An oil sheen that emerged last week in the Pacific Ocean off the Southern California coast does not appear to have been caused by an oil spill, officials with the U.S. Coast Guard said on Monday.

Oil samples collected from the sheen, which was about 2.5 miles long and half a mile wide, did not match anything that would have leaked from one of the oil rigs or ships in the water near Huntington Beach, Calif., according to preliminary tests conducted by the state’s Office of Spill Prevention and Response.

The samples had characteristics of “freshly produced oil” from the local Monterey geological formation and were more consistent with oil that seeps naturally from the sea floor, though the tests did not conclusively determine their origin, the Coast Guard said.

“We have many samples from all the oil rigs and their products, as well as the oil that was in the oil sheen,” Petty Officer Richard Uranga, a spokesman for the Coast Guard, said. “The oil doesn’t seem to be connected with the oil rigs.”

The sheen was first spotted on Thursday evening, about 1.5 miles off the coast of Huntington Beach, where there are significant offshore oil operations.

The discovery prompted concerns that equipment from those operations was spilling oil into the Pacific Ocean — a particularly alarming prospect in the wake of a major oil spill in the same area in 2021, during which 25,000 gallons of crude gushed from a crack in a pipeline and into the sea. The leak resulted in federal charges against three companies.

The Coast Guard, as well as state and local agencies, investigated the new sheen over the weekend. Crews cleaned up about 85 gallons of oil offshore and removed about 1,050 pounds of oily sand and tar balls from the shore, the Coast Guard said. The various agencies have concluded their response to the incident.

In Huntington Beach, popularly known as Surf City, beaches remained open over the weekend and on Monday, despite what seemed to be an increase in the appearance of tar balls, said Jennifer Carey, a spokeswoman for the city.

She said that tar balls had been washing ashore regularly since the oil spill in 2021. Still, the city continued to monitor the situation, Ms. Carey said, adding that if beachgoers see tar on the beach, they should avoid it. If visitors see an area with heavy contamination, she added, they should report the situation to a lifeguard.