Spill reported at Shell Gulf of Mexico drill site

* Site is near the BP Plc oil spill disaster

* Coast Guard says spill estimated at 13,398 gallons

* Shell says Transocean rig leaked drilling fluid only

HOUSTON, Dec 19 (Reuters) - The U.S. Coast Guard was investigating a 13,000-gallon spill from an oil rig leased to Shell, operating about 26 miles (41 km) southeast of last year's BP Plc Macondo oil well disaster, a Coast Guard spokesman said on Monday.

The spill of either drilling fluid or oil mixed with drilling fluid was reported Sunday by Transocean Ltd's Deepwater Nautilus rig, which was drilling a well at Shell's Appomattox discovery.

"Shell can confirm it has a loss of 319 barrels of drilling fluid," Shell spokeswoman Kelly op de Weegh said by email.

The leak was from a booster line, which provides additional drilling fluid and is separate from the well, she said.

"The leak was isolated, stopped and remedial action has been approved by BSEE (the U.S. Bureau of Safety and Environmental Enforcement), which includes temporarily abandoning the well, and making appropriate repairs," op de Weegh said.

The Coast Guard was attempting to determine what material was spilled, Coast Guard spokesman Steve Lehmann said.

"An overflight from New Orleans spotted a very light sheen in the vicinity," Lehmann said.

He did not estimate the sheen's size.

The initial report filed with the U.S. National Response Center described the leak as a discharge of base oil mixed with synthetic-based (drilling) mud with an oil content of 180 barrels.

"Everything's pretty up in the air as to what the actual substance is and what the cause of it is, but that's what we're going off of right now," said Coast Guard spokesman Lehmann, referring to the report.

"The 'oil' referenced in the report is referring to the synthetic fluid," op de Weegh, the Shell spokeswoman, said. "The remaining amount in that discharge is water-based."

The BP Macondo well blew out in April 2010, killing 11 workers, sinking the Transocean Deepwater Horizon drilling rig and spilling nearly 5 million barrels of oil into the Gulf. (Reporting by Bruce Nichols; editing by Erwin Seba, Gary Hill)