Oil washed up on a remote Oahu beach after a tanker ran aground on a coral reef, spilling 117,000 gallons of oil into the ocean and posing the possibility of a "major pollution incident," the Coast Guard said Friday.

The 800-foot Exxon Houston tanker broke anchor at 5:55 p.m. Thursday about 25 miles south of Honolulu as it was unloading oil to the Pacific Resources Inc. Hawaiian Independent Refinery at Campbell Industrial Park.The ship, carrying 90,000 barrels of crude oil, then drifted onto a coral reef about 1,000 yards southwest of Barbers Point lighthouse on Oahu's southwest tip, Coast Guard spokesman Chief Dennis Hubbard said.

Oahu Civil Defense officials said volunteers using flashlights spotted oil on remote Germain's Beach northeast of the vessel's position at 1 a.m. Friday.

"There was no way for them to determine the extent of the spill or what type of oil it was," a spokesman said.

The vessel appeared in no immediate danger of sinking or losing its cargo at this point, Coast Guard spokesman Dennis Hubbard of the Marine Safety Office said.

The ship initially carried 490,000 barrels of crude oil but had unloaded all but 90,000 barrels (3.78 million gallons) before it broke anchor, Hubbard said.

The captain of the vessel estimated 92,400 gallons of Bunker C fuel oil, which powers the ship, spilled when the vessel ran aground on the reef, Hubbard said.

Another 25,200 gallons of crude spilled from the fuel lines when the vessel broke anchor before the tap could be shut off, Hubbard said. The captain's total estimate of the spill was 117,600 gallons, he said.