The sheen was reported to the Coast Guard on Sept. 16 by BP, which was using the Deepwater Horizon platform when it caught fire and sank two years ago, leading to a massive oil spill, The (New Orleans) Times-Picayune reported.
The Coast Guard said in a statement that samples taken from the sheen matched oil from the now-capped well.
The statement added that both BP and Transocean, the owner of the platform, could be financially responsible for the new leak.
The oil could be leaking from storage areas on the platform or from the riser that connected the well to the platform.
The Coast Guard said it was not financially feasible to recover the oil and that did not pose a risk to the shoreline.
The Coast Guard, BP and the National Oceanic and Atmospheric Administration have continued to monitor the leak since it was discovered.
The announcement came on the same day the Wall Street Journal reported that BP and the Justice Department are close to settlement of a federal lawsuit against BP and other parties involved in the drilling operation.