Oil “Fingerprinting” Distinguishes Natural Seeps from Spills off Southern California

By Coastal and Marine Hazards and Resources Program September 12, 2025

Off the coast of southern California, oil naturally seeps from the seafloor, bubbling up to the surface and washing ashore as sticky tarballs. For decades, these natural seeps have complicated the work of regulators and resource managers, who must determine whether tar on beaches originates from human-caused spills or natural processes.

Research led by the U.S. Geological Survey and the Bureau of Ocean Energy Management has advanced the science of oil “fingerprinting,” providing a clearer way to differentiate naturally seeping versus anthropogenic oil sources along the southern California coast, where offshore oil production began in the 1890s and continues today.

By analyzing biomarker compounds and stable carbon isotope ratios, scientists could infer details such as the age, depositional environment, and type of organic matter in crude oils, seep oils, and tarballs collected along coastal California. These insights were used to develop a predictive model of oil source families that can be applied to samples of unknown provenance.

The studies support long-standing evidence that natural seeps account for most tarball accumulation on California beaches, not offshore drilling. However, the research also highlights a challenge: spilled platform oil can, over time, begin to resemble natural seep residues. Within about a month, both sources can appear nearly identical due to biodegradation. Thus, the ability to distinguish between seep-derived oil residues and platform oils within this time span is extremely valuable to regulators responding to an oil spill incident.