Refineries along the Texas Gulf Coast were being secured, a little over a week after Gustav shut down a dozen refineries in Louisiana.
Gustav halted nearly all oil and natural gas production in the Gulf, and Ike is expected to affect output too, particularly from platforms in the western Gulf of Mexico. Ike's projected path has it headed toward south Texas, with landfall expected later this week.
"It's a much more favorable path for where most of the production in the Gulf is -- which is the central Gulf and deep central Gulf," said Lars Herbst, regional director for the U.S. Minerals Management Service, which oversees offshore activity. "But we do have concerns about the western Gulf."
Ike swept across Cuba Tuesday, forcing the evacuation of 1.2 million people.
Exxon Mobil said Tuesday it was evacuating workers from facilities expected to be in Ike's path. ConocoPhillips, BP PLC and Anadarko Petroleum Corp. were doing the same.
The MMS said Tuesday afternoon Gulf oil production stood at about 23 percent, up slightly from Monday, though that figure could drop as producers shut-in operations. Natural gas output was at 35 percent, roughly flat from Monday.
Gustav prompted Exxon Mobil, ConocoPhillips and other refiners to shut down facilities in Louisiana, and widespread power outages have delayed the restart of some of them.
ConocoPhillips said Tuesday that its refinery in Belle Chasse, La., remains without power, while another facility in Lake Charles, La., was ramping back up to normal levels.
Now, as Ike approaches, the focus turns to the extensive refining complex along the Texas coast, including several plants near Corpus Christi.
The U.S. Gulf Coast is home to nearly half the nation's refining capacity, while offshore, the Gulf accounts for about 25 percent of domestic oil production and 15 percent of natural gas output.
To date, neither Gustav nor Ike has caused a significant increase in energy prices. Oil prices fell below $104 a barrel Tuesday after Saudi Arabia suggested OPEC could decide to keep crude output steady. Prices have plunged about 30 percent since surging to a record $147.27 a barrel on July 11.
The average retail price of a gallon of regular unleaded gasoline was at $3.652 on Tuesday, down marginally from Monday and down about 4 percent from a month ago, according to auto club AAA, the Oil Price Information Service and Wright Express.
Darin Newsom, an analyst with Omaha, Neb.-based DTN, which tracks energy markets, said even if gasoline stocks begin to decline, he still expects prices to fall a bit more. The big factor continues to be slacking demand for gasoline, Newsom said.
In its weekly pump spending survey, MasterCard said Tuesday U.S. gasoline demand fell 4.7 percent compared with the same week a year earlier. The year-over-year demand figure has declined for 20 consecutive weeks, MasterCard said.
"Demand continues to come down, and prices should work a bit lower yet before we establish a seasonal low," Newsom said. "I think we could see the national average get to $3.50 (a gallon.)"