The term fossil refers to an earlier geologic age. Fossil fuels were formed many years ago and are not renewable, existing in deposits of finite extent.

Coal

Millions of years ago, descomposed vegetation was covered with earth to varying depths; the heat and pressure transformed it from carbohydrates to carbon and hydrocarbons.

Initially, the material was converted to peat, then peat to lignite, lignite was later changed to subbituminous, bituminous and anthracite coal.

Petroleum 

Petroleum is a naturally liquid that consists principally of a mixture of various hydrocarbons, with small amounts of sulfur, nitrogen, oxygen, and ash. The percent of mass by elements is as follows:

Process to transform wood into petroleum.

While searching for alternative sources to obtain energy, scientists at the Pittsburg Energy Research Center, have made oil from wood, organic waste and dung.

50% of the petroleum demand in 1970 in the United States could be satisfy with this method because 2 000 000 000 tons of waste could produce 2 000 000 000 oil barrels.

Basically, the process implicates the reaction of CO with wood at 371C and 4000 psig. The following are common reactions

NaCO₃ + H₂ + 2CO -> 2HCOONa + CO₂

2C₆H₁₀O₅ + 2HCOONa -> 2C₆H₁₀O₄ + H₂O + CO₂ + Na₂CO₃