My understanding is that the Euro is not robust enough currency to handle the all the capital that is now is petrodollars.  The dollar is like the failing bank that is too big too fail in it is too big to ignore.

General Motors is too big to fail.

I think the USSR is too big to fail also.

Oh wait...

OK, bad example.  The Euro cannot "physically" handle the job.  The stock and bond markets denominated in Euros do not have the necessary capacity.
I believe the Euro bond market might be larger than the U.S.'s.  I've heard that anyway, although I can't provide a link to any substantial source.  If so, it's kind of silly to say the Euro can't handle the job.  

The dollar is not all that important.  Everything could easily switch over to Euros or other currencies.  It will happen, but it will no doubt take many years time.  

The euro float is certainly large enough to handle the oil trade. If money is purchasing power, the financial institutions of this world can provide plenty of Euros.

The weakness of the Euro is political. Not all countries in the Euro market are happy with membership. See Italy for one example. It would prefer a currency it could more easily inflate (devalue or debase).

Hey...a petrocurrency has to start somewhere.  The Euro already acts as a defacto "basket" currency.  Makes it easier to add additional countries/currencies.  I think it would be quite easy to switch over, if/when countries decide to give it value.  All it takes is "agreed upon" confidence.