Krugman doesn't believe that oilprices are driven up because of speculation. Speculation would mean that oil is being stored somewhere. And secretly since it's not showing up in any inventory data. His back-of-the-envelope calculations shows that the amount would equal some 2 million barrels a day and that's a pretty big secret oil-hiding-chamber.
He's probably right!
But what if the speculators were OPEC itself? Then they would have a pretty big pre-built oil-hiding-chamber in the ground. From where they pump up their oil. If the oilproducing countries reduced production with a few percent, then prices would rise and the excess oil could just wait nice and still in the ground. I should probably check supply data in order to falsify my speculation. Because honestly I think demand is driving the price increase. And the depreciating dollar of course!