In other words oil and all commodities are "Fungible resources" bought and sold on the global market.

I modest or even fairly substantial increase in supply won't change the global price.

The Dollar has lost 60% off the Euro since 2001. The Pound a few years ago was on parity with the dollar and now the British Pound trades 2:1 to the dollar.

 So maybe some short term speculation is driving it in the immediate but the long-term view is the supply of dollars that is increasing.

 Most commodities are bough in U.S. dollars on the global market. People buy 100 barrels of crude oil for a certain amount of dollars. But as the dollar devalues, it buys less and less crude (that's why commodities are seen as a protection against inflation provided their isn't some sort of bubble- aka you can still lose money of course.)