Natural Oil Seepage Facts



Natural Oil Seep Information

Natural oil seepage is the greatest source of hydrocarbon pollution in the ocean and atmosphere.

According to a study by the National Research Council (NRC) of the U.S. National Academy of Sciences, “natural seeps are the highest contributors of petroleum hydrocarbons to the marine environment. Of the total load of petroleum hydrocarbons discharged into the sea, natural seeps account for the largest load, nearly 61 percent of the total. In southern California, natural seeps represent 98 percent of oil inputs to the offshore zone. In fact, the extensive system of natural seeps off Coal Oil Point accounts for nearly two-thirds of the total load received by coastal and offshore waters of the Pacific seaboard, including Hawaii.”



Offshore Santa Barbara has the second largest marine oil seeps in the world.

The largest natural oil and gas seeps in the Western Hemisphere lie in the Santa Barbara Channel. According to the California State Lands Commission, they comprise more than 1,200 of the over 2,000 active submarine seeps along the California coast. Half of these occur within 3 miles of an area called Coal Oil Point, located just west of Santa Barbara near the University of California, Santa Barbara (UCSB) campus.

It is estimated that oil seepage for a single 6-mile stretch, including Coal Oil Point, averages 10,000 gallons of oil each day (240 barrels). Every 12 months about 86,000 barrels of oil seep into the ocean—the equivalent of the quantity of oil spilled in the 1969 Santa Barbara oil spill. Since 1970, the quantity of oil that naturally seeps into the Santa Barbara Channel equals ~ 31 “1969” Santa Barbara oil spills.


How Natural Oil Seepage Starts

Crude oil and natural gas seep naturally out of fissures in the ocean seabed and eroding sedimentary rock. These seeps are natural springs where liquid and gaseous hydrocarbons leak out of the ground (like springs that ooze oil and gas instead of water). Whereas underground pools of water feed freshwater springs, natural gas and petroleum seeps are fed by natural underground accumulations of oil and natural gas. Natural oil seeps are used in identifying potential petroleum reserves. (Global Marine Oil Pollution Gateway)

How Natural Oil Seeps Work


How Natural Oil Seeps Spread

The gas and oil originate from a petroleum seepage reservoir 1,500 meters below the sea floor. They seep from the crests of anticlines exposed on the seafloor through as much as 60 meters of water to the surface. On the sea surface, locations of seeps are characterized by patches of bubbles and oil slicks that extend down-current.

Viewed at the sea surface, seeps range from being so diffuse that they are undetectable to appearing on the surface as areas of effervescence or boiling, measuring 1 to 10 meters in diameter. A large amount of seepage takes the form of gas bubbles that emerge from the seafloor, carrying a thin coating of oil on their surfaces. Seepage also occurs as discrete petroleum droplets and as tar that oozes out and forms tar mounds on the seafloor (Santa Barbara County Energy Division).


Effects of Oil Seepage on the Environment

Oil from natural seeps starts to undergo changes as soon as it is released.  Contact with water and air causes physical and chemical changes that may make it hard to recognize a natural oil seep as the source. This section describes what happens when natural seep petroleum and gas contact the ocean, beach, and air.