Coast Guard Petty Officer Bill Coklough says the sheen, about 100 feet wide, was spotted near the platform owned by Houston-based Mariner Energy Incorporated.
He says Mariner had deployed three firefighting vessels to the site and one already was in place fighting the blaze.
The Coast Guard says no one was killed in the explosion and fire, which was reported by a commercial helicopter flying over the site around 9 a.m. CDT.
All 13 people aboard the rig were rescued as they floated in the nearby water in survival outfits called gumby suits.
The platform is in about 340 feet of water and about 100 miles south of Vermilion Bay on the central Louisiana coast.