 They said that that information along with another small spill prompted them to shut down the Prudhoe Bay oil field, which would reduce Alaska North Slope oil production by an estimated 400,000 barrels per day.Later BP decided not to shut the western side of the field, meaning Prudhoe Bay would still produce about 200,000 barrels a day, half its normal total.Oil revenue accounts for 90% of Alaska's tax income.