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The Ecology of Oil Seeps in Central California

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photograph of deer browsing near oil seepage
California Pacific Blacktail doe browsing in the rich vegetation supported by water and oil seepage.
As part of USGS continuing efforts to understand natural oil seeps and their impact on the onshore environment, a multi-agency team investigated the Sargent oil field just south of Gilroy, CA, last summer. This is a cooperative venture among the USGS Coastal and Marine Geology Program (CMGP), and the California Department of Conservation's Division of Oil and Gas and Geothermal Resources (CDOGGR) and Division of Mines and Geology (CDMG). Rick Stanley and Bob McLaughlin (USGS, Earth Surface Processes) and Dave Wagner (CDMG) mapped the rocks. Les Magoon (USGS, Earth Surface Processes) and Tom Lorenson (CMGP) collected samples of oil and gas from seeps and oil wells. Bill Fedasko and Ross Brunetti (CDOGGR) mapped the seeps and located oil wells using GIS equipment. Paul Lillis (USGS, Energy Resources) and Keith Kvenvolden, Fran Hostettler, Jon Kolak, Tom Lorenson, and Bob Rosenbauer of the Menlo Park organic geochemistry group are performing geochemical analyses.

With active oil seeps at its eastern and western ends, the study area is the most extensive area of oil seepage in northern California. Approximately 3 acres of land are covered in black, dense, sticky tar from several seeps. Nearby pump jacks working in the oil field nod approval and bring heavy, biodegraded oil with carbon-dioxide-rich hydrocarbon gas to the surface.

oblique aerial photograph of the western portion of Sargent oil-field seeps map showing location of the Sargent oil-field seeps east of Monterey Bay
The western portion of Sargent oil-field seeps. Note the extensive area of seepage, the trees and shrubs at the upper part of the seeps, and the green grasses and shrubs at the lower part of the seeps.

Surprisingly, we found the areas around seeps to be biologically more active than the surrounding neighborhood, partly because oil and water seep to the surface together.
The thought of oozing tar bubbling out onto the California landscape brings environmental disaster to mind. Surprisingly, we found the areas around seeps to be biologically more active than the surrounding neighborhood, partly because oil and water seep to the surface together. The photographs show that the grass is greener and the shrubs more abundant at the oil seeps. In fact, inhabitants of the area seem to like the tar.

For example, the doe in the photograph at the top of this page is browsing in an area affected by oil seepage. California oil seeps are home to the petroleum fly, Helaemyia petrolei, which relies exclusively on tar seeps to feed its larvae. This fly is a species that depends on oil seeps for its existence. The petroleum fly larvae feed on bacteria that eat oil, and the flies supplement their diet with organic matter that falls into the sticky seeps along with other insects and plant debris. The petroleum fly is just one example of the biota associated with oil seeps.

Contrary to expectations, natural oil in small quantities provides valuable habitat in California. Only when large volumes are rapidly spilled in areas unaccustomed to oil do we see the negative effects of oil pollution.

Related Web Sites
Natural Oil and Gas Seeps in California
U.S. Geological Survey (USGS)

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