Tom, supply and demand are not the same.  They intersect to create price.  If demand is high and supply is low, price is high... etc.  Petroleum can be stored in many places, such as in the US Strategic Petroleum Reserve (727 million barrels).  Global strategic petroleum reserves hold over 4 billion barrels.  Supply, which is the refined petroleum (and other liquids) available in the open market, is affected by releases of strategic reserves (note that production is not the same as supply).  Depending on the source you read demand and usage might not even be the same thing, as demand might include buyers for strategic reserves.  

Most of the big issues with Peak oil begin with the fact that until recently demand has grown very steadily for a long time, catching up with production.  When production can not go up anymore (and begins to decline), the strategic reserves will slowly be used up until supply also begins to decline.  Then usage will be forced to decline, but demand could theoretically keep going up (imagine a room with one cupcake, full of people yelling for more).  I hope this helps... let me know if part of this is confusing.