To begin with, oil is not a fossil fuel. This is a theory put forth by 18th century scientists. Within 50 years, Germany and France's scientists had attacked the theory of petroleum's biological roots. In fact, oil is abiotic, not the product of long decayed biological matter. - Oil fields are depleting, and new sources of oil are harder to get at.-Therefore, the rate of this abiotic synthesis of hydrocarbons is likely much much smaller than the rate we are consuming fuel.-Therefore, the oil and gas we have consumed in the last century was likely produced over some much longer time period. Fossil fuel is a general term for buried combustible geologic deposits of organic materials, formed from decayed plants and animals that have been converted to crude oil, coal, natural gas, or heavy oils by exposure to heat and pressure in the earth's crust over hundreds of millions of years.