The fossil fuels have been formed on the basis of organisms that have died, mostly producers. These organisms have carbon stored in them. As they died and got covered by sediments, the carbon has been stored in the lithosphere. Since more and more sediments have been covering the stored carbon, it has been isolated and stored into the lithosphere for tens or even hundreds of millions of years, thus contributing to lowering the amount of carbon in the atmosphere for quite some time, and removing it from its natural cycle.
