Natural Oil Seeps
http://www.whoi.edu/oil/natural-oil-seeps
As much as one half of the oil that enters the coastal environment comes from natural seeps of oil and natural gas. These geologic features are known to occur in clusters around the world, such as off the southern coast of California and in the Gulf of Mexico, but are still relatively unstudied. In recent years, advances in remote sensing have enabled more accurate detection and estimates of natural oil flows in the ocean.
In locations where seeps are found, oil flows slowly up through networks of cracks, forming springs of hydrocarbons similar to the La Brae tar pits on land. Lighter compounds rise buoyantly to the water’s surface and evaporate or become entrained in ocean currents; others fall to the seafloor and collect over hundreds or thousands of years.