HOUSTON -- Oil and gas supermajor BP PLC and Transocean Ltd., the world's largest offshore drilling rig operator, have agreed to investigate the origin of an oil sheen floating on the surface of the Gulf of Mexico that was found to be leftover crude oil from the 2010 Gulf spill.

The office of the Federal On-Scene Coordinator (FOSC) for the Deepwater Horizon oil spill response in New Orleans said on late Friday that it had approved a joint plan from BP and Transocean for them to inspect the sheen and survey the wreckage of the accident still lying on the bottom.

Last week, the FOSC reported that U.S. Coast Guard testing had confirmed that the large sheen, first reported by BP representatives on Sept. 16, consists of the same crude oil and leaked from the Macondo well. The record spill is estimated to have released about 5 million barrels of crude into the Gulf's waters.

BP said its initial tests strongly suggest that the oil comes from the wrecked riser pipe and not the well itself. The sheen includes chemicals found in drilling mud fluid, an indication that drilling mud mixed with oil coming from wreckage is what is causing the sheen, and not a new leak from the well itself, a company representative explained.

A September investigation showed that the plugged Macondo well is secure and not leaking anything, BP said. Nevertheless, the FOSC report issued last week put BP and Transocean on notice that they would be held responsible for monitoring the sheen, as it was caused by the Deepwater Horizon accident, and for any cleanup that might have to occur.

Thus far, the government has said the sheen does not threaten coastal habitats and that a cleanup is infeasible.

FOSC says that the two companies will employ satellites and robotic remotely operated vehicles (ROVs) to determine what precisely caused the oil sheen to form.

"The joint plan was required by Coast Guard Capt. Duke Walker to identify the source of a sheen located in the Gulf of Mexico that has been associated with Macondo 252 oil," the FOSC said Friday. "The Notice of Federal Interest informed BP and Transocean that they may be responsible for costs related to identifying the source of the sheen and cleanup."

The Coast Guard has been tasked with overseeing the investigation. The government said the joint plan by BP and Transocean would see an ROV deployed Sunday, depending on weather conditions.

Last week, BP acknowledged that the oil slick is leftover contamination from the Macondo well blowout and spill (EnergyWire, Oct. 11). But a chemical found in samples of the sheen tested by that company pointed to the wrecked riser pipe. The company theorizes that a mixture of oil and mud was probably left trapped in the pipe when it bent and collapsed.

Meanwhile, a Florida senator has asked BP to share video from the initial September investigation of the stricken Macondo offshore oil well. In a letter sent to BP CEO Bob Dudley, Democratic Sen. Bill Nelson asked the company to hand over the most recent video taken from inspections of the site (EnergyWire, Oct. 12).