Fossil fuels are complex mixtures of hydrocarbons formed in a process lasting millions ofyears by decomposition ofbiological matter. Carbon and hydrogen are the main components, but other elements such as sulfur, nitrogen, oxygen and metals are also present in small amounts. At normal temperatures and pressures, these compounds may be gaseous, liquid or solid, depending on the complexity of their molecules.
All fossil fuels are derived from biological materials, either marine plankton or terrestrial vegetable matter. The formation of oil and natural gas was initiated in shallow ocean basins, under conditions that favored the growth of plankton and algae. When large numbers of dead marine organisms sank to the ocean floor and were subsequently covered by sediments, they were slowly transformed by processes that were partly due to the biological activity of anaerobic bacteria on the seafloor and partly chemical processes taking place at elevated temperatures and pressures. The sedimentary layer in which this transformation took place is generally referred to as source rock. Crude oil can only be formed within a precisely defined temperature and pressure window. If temperatures exceed a critical limit, the liquid hydrocarbon molecules break down to form natural gas. But also at low temperatures of up to about 80 °C, crude oil is often biologically degraded, over geological timescales, by micro-organisms that destroy hydrocarbons and other components to produce altered, denser heavy oils .
Due to their low density, freshly formed oil and natural gas deposits migrate upwards through porous strata. Eventually, a large proportion of the hydrocarbons that were formed in the geological past reaches the surface, is lost to the atmosphere and degraded in an aerobic environment. Only those fractions that were trapped beneath an impervious layer of sedimentary rock remain to form oil and gas reserves, captured within the microscopic pore spaces of so-called reservoir rocks.
Crude oils are liquid compounds composed mainly of saturated hydrocarbons. Sulfur is present in almost every crude oil at concentrations varying between less than 1 % and over 4 %. Further components include organic and inorganic nitrogen and oxygen species (both usually less than 0.5 %) and traces of sodium, potassium, vanadium and nickel . The physical properties of crude oils differ widely. These variations are mainly due to the different proportions ofvarious hydrocarbon compounds. Depending on their origin, oils may range from yellowish, mobile liquids to black, viscous semi-solids. Densities range from about 0.7 to 1.00 kg/l. Estimates suggest that about 9000 different crude oils can be found worldwide, of which only about 200 are traded.
Natural gas consists mainly of methane, but also contains higher hydrocarbons together with traces of nitrogen, carbon dioxide and hydrogen sulfide. The precise composition of natural gas varies greatly depending on the geological location and the history of the gas source. Natural gas may be found associated with crude oil as a gas-cap above the oil or on its own, unassociated with oil reserves. It can be derived directly from vegetable material, from the breakdown of crude oil at high temperatures, or from the decomposition of coal. This latter process results in a gas very rich in methane, containing only small quantities of other hydrocarbons.
Where terrestrial vegetable matter accumulates, peat is formed which, after burial, is transformed in various steps into lignite, bituminous coal and, eventually, anthracite. The coalification process is initiated by anaerobic bacteria and continues under the action of temperature and pressure over a period of millions of years. Upon maturation, the volatile gaseous and liquid hydrocarbons are progressively lost, leaving only solid carbonaceous compounds. Consequently, coals are solid compounds with a relatively high carbon and low hydrogen content.
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