Conventional fossil fuels include coal, natural gas, and liquid petroleum that has accumulated in Earth’s crust. The geologic time scale over which fossil fuels form—hundreds of millions of years—lies outside human time horizons, so these resources are considered for practical purposes to be finite and exhaustible. Synthetic forms of petroleum and natural gas can also be fabricated. For example, synthetic gasoline or synthetic diesel can be made from coal or natural gas through processes known as coal-to-liquids and gas-to-liquids. In addition, anaerobic decomposition of organic matter such as food waste or manure from agricultural operations produces renewable natural gas (RNG) also called biogas.
Unconventional fossil fuels include nonliquid forms of petroleum, such as oil shale, shale oil, oil sands, tar sands, and heavy oils. Unconventional forms of natural gas include shale gas and coalbed methane. Oil shale is a form of solid kerogen rock found in Utah and Colorado that releases energy when burned. Shale oil, also known as tight oil, is the liquid produced from impermeable shales.
Fossil fuels gained significant market share in the 1860s with the Second Industrial Revolution. Despite the diversity of fuel options, fossil fuels—coal, petroleum, and natural gas—remain the dominant primary energy sources today and still provide approximately 85% of the world’s energy.
In addition to their role as fuels in combustion, coal, petroleum, and natural gas serve as feedstocks for manufacturing materials. This role is similar to how wood can serve as a fuel in the form of firewood or as a building material in the form of lumber and beams. Plastic, pesticides, pharmaceuticals, cosmetics, paints, dyes, and cleaners are formed from petroleum. Fertilizer, ink, glue, and paint are formed from natural gas. Steel and iron production and the cement making process incorporate heat and carbon from coal. Even some solid wastes from coal combustion, including bottom ash found at the bottom of coal boilers and fly ash that rises through the smokestack, can be used to make drywall for buildings and aggregate for roads.