Fossil Fuels

Coal, petroleum and natural gas are the only three fossil fuels, and everything else described as one is either a product refined from them or is not a fossil fuel at all. That single fact answers most classification questions, and the test behind it is simple: did this substance form from organisms buried underground over millions of years?

The section covers how they formed, why they are classed as exhaustible, why energy from them is not green energy however cleanly it burns, and the honest counterweight — why they still supply roughly 80% of world primary energy. That last part matters. Fossil fuels dominate because of energy density, storability and dispatchability, not merely inertia, and understanding those properties is what turns 'why don't we just stop' into a question with a real answer.

There is also a distinction here that carries more weight than it first appears: classification follows the origin of the carbon, not the molecule. Biogas and natural gas are chemically identical methane with entirely different consequences.