Oil prices held firm on Monday after the world's No. 5 oil producer Mexico shut down its biggest oilfields as Hurricane Emily roared toward the southern Gulf of Mexico. U.S. light, sweet crude oil futures were trading four cents a barrel higher at $58.13, while London Brent crude was three cents lower at $57.58 a barrel. "The weather in the Gulf seems to be dominating people's minds at the moment," said David Thurtell, a commodities strategist at Commonwealth Bank of Australia. "The risk of something like this had been built into the market already, but traders don't assign a 100 percent chance to it so they'll bid up the price when it happens."The season's second major hurricane began to weaken over the Yucatan Peninsula and the National Hurricane Center said it was expected to continue to lose force while over land.But hurricane watchers were concerned it could strengthen again when it heads out over the Gulf of Mexico. State oil firm Pemex cut off most oil production in the Campeche Sound, the southern Gulf of Mexico basin that produces 80 percent of Mexico's 3.44 million barrels per day (bpd) of crude output. Mexico, the world's ninth-largest exporter, also began shutting its oil terminals in the southern Gulf. It exports more than 1.8 million bpd, almost all of which heads north to the United States, where it was the top foreign supplier in May.