Update- Shell Oil Slick in Gulf

* Shell says 'light' sheen 10 square miles in size on water

* Sheen spotted near Shell's Mars and Ursa oil platforms

* Shell says to issue update after sunrise

* Shell shares down 5 percent in London

HOUSTON/LONDON, April 12 (Reuters) - Shares in Royal Dutch Shell fell on Thursday as the company stepped up efforts to monitor an oil spill in the Gulf of Mexico, in a market still jittery after BP's 2010 Deepwater Horizon disaster.

Shell has sent a spill response vessel to the area and is seeking aircraft to monitor an oil sheen - a layer of oil floating on the sea - near its platforms in the central Gulf of Mexico, it said in a statement on Thursday.

"While the extent of the sheen, reported at 1 mile by 10 mile, is large and will be a concern, the sheen is light and could result from volume of only a couple of barrels," RBC Capital Markets analyst Peter Hutton said.

"First light will provide opportunity to assess whether there has been any increase in the visible sheen," he added.

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 keeps up with marine oil spills, pipeline leaks and other incidents of pollution.

Europe's largest oil company by market capitalisation said the source of the one-mile by 10-mile sheen was unknown, but added that there was had no indication that it had originated from wells in either the Mars or Ursa projects.

"Given the location of the sheen, we expect sensitivities to be high," Hutton said, adding that he expected a swift response from the company.

Shell said it had activated the Louisiana Responder, a Marine Spill Response Corporation vessel, and requested flights to monitor the sheen closely.

The Marine Spill Response Corporation is a non-profit organization created in 1990 by the oil and shipping industries to enable members to fulfil requirements of the U.S. Oil Pollution Act of 1990.

http://www.reuters.com/article/2012/04/12/oil-spill-gulf-idUSL2E8FBO4B20...

(Reuters) - An oil sheen spotted near Royal Dutch Shell platforms in the central Gulf of Mexico has caused the company to send a spill response vessel and seek aircraft over flights, a Shell spokeswoman said on Wednesday.

The source of the one-mile by 10-mile sheen between Shell's Mars and Ursa projects is unknown and not linked to the facilities, but Shell is responding "proactively," a company statement said.

http://www.reuters.com/article/2012/04/11/us-oil-spill-gulf-idUSBRE83A1H...

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