Hydrogen, oxygen and carbon. The basis of all known life. These elements are abundant on this planet. It could be a store of these constituent components from the large quantities available at the time the Earth was formed. Not much ever leaves this planet, therefore it stands to reason that the Earth is a huge recycling system. The releasing of these components through the chemical processes make these constituent elements available for the creation of life. It also stands to reason that when life forms die, these constituent components break back down and recombine in different ways, one would assume that if it isn't recycled as another life form, then perhaps it can be stored again as petroleum under the right conditions. Perhaps there is merit to both theories? It is possible that in our efforts to classify things in nice neat categories that we are missing the big picture and that H,C, and O are both available in a stored form and also in a recycled form. Just a possibility.