The original creation of oil or petroleum is not well understood. There are several theories, but the matter is still one of scientific controversy. It is generally accepted however that the origin of oil begins with plant fossils, just as with coal. The study of fossils is called paleontology. The creation of oil is part of geology.
Petroleum is a mixture of liquid hydrocarbons (chemical compounds containing only hydrogen and carbon) plus various impurities such as sulfur. Unprocessed petroleum is usually called crude oil, although it has been called mineral oil and Seneca oil, named for the Seneca Indians of Western Pennsylvania. The name petroleum is from a combination of Latin words meaning "rock oil". We refer to it here simply as oil.
As found in the earth, oil may have a variety of properties. Some forms are black, others dark green, and some light like kerosene. The liquid ranges from very viscous to easy-flowing. Crude oil usually consists of a mixture of hydrocarbons having varying molecular weights and differing from one another in structure and properties.
These various species are separated into groups, or fractions, by a process of distillation called refining. Oil fuel, in all of its usable forms, is a refined product, unlike coal and natural gas which can often be burned in their natural condition.
The initial search for oil is carried out mostly by geologists. Many techniques are used, especially the study of landforms and seismic analysis. Where oil is thought likely, test wells will be drilled. Many exploration companies are small, and are called "wildcats."
Oil occurs in certain geologic formations at varying depths in the earth's crust, and in many cases elaborate, expensive equipment is required to get it from there. The oil is usually found trapped in a layer of porous sandstone, which lies just beneath a dome-shaped or folded layer of some non-porous rock such as limestone. In other formations the oil is trapped at a fault, or break in the layers of the crust.
In the dome and folded formations natural gas is usually present just below the non-porous layer and immediately above the oil. Below the oil layer the sandstone is usually saturated with salt water. The oil is released from this formation by drilling a well and puncturing the limestone layer on either side of the limestone dome or fold. If the peak of the formation is tapped, only the gas is obtained. If the penetration is made too far from the center, only salt water is obtained. Since the formation may be several miles below the surface this is clearly a difficult business.
The oil in such formation is usually under such great pressure that it flows naturally, and sometimes with great force, from the well. However, in some cases this pressure later diminishes so that the oil must be pumped from the well. Natural gas or water is sometimes pumped into the well to replace the oil that is withdrawn. This is called "repressurizing" the oil well.
Oil wells may be either on land or under water. In North America many wells are "offshore" in the shallow parts of the oceans, especially in the Gulf of Mexico. The "crude" or unrefined oil is typically collected from individual wells by small pipelines.
Many scientists believe that natural gas was created from fossil plants along with coal, as well as where there were large organic deposits that did not become coal, such as at the mouths of rivers. Some, however, have argued that methane, the principle component of gas, was created along with the earth's crust. The study of fossils is called paleontology. The creation of natural gas is part of geology.
The chief deposits of "dry gas" (not associated with oil) are in Texas, Louisiana, Kansas, Oklahoma, Kentucky and West Virginia. Considering both "wet" and "dry" gas, the leading states are Texas, California, West Virginia, Louisiana, Kansas, Oklahoma, Pennsylvania, Kentucky and Ohio. Many other states have relatively large scattered supplies. So-called "proved reserves" in the US are around 160 trillion cubic feet of gas.
Natural gas typically flows from wells under its own pressure. It is collected by small pipelines that feed into the large gas transmission pipelines. In the US about 20 trillion cubic feet of gas is produced each year.
Most natural gas is processed very little to render it useable. Sometimes components called natural gas liquids, such as propane, are extracted from the gas before it is transported or delivered to the consumer. The local gas utility that delivers gas to customers is typically not the same as the pipeline company that collects the gas from the wells and brings the gas into the local area. The local utility may be a private firm or a municipal agency. In many cases they provide electricity as well as natural gas. Some supply propane as well.
Use of Natural Gas
As a fuel, natural gas is convenient and efficient. It is used primarily for heat, in industrial, commercial and residential settings. In many homes the house and water are heated by gas, the food is cooked with it and clothes dried. Gas is also used to produce electricity, in some cases using gas fired turbines that are similar to jet engines, in others to fire steam boilers. Small gas-fired generators, called micro turbines, have recently been introduced. Gas is often used to heat industrial and commercial boilers and pressure vessels.