SOUTH PORTLAND — Nearly 500 gallons of heating oil leaked from a barge docked in Portland Harbor on Wednesday, but authorities managed to contain the spill to the area immediately surrounding the tanker.

“Everything has been recovered,” Coast Guard Lt. Regina Callies said Wednesday afternoon.

About 332 barrels of the No. 2 oil were still unaccounted for but believed to be in some part of the vessel. “That doesn’t mean it’s in the water. It could be in the ballast tanks,” Callies said.

The spill, which occurred about 2:45 a.m., was noticed as the fuel was being transferred from the New York barge to the terminal.

“It was nothing they were doing in the transfer itself,” Callies said. She said the leak was from a tank on the barge. Divers conducting a survey of the vessel determined that the hull had been damaged and the oil seeped out through a hole.

“We don’t know the cause of it [the damage] yet. There are some cracks in the hull, and that’s where the release came from,” she said.

Callies said the situation has been stabilized. The next task is to offload the remaining oil on the barge and patch the hole.

Birds or other wildlife were not affected by the spill, unless they landed within the booms during the day. “They’d have to land right in there,” Callies said.

Two companies, Clean Harbors Environmental Services and Clean Casco Bay Inc., worked throughout the day removing oil from the water.

The Coast Guard initially closed the Fore River upriver from the Casco Bay Bridge with no boat traffic allowed. Later in the afternoon, the river was reopened, with a safety zone surrounding the barge.

The red-and-black barge, called RTC-503, sat at a dock surrounded by booms, a scene similar to September 1996, when the Julie N tanker crashed into the old Million Dollar Bridge, spilling 180,000 gallons — or 2,000 barrels — of oil.

However, this spill was not nearly as bad as the Julie N disaster.

“This is a whole different type of incident. The Julie N hit the bridge and everything was spilled as it moved through the river. This spill was at a facility. It’s a different type of oil, too,” Petty Officer Jared Ruthman said.

Ruthman said heating oil is very light and floats on the water’s surface. The Julie N oil was black and gooey, clinging to birds, rocks and tidal areas.