Royal Dutch Shell says it is confident a 16-kilometre oil sheen spotted in the Gulf of Mexico off the Louisiana coast didn't originate from its operations in the area.

In a statement Thursday, the company says it found no sign of leaks and ruled out any well-control issues associated with its operations. The sheen was spotted late Wednesday.

A Coast Guard helicopter with a pollution officer on board was heading out Thursday morning to the site, about 200 kilometres southeast of New Orleans, in an effort to determine the source of the oil.

Shell estimates the sheen at six barrels of oil, or just under 1,000 litres. The company sent a response ship to skim the area as a precaution.

The company has two production platforms in the area, called Mars and Ursa.

Shares in Royal Dutch Shell PLC fell in European trading early Thursday after the sheen was reported. After New York trading opened, Shell's U.S. shares were up 32 cents at $68.07.

Early reports could not determine whether oil was continuing to flow at the site near the Shell platforms.

The Mars and Ursa fields are producing oil and natural gas from huge tension-leg production platforms. The Mars platform is in 2,900 feet of water while Ursa is in 4,000 feet of water.

Shell operates six major offshore facilities, 13 crewed platforms and numerous subsea systems in the Gulf.

The sheen was reported in an area about 80 kilometres 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.

Other possible sources, aside from oilfield equipment, could include natural seepage from the Gulf bottom and fuel discharged by passing ships headed into or out of the Mississippi River.