HOUSTON (Reuters) - A 2,100-barrel oil spill in the U.S. Gulf of Mexico forced Royal Dutch Shell on Thursday to shut in all wells that flow to its Brutus platform, federal regulators said.

The U.S. Bureau of Safety and Environmental Enforcement (BSEE) said a 2 mile by 13 mile (about 3 km by 21 km) sheen was visible in the sea about 97 miles off the Louisiana coast.

About 88,200 gallons was reportedly released from the pipeline, the Coast Guard said, adding the source of the discharge was reported as secured.

The sheen is near Shell's Glider Field, a group of four subsea wells whose production flows through a subsea manifold to the Brutus platform, which sits in water with a depth of 2,900 feet (884 m).

In a statement, Shell spokesman Curtis Smith said a company helicopter observed the sheen on Thursday, and that the wells were under control after it isolated the leak and shut in production.

"There are no drilling activities at Brutus, and this is not a well control incident," Shell said.

Brutus began operation in 2001 and was designed with top capacity of 100,000 barrels of oil and 150 million cubic feet (4.25 million cubic meters) of gas per day, according to industry trade publications.

Shell said the sheen likely came from a release of oil from subsea infrastructure, though it is still determining the exact cause of the release by inspecting subsea equipment and flowlines.

Authorities said the cause of the incident is under investigation.

BSEE has tightened regulations for offshore operators since the 2010 BP Plc Macondo well blowout that spilled more than 3 million barrels of oil, making it the worst disaster of its kind in U.S. history.

Shell said no injuries resulted from the incident and that the company has mobilized response vessels, including aircraft, to see if it could recover the spilled oil.

(Reporting by Terry Wade and Luc Cohen in Houston and Arpan Varghese in Bengaluru; Editing by Bill Rigby and David Gregorio)