A trio of oil companies led by Chevron Corp. has tapped a petroleum pool deep beneath the Gulf of Mexico that has the potential to boost the nation's reserves by more than 50 percent. A test well indicates it could be the biggest domestic oil discovery since Alaska's Prudhoe Bay more than a generation ago. But the vast oil deposit about four miles beneath the ocean floor won't significantly reduce the country's dependence on foreign oil and won't help lower prices at the pump anytime soon. "It's a nice positive, but the U.S. still has a big difference between its...