OsideNews:

Oceanside CA— Tarballs that washed ashore near Oceanside Pier in mid-May are likely from natural seepage, the city recently reported.
Coast Guard and California Fish and Wildlife crews took samples of the tar on May 17 after city lifeguards reported seeing them in the water and on the beach around the pier, according to the city’s report.
Fish and Wildlife’s Office of Spill Prevention and Response tested the samples and found indicators of natural seepage, not from an oil spill. A cleanup of the tarballs followed.
“Seeps occur when crude oil leaks from fractures in the seafloor or rises up through seafloor sediments, in much the same way that a freshwater spring brings water to the surface,” according to a National Ocean Service factsheet on oil seeps.
Last October, Oceanside and other coastal San Diego cities monitored their beaches for oil from a spill that occurred in Huntington Beach.
Tarballs from the spill, which occurred on Oct. 1, washed up on Oceanside beaches throughout the month but subsided by early November.