Economics are outside my field but your questions are interesting so I'll have a go.
A few Points:
- You hinted at this with your response to AlXtreme. I suppose AlXtreme's answer better describes potential wealth. Vast oil reserves represents potential wealth equal to what others will give for it.
- We have a workable definition if we adopt a variant definition of wealth: Wealth is what I have that I want. With this definition OPEC's oil reserves still represents wealth to it(They burn fossil fuels and thus want oil). Of course they have many other wants that need filling before they will feel wealthy.
- Up to a point, Oil is worth more to a big consumer(such as the US) than it is to an OPEC nation. There will be things that are worth more to an OPEC nation than oil(perhaps food and knowledge).
- This unequal valuation of goods among countries encourages trade.
I fear any explanation of economic processes reduces the problem to near idiocy but it is interesting none-the-less. Thanks for your intersting questions.