An oil sheen in the Gulf of Mexico last week came from a natural sea floor seep and not from Royal Dutch Shell offshore production platforms, the U.S. Bureau of Safety and Environmental Enforcement (BSEE) said on Monday. \"A modest amount of oil is being released from a natural sea floor seep previously suspected as the source of an oil sheen reported last week in the Gulf of Mexico,\" BSEE said in an emailed news release. BSEE and natural seeps experts from the Bureau of Ocean Energy Management (BOEM) viewed videotape from a remotely operated vehicle survey deployed by Shell to make the final determination, the news release said. The 10-square-mile sheen, estimated to be about six barrels of oil, was spotted on Wednesday near Shell\'s Ursa and Mars platforms. It prompted Shell to launch a spill response vessel and request overflights of the area. The sheen later dissipated.