TL;DR: coal, oil, and gas and/or the strata in which they form contain fossils and chemical signatures of ancient plant life, and are found in zones (ancient marshes/forests for coal, ancient shallow seabeds for oil) which correspond to a biotic rather than abiotic origin.
The mantle exists at depths too great for fuel formation --- the materials would be broken down at the temperatures and pressures found. Kerogenesis is depth-limited on the same basis.
TL;DR: coal, oil, and gas and/or the strata in which they form contain fossils and chemical signatures of ancient plant life, and are found in zones (ancient marshes/forests for coal, ancient shallow seabeds for oil) which correspond to a biotic rather than abiotic origin.
The mantle exists at depths too great for fuel formation --- the materials would be broken down at the temperatures and pressures found. Kerogenesis is depth-limited on the same basis.