A U.S. Army soldier atop a Humvee armed with a heavy machine gun secures an area by a burning oil well in Iraq's vast southern Rumaila oil fields. U.S. engineers moved through the oil fields on Sunday shutting down wellheads in an operation that could take months to complete. Having discovered a cache of arms and a minefield, U.S. troops must tread carefully in their mission to safeguard the region's oil fields, which pumped more than half of Iraq oil exports before the war began.