Relative to daily US/global petroleum demand, the stategic oil reserve really isn't that big. I don't think it will cause a noticeable impact in prices.
With an inelastic good like oil, small changes in supply can cause big price swings. The reason is people don't curtail their use much as the price rises.
It's not the magnitude of demand that affects price - it's the price elasticity - the demand and supply curves matter. Demand for petroleum (e.g. gas, etc.) is very stiff - so small changes in supply can have large impacts on price.