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Coal, oil, and natural gas are called fossil fuels because they are the fossilized remains of ancient photosynthetic organisms that converted the radiant energy in sunlight to chemical energy. That energy is stored in the form of molecular chains made up of hydrogen and carbon. Burning these fuels breaks up those molecular chains and liberates smaller molecules, including water and carbon dioxide, along with pollutants and ash particles known as black carbon. By burning them, the stored energy is released, primarily in the form of heat.

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As the leaves, branches, and trunks of ancient plants accumulated in freshwater swamps some 100 to 300 million years ago, they formed organic-rich bottom sediments (Figure 9.1a). Inorganic material, including clays and sand, were then deposited in these ancient swamps, first mixing with and then burying the organic-rich layer. Over the course of millions of years, the rock and soil overlying the swamp sediments increased to great depths (Figure 9.1b). With the increasing pressure and heat that resulted from the thick accumulation of rock and soil, the organic-rich material that began as buried swamp sediment was gradually converted into the sedimentary rock that we call coal (Figure 9.1c).

Geologists classify coal into four major grades, which differ in carbon and energy content (Figure 9.2). The differences among coal grades are mainly the result of variations in their age and the amount of heat and pressure to which they were exposed during development. Lignite, the youngest of the coal grades with the lowest carbon and energy content, was subjected to less heat and pressure during development than were the other grades. The other coal grades, listed in order of increasing carbon content, are sub-bituminous coal, bituminous coal, and anthracite. Energy content increases from lignite to bituminous coal, then the increase in energy drops slightly from bituminous coal to anthracite.

Humans have used coal as a source of heating since prehistoric times. Native Americans once burned it in their pottery kilns, and the steamships and railroads of the Industrial Revolution were powered by coal boilers. Today, coal is most commonly used to generate electricity. For example, more than 90% of coal mined in the United States is used in coal-fired electrical power stations. Coal also runs various industrial processes, such as the manufacture of paper or cement. Coal serves as a raw material to manufacture plastics, tar, fertilizer, and medicines. When baked in a hot furnace, coal is transformed to coke, a key material used in the manufacture of steel.

In 2008 energy experts estimated that about 93% of the world’s known coal reserves occur in three regions of the world: Northern Eurasia, Asia Pacific, and North America. Meanwhile, Africa, South and Central America, and the Middle East collectively are estimated to hold just 7% of world reserves. Closer analysis shows that proven coal reserves are even more restricted geographically. Just nine nations harbor more than 90% of the world’s coal reserves (Figure 9.3). At 28% of the world total, the coal reserves in the United States exceed all others.

Petroleum, or crude oil, formed in the oceans from the accumulated remains of algae and zooplankton deposited on the sea floor over millions of years (Figure 9.4). These deposits mixed with sands and silt, and eventually this organic-rich sediment was buried under deep layers of sedimentary rock. Trapped under a cap of heavy rock, the developing petroleum was subjected to increasing pressures and heat, which gradually transformed the organic materials into a waxy substance called kerogen. With increasing heat and temperature applied over the course of millions of years, kerogen was converted to crude oil.

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Chemically speaking, crude oil is a mixture of hydrocarbons, molecular chains consisting of only carbon and hydrogen. The smallest hydrocarbon, made up of four hydrogen atoms bonded to a single carbon atom, is methane (CH₄). To be useful and safe, crude oil must be refined, a process that involves separating the hydrocarbons into fractions containing hydrocarbons with the same number of carbon atoms.

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The main principle behind the refining of oil is that different sizes of hydrocarbons have different molecular weights and will boil (or condense) at different temperatures. To separate these hydrocarbons, heated crude is pumped into the refinery column, as shown in Figure 9.5. Because the temperature within the column decreases from bottom to top, the heaviest hydrocarbons (e.g., heating oil) condense and flow out in the lower layers. Meanwhile, the lightest hydrocarbons (e.g., methane) rise to the very top of the column, where they are collected. These gases are typically pressurized until they turn to liquid, resulting in liquefied petroleum gas, or LPG.

Petroleum ends up in our cars and trucks in its most familiar forms as gasoline (petrol) and diesel fuel. However, various oil products are in high demand to lubricate engines, tar our roads, or heat our homes (Figure 9.5). For many of these applications, alternatives do not work as well or are currently more costly.

The Middle East has oil reserves that account for over half of the world total and dwarf those of all other regions. After the Middle East, South and Central America, North America, Africa, and Northern Eurasia, each control from approximately 7% to 16% of global reserves. Within the world’s regions, just 15 countries control more than 90% of the world’s known oil reserves (Figure 9.6).

Natural gas is a mixture of gaseous hydrocarbons—primarily methane, but also ethane, propane, and butane (Figure 9.7). Natural gas forms in both petroleum deposits and coal beds. The long hydrocarbons in petroleum break down into shorter hydrocarbons of natural gas at temperatures above 100°C. These volatile gases then migrate up through porous rocks until they reach a nonporous impermeable rock, called a cap rock (Figure 9.8). Significant deposits of natural gases occur, especially where dome-shaped cap rocks trap a reservoir of gas overlying crude oil deposits. If drilling ruptures such cap rock, the natural gas under it flows to the surface under pressure. In coal beds, natural gas, mainly methane, is formed from coal either under heat and pressure (as in crude oil) or by methane-producing bacteria operating at lower temperatures.

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Natural gas associated with oil deposits and coal beds was once considered a hazard, because it could explode during drilling, and a waste product, because it was difficult to store and transport. Now, however, natural gas is used widely in residential, commercial, and industrial settings (Figure 9.9), where it accounts for about one-fourth of the energy used in the United States. As was the case with oil and coal, natural gas is unevenly distributed around the world. Just three countries, Russia, Iran, and Qatar, control over 50% of the known reserves of natural gas (Figure 9.10).

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