Crude oil (petroleum) is a mixture of hydrocarbons formed from ancient organisms buried under intense heat and pressure over millions of years. It collects in underground pores and cracks, often capped by impenetrable rock.

Geologists locate deposits using seismic shock waves—large machines pound the ground and computers convert the reflected wave data into 3-D maps. Once a deposit is confirmed, wells are drilled and oil is pumped to the surface.

Crude oil must be processed at a refinery through a process called refining—heating the oil to separate it by boiling point into fuels (gasoline, diesel, jet fuel, heating oil) and other products. About 2% of refined products are petrochemicals, used to make plastics, medicines, and cosmetics. A less desirable by-product is petcoke, a black powder that can become an air pollutant.

After roughly a decade of pumping, well pressure drops and output begins to decline—this point is called peak production. Proven oil reserves are deposits profitably extractable at current prices. Technologies like horizontal drilling and hydraulic fracturing have expanded proven reserves by reaching previously inaccessible shale formations.

Natural gas is a fossil fuel composed mostly of methane (CH₄)—50–90%—with smaller amounts of propane, butane, and traces of toxic hydrogen sulfide. It is widely used for heating, cooking, electricity generation, and industrial processes.

Natural gas is the cleanest-burning fossil fuel—it produces less CO₂ and fewer air pollutants than oil or coal. In 2020 it supplied about 34% of U.S. energy.

Hydraulic fracturing (fracking) uses high-pressure pumps to inject water, sand, and chemicals into shale rock, fracturing it to release trapped gas. Fracking has dramatically expanded U.S. natural gas production, but raises serious concerns: water contamination (methane entering drinking water wells), air pollution from methane leaks, and increased seismic activity from injecting wastewater into deep disposal wells. Despite these risks, fracking is currently exempt from key laws including the Safe Drinking Water Act and Clean Water Act.

Natural gas can be liquefied (LNG) and shipped across oceans, but liquefying greatly reduces net energy and increases cost.

Coal is the world's most abundant fossil fuel, formed from plant matter buried under heat and pressure for 300–400 million years. Four main types exist, in order of increasing heat content and decreasing moisture: peat (not true coal, low heat), lignite (brown coal, low sulfur but limited supplies), bituminous (soft coal, widely used but typically high sulfur), and anthracite (hard coal, highest heat content and lowest sulfur—most desirable but rarest).

Coal is burned to generate electricity: crushed coal heats water → steam spins a turbine → the turbine drives a generator. It is also used industrially to make steel and cement.

Coal is the dirtiest fossil fuel. Burning coal accounts for 46% of global CO₂ emissions. It also releases sulfur dioxide (SO₂)—which reacts in the atmosphere to form acid rain that damages ecosystems and human health—and soot, a mixture of fine particles that collect deep in the lungs, causing breathing problems, heart attacks, and strokes. Coal ash contains toxins including arsenic, lead, mercury, cadmium, and radioactive radium.

Some plants install scrubbers to capture pollutants before they exit through the smokestack, but this reduces coal's net energy. Coal use in the U.S. has declined sharply as cheaper, cleaner natural gas has taken over electricity generation.