An oil sheen that appeared off the coast of Huntington Beach late last week was caused by a natural seep from the ocean floor, Coast Guard officials said Monday. Seeps occur when crude oil leaks from fractures in the seafloor or rises up through seafloor sediments.

The sheen was first reported Thursday evening about 2.5 nautical miles off Huntington Beach near two oil platforms, Emmy and Eva. By Sunday morning, officials were no longer seeing a sheen in the water, according to the Coast Guard, but they had skimmed about 85 gallons of oil from the ocean and removed about 1,050 pounds of oily waste and tar balls from the shoreline.

The location of the seep is not far from the site of a large spill in 2021 that occurred when a ship’s anchor punctured an underwater oil pipeline in San Pedro Bay, sending 25,000 gallons of crude gushing into the waters off Huntington Beach. Cleanup from that spill spanned months and resulted in criminal charges and years of litigation.

“This situation isn’t even remotely close to what we saw in 2021,” said Jennifer Carey, a Huntington Beach spokesperson. “The Coast Guard is still investigating where it came from, but they were able to get it cleaned up quickly.”

Investigators used technology that allows them to basically fingerprint the oil they gathered in the ocean and match it to either a platform, a vessel, a pipeline or determine whether it was the result of something naturally occurring, Orange County Supervisor Katrina Foley said.

“They have a database of all of the different kinds of crude and petroleum and oil generated from these different rigs and vessels, so they can pretty much match it to a particular operator,” she said.

Coast Guard spokesperson Richard Uranga said Monday the testing revealed the oil was from natural seepage. Officials said that although seeping from the ocean floor is not unusual, it’s uncommon to see such a large amount.

“They’re used to seeing about a cup worth of oil, not 85 or more gallons,” Uranga said. “We don’t know what caused the natural seepage to be so much.”

Oil seeps account for nearly half of the oil released into the ocean each year, according to the National Oceanic and Atmospheric Administration.

Tar balls on Monday continued to wash up on the sand in Huntington Beach, including at the popular dog beach. City officials do not have plans to close the beaches but advise visitors not to touch any tar that washes up along the shore.

One bird — a Brandt’s cormorant — that had been oiled died over the weekend. An unoiled, but injured, snowy plover that was captured also died. Officials are caring for a common loon and a western grebe that were recovered with oil on their bodies.

Neighboring beach cities have not reported any signs of oil residue on their shores.