Oil naturally seeps out of the ocean floor in areas known as seeps. One in particular, off the coast of California is well known. It is the Santa Barbara Oil Seep, and was known by the Indians that lived on the coast.
The oil seep has been quantified by researchers in this paper. The authors claim that 100 barrels of oil per day seep into the ocean.
According to Wikipedia, the Valdez reportedly released 10.8 million gallons of oil into Prince William Sound. This is equivalent to 257,000 barrels.
Thus, the Santa Barbara oil seep leeks equivalent to the Valdez every 2600 days, or 7.0 years.
And check out this article from the Christian Science Monitor on the same subject.