Natural Oil and Gas Seeps
Oil and natural gas have seeped naturally from underground sources to the surface of the earth for millions of years in onshore and offshore areas. The seeps are mixtures of crude oil, asphaltum (tar), natural gas, and water.
Water salinity ranges from fresh to supersaturated brine depending on the source rock, path of migration, and geologic setting. Seeps sometimes provide water to plants and animals. For example, an insect known as the petroleum fly lives exclusively in and around seeps. Fossil records from the La Brea Tar Pits in Los Angeles show that entire food chains can exist by seeps.
Several hundred oil and gas seeps have been found in 28 counties throughout California. They’ve played important roles in human social and economic history.
California Indians living near the seeps hand-formed cakes of asphaltum to make trade goods. As a symbol of mourning, crude oil was sometimes smeared on the faces and hair of widows and grieving female relatives. Some widows wore lumps of asphaltum strung into necklaces. Shamans painted their faces with heavy oil before dancing, believing the oil had supernatural powers.
Many items—including bowls, jewelry, knives, and canoes—were decorated with asphaltum inlaid with chips of abalone and other shells. Baskets and water bottles were waterproofed inside and out with asphaltum. Chumash Indians used refined asphaltum to build their plank canoes, a crowning achievement of Chumash technology.
Early settlers used oil from seeps for waterproofing, lubrication, and lamp oil. Eventually some natural seeps inspired early pioneers to drill for oil, leading to the discovery of over 50 California oil fields.
For more on California’s oil and gas seeps, see two Division publications, Offshore Oil and Gas Seeps in California and Onshore Oil and Gas Seeps in California. You may visit the joint Division/USGS seep website.