Human Use Causes Much More Ocean Oil Than Drilling Accidents

From "Oil and Pollution in the Ocean" by the National Academy of Science, National Research Council:

accidental spills from platforms represent about 1 percent of petroleum discharged in North American waters and about 3 percent worldwide.

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...surprising to many, is that oil from individual cars and boats, lawn mowers, jet skis, marine vessels, and airplanes contribute the most oil pollution to the ocean. This includes land runoff from oil slicks on urban roads and hydrocarbons deposited from the atmosphere. According to the report's estimates, use-related oil pollution dwarfs that from oil and gas production activities, accounting for about 87 percent of the oil from human activity in North American waters.

Earlier in the report:

natural seeps are the largest single source of oil in the sea, accounting for about 60 percent of the total in North American waters and 45 percemt worldwide. Seeps form when crude oil oozes into the water from geologic formations beneath the seafloor. Oil and gas extraction activities are often concentrated in regions where seeps form.