Opec Article on Euros
There's a lot of reading there, and i haven't read it all, but it seems very interesting. It is by the then Secretary General of OPEC essentially building a case for oil to at least partly be traded in Euros.
One thing is some of the weaknesses he points out about the Euro have been turned around since then. For example the actual relative weakness of the Euro to USD.
While the USD is losing some of it's previous strengths. e.g seens as a reliable store of value, and general goodwill around the world.
If Norway and the UK join the Euro you can pretty much kiss goodbye US dominance of oil trade. Best case scenario there is dual pricing.