HOUSTON/LONDON (Reuters) - Royal Dutch Shell said a sheen discovered in the water near its offshore Gulf of Mexico oil wells was estimated to total about six barrels of oil and that its facilities showed no signs of leaks.
The statement appeared to calm investors' nerves, with Shell's London-listed shares trimming losses to trade down 1.3 percent to 21.13 sterling, after sliding as much as 5 percent earlier in the day.
News of the sheen comes almost exactly two years after BP's deep sea Macondo well disaster that saw nearly 5 million barrels of oil escaping into the Gulf of Mexico, rocking BP's reputation.
Shell had asked for a spill response vessel to be sent to the area to investigate the sheen, which was discovered on Wednesday.
"Although we are confident at this time that the sheen did not originate from Shell operations, out of prudent caution we will continue to respond to the sheen," the company said in a statement.
Shell deployed two remote operating vehicles to inspect the oil equipment in the area and to search for naturally-occurring seeps in the ocean floor.
"We're checking the whole area," including seabed wells for both the Mars and Ursa platform, said Shell spokesman Kelly Op de Weegh. Those checks have so far shown no leaks or plumes.
A U.S. Coast Guard helicopter was sent to investigate the sheen at about 9 a.m. local time (1400 GMT).
"They were going to go out at first light, but some weather issues delayed them," Coast Guard Petty Officer Bill Colclough said.
The earlier drop in the company shares price showed investors remained anxious over potential oil accidents two years after the worst-ever offshore spill in the United States.
Shell said it had reported the sheen, between its Mars and Ursa projects, to the National Response Center, which is run by the Coast Guard and monitors marine oil spills, pipeline leaks and other incidents of pollution.
Europe's largest oil company by market capitalization said the source of the one-mile by 10-mile sheen was unknown, but added there was no indication that it had originated from wells in either the Mars or Ursa projects.
Shell owns a 71.5 percent interest in the Mars project and a 45.4 percent stake in the Ursa project, and is the operator of both projects. BP owns a 28.5 percent stake in the Mars project and a 22.7 percent interest in the Ursa.
Shares in BP - which was facing environmental activists still angry about its 2010 spill at its annual shareholders' meeting in London on Thursday - were down 2.
The Marine Spill Response Corporation is a non-profit organization created in 1990 by the oil and shipping industries to enable members to fulfill requirements of the U.S. Oil Pollution Act of 1990.
(Reporting by Matt Daily in New York, Kristen Hays in Houston and Adveith Nair in London; Editing by Helen Massy-Beresford and Sofina Mirza-Reid)
