NEW YORK ( Real Money) -- Oil is the global currency, and the Organization of the Petroleum Exporting Countries (OPEC), led by Saudi Arabia, has operated in essence as the world's central bank for oil since its founding in 1961.OPEC has attempted to maintain a consistent supply and price of oil in balance with the demand for it, similar to the way that central banks attempt to manage the supply of monetary capital to the economies they serve. The principal difference between OPEC and individual central banks is that OPEC is a global organization, so it is dedicated also to preventing competition among its members. The Bank for International Settlements (BIS), essentially the central bank of central banks, does not have this mandate, so monetary policy among central banks is partially dedicated to competitive depreciation, although it is politically taboo to discuss it in those terms. The process of competitive depreciations among the world's three dominant central banks -- the Bank of Japan, the European Central Bank and the U.S. Federal Reserve -- is, however, increasingly evident in the expansion of the balance sheets of all three, although it is discussed in terms of domestic economic need rather than as a reflection of competition with other central banks. The zero-interest-rate policies in Japan and the U.S., and Europe's migration toward zero interest rates, and the failure of these policies to spur economic activity, provide insight into the pressures that are being exerted upon OPEC and its ability to maintain cohesion among its members. On Monday, Glenn Williams noted that the price band within which oil prices may operate and be economically viable is shrinking. A few weeks ago, I made note of this same phenomenon from a different vantage point. As the upper and lower economically viable price limits for oil shrink to a convergence point, the result is that OPEC's ability to affect the price of oil by managing supply shrinks as well, and at the point of convergence it becomes zero. At that point, the need to monetize oil to meet individual OPEC members' domestic financial needs becomes the sole driver of supply, irrespective of demand.