While it pisses me off that he did this, from looking at the oil production records in Utah, it appears to be more a "symbolic" gesture than anything else. I doubt it will affect production much. According to the Utah Oil & Gas website, there are 3,005 wells producing about 21,000,000 barrells per year. I can't imagine that this will have too big an impact on production.
Link to Utah production here
Having said all that, I DO believe that this will show the oil market folks that Obama is no friend of fossil fuel energy and cause the price of oil to go up a little.
That’ll teach them Mormons to support Prop. 8!!!