There's quite a few studies, but generally it is believed that around half of all oil/gas "pollution" in the oceans is from natural seepage. Some studies estimate that natural seepage may significantly outpace human caused oil/gas pollution in the oceans.
Here's one such study:
http://www.springerlink.com/content/bya6g7r7ceebanrl/fulltext.html
Yes, I noted that a Chevron petroleum engineer is one of the authors, the other is from the USGS. This study appears to be well supported and is corroborated by similar studies.
Abstract:
Quote:
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Recent global estimates of crude-oil seepage rates suggest that about 47% of crude oil currently entering the marine environment is from natural seeps, whereas 53% results from leaks and spills during the extraction, transportation, refining, storage, and utilization of petroleum. The amount of natural crude-oil seepage is currently estimated to be 600,000 metric tons per year, with a range of uncertainty of 200,000 to 2,000,000 metric tons per year. Thus, natural oil seeps may be the single most important source of oil that enters the ocean, exceeding each of the various sources of crude oil that enters the ocean through its exploitation by humankind.
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