Shell said the sheen is about a mile wide and 10 miles long and is between its Mars and Ursa drilling platforms in the middle of the gulf, CNNMoney reported Thursday.
Shell said Wednesday it reported the oil sheen to the National Response Center and activated an oil spill response vessel.
The company said it had "no current indication that the sheen originates from wells in either the Mars or Ursa projects."
One of the worst spills in U.S. history occurred in the Gulf of Mexico two years ago, when BP's Deepwater Horizon rig exploded, killing 11 workers. The months-long leak fouled the coastlines of Louisiana and other gulf states, damaging the environment and the economy.