If it were plants and animals that made oil you'd have to ask where they got it from in the first place. It's not like it magically appears inside of them is it? It has to be made from elements available to them from their food which inevitably falls back on plant life which uses sunlight and water which we can exclude as possible sources of the oil. That means it has to come from the minerals available to the plants and through some biological process is converted into oil.

That biological conversion might be a faster process than what goes on down below but it would just be a replication of the chemistry taking place beneath our feet which would be constantly taking place. This is something that really should be studied in depth because once you learn the normal and biological processes of converting those minerals into oil you can artificially replicate the system that works best to create virtually unlimited oil which would be like planting a money tree.