Fossil fuels are fuels formed by natural processes such as anaerobic decomposition of buried dead organisms.The age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years.Fossil fuels contain high percentages of carbon and include coal, petroleum and natural gas. Other more commonly used derivatives of fossil fuels include kerosene and propane.lthough fossil fuels are continually being formed via natural processes, they are generally considered to be non-renewable resources because they take millions of years to form and the known viable reserves are being depleted much faster than new ones are being made.