Shell reported it saw an oil sheen floating between two of its offshore platforms some 130 miles southeast of New Orleans.
An estimated six barrels of oil leaked from a source, or sources, that Shell officials are still trying to locate.
Shell officials confirmed on Thursday that the company did not lose control of any of its drilling operations in the area.
The sheen was spread out over 10 square miles, but has started to dissipate on Friday, reported the Bureau of Safety and Environmental Enforcement
We remain very confident that the sheen did not originate from Shell operations, the company said on its site. It is well studied and documented, most recently by BSEE, that the Gulf of Mexico has a long history of naturally occurring seeps, which can on occasion produce sheens. Shell's subsea surveillance today and tomorrow will continue to determine if there is a connection between natural seeps and this orphan sheen.
On Thursday, the company mobilized an ocean-skimming vessel and conducted arial surveys of the oil slick. The company is working with the BSEE to find the source of the oil leak.
If the spill is found to be linked to Shell's drilling operations, it will be the second such leak in the Gulf since BP's Deepwater Horizon spill in 2010.
In December, an offshore rig off the coast of Alabama spewed thousands of gallons of synthetic and biodegradable drilling fluid into the Gulf. That rig was temporally abandoned to allow for repairs.
Roughly 319 barrels of drilling fluids leaked into the Gulf.
In Firday trading in New York, Shell's shares fell 42 cents to $67.44.