Fossil fuels are formed when organic matter that has been buried deep within the earth are subject to heat and pressure over millions of years.
are formed when organic matter that has been buried deep within the earth are subject to heat and pressure over millions of years.
The image below shows how oil and natural gas form:
This image shows how coal forms:
In both cases heat and pressure are critical to the formation of the fossil fuel. In the case of oil and natural gas the organic material is marine in origin whereas coal is formed from ancient peat forests. In both cases the dead organisms are buried over time and the extreme heat and pressure converts these dead organisms into either coal natural gas or oil.