂H6), propane (C3H8), and butane (C4H10) with each heavier molecule adding one CH₂ molecule to the preceding compound.

Australia: An example of
a very light crude oil.

Another type of crude oil is that belonging to the naphthene series. It contains carbon ring compounds having the general formula CnH2n. These are called cycloparaffins with the most common being cyclopropane (C3H6) and cyclobutane (C4H8). However, the molecular structures for this group of hydrocarbons can contain long, branched chains with complex rings, such as C30H50OH.

Of the two types of crude oil, paraffin or methane series oil is the most prized, but it comprises less than 2% of the total world supplies. The "Industry Standard" is "Pennsylvania Crude" derived from oil wells in the state of Pennsylvania. Crudes dominated by naphthenic components are called asphalt-based oils. About 15% of the total world supply of crude is this type; it is also called the black oil. Most of these supplies are

Utah: An example of petroleum
in solid state
at room temperature.

derived from Mexico and Venezuela, California, Gulf Coast region, and Russia. However, the vast majority of crude oils are a mixed base of some combination of paraffin and naphthene with the principal source for these crudes being the Middle East, Mid-Continental (USA), and the North Sea.

Natural Gas consists of hydrocarbons not condensable at atmospheric temperatures (68^oF) (20^oC) and atmospheric pressure. Natural gas comprises the first four members of the paraffin series. If the natural gas is comprised almost entirely of methane, it is referred to as dry gas. But if the ethane content in methane exceeds 5%, it is referred to as a wet gas.

There are three (3) distinct types of natural gas based upon their origin:

1. Petroleum gas - formed as a natural by-product during the generation of petroleum. It is often dissolved within the liquid hydrocarbon or in a free gas phase associated with the oil pool.

   Click for an illustration of "free gas phase."

2. Coal gas - formed by the modification of coal through the introduction of heat, pressure or other natural processes. Coal gas is the source of most of world's supply of commercial natural gas. And,
3. Bacterial gas - forms during the low temperature alteration of organic matter at or near earth's surface.

Petroleum, a mixture of carbon, hydrogen and oxygen (called hydrocarbons), is seldom found in nature in a pure form without impurities. Of the three major impurities, sulfur, nitrogen, and oxygen, sulfur is seldom absent from crude oil and can be found even in minute amounts. However, crude oil seldom has surfur concentrations greater than 3% with the higher concentrations restricted to heavy crude. Some sulfur may be in elemental form in solution or as H₂S (hydrogen sulfide). Crude containing hydrogen sulfide (H₂S) is referred to as sour crude. High sulfur crude oil costs more to refine which means that future oil supply costs will increase the price paid for gasoline at the pump. Moreover, compounding this problem, most refineries are not able to process the higher sulfur crude oil without major refinery modifications. Sweet crude is crude oil that contains less than 0.1% sulfur but such deposits have declined significantly since 1960's. Sour gas refers to natural gas containing hydrogen sulfide (H₂S). Similar restrictions apply to this type of natural gas as well. The H₂S must be extracted during processing of gas before the gas can be utilized by both industrial and domestic users. Sour gas is typically found in carbonate reservoirs; concentrations greater than 100 ppm are considered destructive because of the corrosive nature of H₂S .

Nitrogen (N) is another major impurity in petroleum, though it is related primarily to asphalt content. Concentrations greater than 0.02% N are considered high, but in petroleum removed from some areas of the world such as in parts of Europe, it can exceed 90%. Nitrogen must also be removed during refining process; this requires low temperatures. The nitrogen can be recovered, and often is, and used as an injectant in the secondary recovery of light oils (see Hide and Seek - Exploration: How Oil and Gas are Found).

Oxygen is the third major impurity found in petroleum. Oxygen compounds in crude oil form acids while such compounds in natural gas are in the form of carbon dioxide (CO₂). The latter is, in itself, a natural gas, but it is non-combustible. Concentrations of oxygen vary widely: Some gas fields in the United States (Utah, Wyoming) for example, contain greater than 90% CO₂. Excess oxygen must be removed during processing and refining.

Another major impurity found in petroleum is heavy metals. These are primarily Vanadium and Nickel and are the most peculiar constituents of crude oil. Vanadium, a high strength alloy in steel, is the most common and is found in concentrations typically varying from 30 - 1100 ppm. Venezuela crude contains more vanadium (by volume) in its crude oil than total world demand for the alloy. Burning the crude oil yields a vanadium-rich ash. Vanadium is generally associated with high sulfur content (H₂S) petroleum.

Nickel, the second heavy metal impurity, is generally associated with crude that is low in sulfur content. Both impurities, vanadium and nickel, must be removed during the refining process.

The physical properties of petroleum are controlled by the chemical composition. The most readily observed parameters are: specific gravity, viscosity, and color.

Specific Gravity is defined as the ratio between weight of a given volume of material and weight of an equal volume of water at 4^o C. Oils generally lie between 0.73 - ~>1.0 with paraffin-based oils being commonly light. Conversely, asphalt-based (naphthenic components) oils are almost always heavy. The units of measure are degrees and are read directly from a hydrometer. The symbol for specific gravity is the Greek letter (rho). The petroleum industry uses an API (American Petroleum Institute) scale to grade crude oil. The relationship between specific gravity, density, and traditional oil industry volumetric measurements are as follows:

In the table above, notice that as the API values increase (top row), specific gravity decreases, but the volume of product per long ton increases. Why is this important? Because, more product per long ton generates more revenue for the same fixed cost of extracting the product from the earth.

An API value greater than 30^o means oils are considered light, API values between 30^o and 22^o mean an oil is considered to be of medium weight, while a value of below 22^o implies the hydrocarbons are heavy crudes. API values below 10^o API are called extra-heavy crude while API values above 50^o are not really oils but rather condensates or distillates. The most favored grade of crude oil is about 37^o API which is common in the Middle East, Mid-Continent & Appalachian provinces (USA), Libya, Alberta (Canada), and the North Sea. Water under STP (Standard Temperature, Pressure) conditions has an API value of 10^o.

Viscosity is defined as the internal friction of a liquid causing its resistance to change form; it is the ratio of stress to shear per unit time. Shear in liquids is not constant but is proportional to time. The symbol is the Greek letter (eta). Viscosity varies directly with density; it is a function of the number of carbon (C) atoms and the amount of gas dissolved in the oil. As the gas content increases, viscosity decreases. A high viscosity means less fluid (flows like cold syrup) while a low viscosity means more fluid (flows like warm motor oil). This parameter is important when transporting petroleum products through pipelines.