Petroleum sheens on the ocean off a stretch of Southern California coast are consistent with oil from local natural seeps, but abandoned old oil wells could be a factor, authorities said Saturday.

The sheens have been observed off Santa Barbara County’s Summerland Beach since early this month, and have been investigated by the U.S. Coast Guard and state and local agencies under a unified command.

Lab tests from multiple locations offshore and onshore showed that the petroleum is consistent with oil from the area’s very active natural seeps, a command statement said.

But there are also about 200 “legacy wells” near the beach that were drilled in the late 1800s and early 1900s and were not properly abandoned.

The wells are believed to be relatively shallow compared to modern wells and that makes their oil nearly impossible to distinguish from the oil released by natural seeps, so one or more of those wells could be a factor, the command said.

Extreme weather and tidal activity could also be a factor. Extraordinary storm surge in recent weeks has eroded large amounts of sand at Summerland Beach.

The State Lands Commission’s Coastal Hazard and Legacy Well Remediation Program will investigate whether a well is discharging petroleum, and crews will continue to monitor the shoreline for impacts, but the unified command will demobilize.