Time is certainly a factor.
Any organic compound is subject to oxidation under proper conditions. It is assumed that if organic material was ever there it has long since been oxidized out. A standard procedure for releasing pollen and spores from coal macerals is to treat them with a mixture of concentrated nitric acid saturated with potassium permangenate. I cannot imagine a stronger oxidizer than that. This stuff completely oxidizes the carbonaceos material in the coal leaving essentially sporopollenin as the surviving organic.