Major sources of alkanes are:
- Petroleum. Petroleum is a complex mixture of alkanes and other hydrocarbons. It usually contains alkanes having up to forty carbon atoms in their molecule. The natural wax found near petroleum wells is a mixture of higher solid hydrocarbons.
- Natural gas. Methane is the major constituent of the natural gas and occurs along with petroleum in the earth’s sedimentary traps.
- Destructive distillation of wood and coal. Methane is produced as one of the by-products in the manufacture of wood charcoal by destructive distillation of wood and coke by destructive distillation of coal.
- Anaerobic decomposition of organic matter, in the presence of bacteria, liberates methane. For example, bio-gas produced by anaerobic decomposition of animal dung, excreta and plant wastes is rich in methane gas. Methane is produced in marshy lands by the bacterial decomposition of plant and animal matter under water and is observed bubbling through the surface. Since methane gas is produced in marshy lands, it is also known as marsh gas.
Decomposed wastes from rice fields and termites are other sources of methane.
Methane gas is also present in coal mines where it is known as damp fire. It is formed as a by-product during the formation of coal.