We have determined the absolute upper limit of hydrocarbon compositional space for petroleum and other fossil oils. The upper limit for the double-bond equivalent (DBE) to carbon is 90% of the carbon number. The DBE lower limit is zero. Any, hydrocarbon molecular formula outside of those boundaries is not observed in hydrocarbon resources, petroleum, coal, and oil shale. These boundaries provide working limits for elemental compositions of fossil fuels for computer searching and model building.