Fossil fuels consist of deposits of once living organisms. The organic matter takes centuries to form. Fossil fuels principally consist of carbon and hydrogen bonds. There are three types of fossil fuels which can all be used for energy provision; coal, oil and natural gas. Coal is a solid fossil fuel formed over millions of years by decay of land vegetation. When layers are compacted and heated over time, deposits are turned into coal. Coal is quite abundant compared to the other two fossil fuels. Analysts sometimes predict that worldwide coal use will increase as oil supplies become scarcer. Current coal supplies could last for 200 years or more.