I thought it was animal matter that made petroleum, plant matter makes coal. I doubt it would take as long as you think it does. Hundreds, at most a couple thousand years should be sufficient but very possibly much less, even within a human lifetime with the application of enough pressure in airtight conditions. Either coal or petroleum. Of course you'd have to round up some dead animals, or just try plants. We aren't going to be able to make enough to get rich on but perhaps enough to demonstrate that you CAN make the stuff. The pre-Flood world was LOADED with vegetable matter, beyond our ability to imagine. Animals too. Extraordinary fecundity. Press some of whatever you can gather together between huge heavy rocks -- about a ton each. Pack a bunch of mud around your organic matter too. And wait a while. Shouldn't take anywhere near as long as you think. Give it ten years.