Fossil fuel formation

Fossil fuels are carbon-based energy sources formed over millions of years from the remains of ancient living organisms. The three main types are coal, oil, and natural gas. They are considered 'energy-dense' because a small amount of fuel releases a large amount of energy when burned (combusted). Because they take millions of years to form and we are using them much faster than they are created, they are classified as a non-renewable resource.

Coal is a solid fossil fuel formed from the remains of land-based plants, like trees and ferns, that lived in vast swamp forests around 300 million years ago. When these plants died, they fell into the swampy water and did not fully decompose due to a lack of oxygen. This created a thick layer of partially decayed organic matter called peat. Over millions of years, this peat was buried deeper and deeper under layers of sediment (like sand and mud). The immense heat and pressure from these overlying layers compressed the peat, squeezing out water and gradually transforming it into coal.

Oil and natural gas were formed in a similar way to each other, but from different original material than coal. They originated from vast numbers of tiny marine organisms, such as plankton and algae, that lived millions of years ago. When these organisms died, they sank to the bottom of the sea and were buried by layers of silt and sand. In these anaerobic (oxygen-free) conditions, the organic material was subjected to intense heat and pressure. This 'cooked' the material, transforming it into a waxy substance called kerogen, which then broke down further to form liquid oil and natural gas. Being less dense than the surrounding rock, the oil and gas migrated upwards through porous rock layers until they were trapped by a layer of impermeable (non-porous) rock, forming a reservoir.

Extracting fossil fuels requires accessing deposits deep underground. For coal, two main methods are used. Surface mining (or opencast mining) is used when the coal seam is close to the surface; the overlying rock and soil (overburden) is removed to expose the coal. For deeper seams, deep shaft mining is used, where vertical shafts and horizontal tunnels are dug to reach and extract the coal. Oil and natural gas are extracted by drilling. A long, vertical well is drilled down through layers of rock to reach the reservoir. The pressure in the reservoir often forces the oil and gas to the surface initially. A controversial modern technique called hydraulic fracturing, or 'fracking', involves pumping water, sand, and chemicals at high pressure to fracture shale rock and release trapped gas and oil.

While fossil fuels are a valuable energy source, burning them has significant environmental consequences. The combustion process releases gases into the atmosphere. The most significant is carbon dioxide (CO2), a greenhouse gas. An increased concentration of CO2 in the atmosphere traps more heat, leading to the enhanced greenhouse effect and contributing to global climate change. Additionally, many fossil fuels, especially coal, contain sulfur impurities. When burned, this sulfur reacts with oxygen to form sulfur dioxide (SO2). In the atmosphere, SO2 dissolves in water droplets in clouds to form sulfuric acid, which then falls as acid rain, damaging buildings, forests, and aquatic ecosystems.

The extraction and transport of oil carry the risk of major pollution, most notably from oil spills caused by tanker accidents or drilling rig failures. Oil spills have devastating effects on marine ecosystems. The oil coats the feathers of seabirds, destroying their waterproofing and insulation, leading to death from cold or drowning. It can block sunlight, killing phytoplankton and coral reefs below. It also poisons filter-feeding organisms like mussels and clams. Cleaning up oil spills is difficult and expensive. Methods include: using floating booms to contain the spill; using skimmers to remove oil from the surface; applying chemical dispersants to break up the slick into smaller droplets; and bioremediation, which uses microorganisms to break down the oil.