Where does most of the oil that pollutes the ocean come from?
Oil tankers and newly drilled or old wells uncontrolled
Oil tankers, if they are not careful, pollute sea environment due to spills of oil on ports (loading and unloading operations), deliberately cleaning up operations of their loading areas, and accidents. Some sunken tankers also pollute sea/ocean environment too.
Another pollution issue is new wells drilled. These wells cause pollution if they are not careful enough. One of the worst disasters occurred a couple of years ago in South of the USA. (I do not want to give the name of the company and location).
Old wells, if they are not taken care properly, cause oil pollution too.
Natural seeps of oil underneath the Earth's surface account for 60 percent of the estimated total load in North American waters and 40 percent worldwide, according to the National Academy of Sciences.