Dear Student,
Fossil fuels require millions of years to form from the dead vegetation and animal that get buried deep inside the earth's surface.
The requirements for a fossil fuel to form are high temperatures and pressure which cannot be obtained in a laboratory.
The fossil fuels are not in abundance and are present in limited quantity in limited quantity beneath the earth and thus will come to an end one day. This is why fossil fuels are regarded as exhaustible natural resources.
Regards