[Nasrudith]

If you define far future as far enough then it may well be coal or oil. The same thing with lignin is what made coal - it was a hydrocarbon "plastic" of its day that bacteria hadn't figured out how to consume.

Of course if we assume a remotely industrial era progression of advances oil reserves may be near-worthless. Why dig down say fifty feet for enough oil to fill the shelves of a Walmart camping section?