Petroleum
Petroleum (crude oil) was formed from the remains of animals and plants (algae) that lived millions of years ago in a marine (water) environment before the dinosaurs. Over millions of years, the remains of these animals and plants were covered by layers of sand and silt. Heat and pressure from these layers helped the remains turn into what we today call crude oil. The word "petroleum" means "rock oil" or "oil from the earth".

After crude oil is removed from the ground, it is sent to a refinery by pipeline, ship, or barge. At a refinery, different parts of the crude oil are separated into useable petroleum products. Crude oil is measured in barrels (abbreviated "bbls"). A 42-U.S. gallon barrel of crude oil provides slightly more than 44 gallons of petroleum products.

In 2008, about 37% of energy used in the United States came from petroleum when the United States used 19,498,000 barrels/day. The amount of key petroleum products consumed in 2008 is shown in the table below.

An overview of how petroleum is refined into these various products can be found at "How Stuff Works".

Natural Gas
Natural gas is found in reservoirs deep in the earth. Natural gas, like coal and petroleum, is believed to have been formed over a period of millions of years from decayed and compressed organic matter. Natural gas is composed chiefly of a mixture of gaseous hydrocarbons (compounds of hydrogen and carbon). The exact makeup of natural gas varies from region to region. The major ingredient in most natural gas is a hydrocarbon called methane. Other substances found in natural gas include hydrogen sulfide, nitrogen, carbon dioxide, water, and helium. Natural gas containing hydrogen sulfide generally has a strong, disagreeable odor and is known as sour gas.

Before natural gas is supplied to consumers, any hydrogen sulfide and carbon dioxide it contains is removed. The resulting gas is odorless. To enable the consumer to detect gas leaks, strong-smelling substances, called thiols, or mercaptans, are added to the gas.

In 2008, about 24% of energy used in the United States came from natural gas when the United States used 23.8 trillion cubic feet (Tcf) of natural gas.

The major consumers of natural gas in the United States in 2008 included:

• Electric power sector - 6.7 trillion cubic feet (Tcf)
• Industrial sector - 7.9 Tcf
• Residential sector - 4.9 Tcf
• Commercial sector - 3.1 Tcf
  Source: Energy Information Administration

An overview of how natural gas is produced and used can be found at "How Stuff Works".

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