A 10-mile sheen of oil off Louisiana's coast appeared to be dissipating as unmanned underwater vessels continued to search for its source, a federal agency and Royal Dutch Shell PLC said Thursday.
The sheen, described by the Coast Guard earlier Thursday as about a mile wide and 10 miles long, was spotted Wednesday afternoon in the area of two Shell production platforms. But the oil giant said it's confident the oil didn't come from its operations.
The company was aiding federal agencies by using two remotely controlled underwater vessels to check for any leaks in plugged wells or naturally occurring seepages from beneath the sea floor.
"Through initial reports from Shell's remote operated vehicle surveys, the permanently plugged wells in the vicinity do not appear to be the source of the sheen," a statement from the federal Bureau of Safety and Environmental Enforcement said. "With oversight from BSEE, Shell continues to survey possible sheen sources in the area including known natural seafloor seeps."
Shell said the surveillance would continue Thursday and Friday.
Shell estimates the sheen at six barrels of oil, or about 252 gallons.
BSEE also said it had instructed operators of pipelines in the area to survey their lines.
The sheen was about 130 miles southeast of New Orleans.
Shell has two production platforms in the area, called Mars and Ursa.
"We are treating this very seriously, as we do all reports of possible pollution. And, in consultation with our state and local partners, we will ensure that all measures are taken to fully investigate and, if necessary, mitigate any impact this could potentially have," Coast Guard Capt. Jonathan Burton said in a news release.
Shell operates six major offshore facilities, 13 crewed platforms and numerous subsea systems in the Gulf.
The sheen was reported in an area about 50 miles from the site of BP's Macondo well, which blew out in April 2010 and created the nation's worst offshore oil disaster. The now-plugged Macondo well is in about 5,000 feet of water.
Sheens spread quickly as oil breaks down and a small amount can cover a large area. Earlier this week, a tanker in the Mississippi River south of New Orleans spilled an estimated 50 gallons of oil. The sheen from the discharge extended almost 30 miles downriver.