The Mississippi River remained closed to ships Thursday from New Orleans to the Gulf of Mexico after a barge split in half in a collision with a tanker, spilling thousands of gallons of heavy oil.

It was unclear when the stretch would reopen.

By dawn Thursday, an oil sheen extended from New Orleans almost to the Gulf of Mexico about 97 miles away, Young said. About 45,000 feet of containment boom had been placed on the river, and workers planned to double that and use vacuum skimmers to pick up the oil, she said.

"It could take a few days to open the river and a few weeks to clean up the spill," Coast Guard Petty Officer Jaclyn Young said Thursday.

The tugboat towing the barge before Wednesday morning's crash did not have a properly licensed pilot, the Coast Guard said.

The person operating the boat had an apprentice mate's license and there was no one else on the vessel properly documented to guide it, said Lt. Cmdr. Cheri Ben-Iesau from the Coast Guard in New Orleans. The operator's name was not released and the Coast Guard and National Transportation Safety Board were investigating.

The barge held more than 419,000 gallons of heavy fuel oil in three tanks. Investigators don't know whether all three tanks broke but "are assuming the worst-case discharge of all 9,980 barrels," said Capt. Lincoln Stroh, Coast Guard captain of the port of New Orleans.

The double-hulled tanker Tintomara was loaded with about 4.2 million gallons of biodiesel and nearly 1.3 million gallons of styrene, but did not leak, said Michael Wilson, president of ship management company Laurin Maritime (America) Inc. in Houston.

The crews of both vessels reported the accident about 1:40 a.m. CDT Wednesday, just upriver from the twin bridges between New Orleans' east and west banks, the Coast Guard said.

American Commercial Lines Inc. of Jeffersonville, Ind., which owns the barge, brought in four oil spill cleanup companies with about 250 people and 11,500 feet of boom to keep oil away from water intakes and environmentally fragile areas, said Paul Book, vice president of operations facilities.

Officials are urging residents of Algiers and Gretna and in St. Bernard and Plaquemines parishes to conserve water because their system intakes have been shut down.

The barge had just been filled at Stone Oil Co. in Gretna, across the river from the accident site, and was on its way to Memphis, said W. Norbert Whitlock, executive vice president of American Commercial Lines.

The tanker was fully manned with a crew of 22 and was heading downriver, said Wilson, who heads the U.S. subsidiary of Laurin Maritime AB of Goteborg, Sweden. The tanker is owned by Whitefin Shipping Co. Ltd. of Gibraltar, and Laurin America's technical director was sent to the scene, he said.

The Liberian-flagged tanker had only minor damage, company spokesman Darrell Wilson said. He said the styrene, taken on at Carville, and some of the biodiesel taken on at St. Rose were bound for Rotterdam, the Netherlands, and the rest of the biodiesel was going to Hamburg, Germany.

State Department of Environmental Quality spokesman Rodney Mallett said the spill poses minimal risk to wildlife because environmentally sensitive areas have been boomed off.

"There's not a whole lot of wildlife on the Mississippi," he said.

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Associated Press writers Janet McConnaughey, Alan Sayre and Becky Bohrer contributed to this report.

(Copyright 2008 by The Associated Press. All Rights Reserved.)

APTV 07-23-08 2037CDT