Oil spills mostly occur when a tanker goes on a reef or something and this makes an enormous leakage. One of the biggest releases of oil ever was during the gulf war when Iraq released an estimated amount of 11 million barrels of oil into the Persian Gulf; the oil formed a slick 45 km long and 12 km wide. This particularly slick ahs killed an estimated 30,000 birds.
The biggest problem wasn't all the oil itself, but the fact that The Persian Gulf is only 35 meters deep, and only has a narrow connection to the Indian Ocean, this means that it takes five and a half years for all the water in the gulf to be changed, so there was no hope for the oil dispersing.
The ecological impacts aren't the facts that the country looses money because of loss of tourism, but the fact that the thick oil goes in the skin of many birds. The oil simply destroys the waxed layers of a bird and this causes the bird not to be able to keep itself warm, and that's why most of the affected birds died of freezing
Long-term effects may be equally devastating. The soluble fraction of the spilled oil may spread over vast areas, and toxic components may create chronic damage to life. A study of spill effects in the Caribbean found that coral organisms were severely hurt and coastal environments such as birds were wiped out along with the creatures that inhabited them. A study of the Brittany coastline in the years following the Amoco Cadiz spill found massive death rates for such bottom-dwelling species as sea urchins, the practical elimination of some other species, and the overall reduction of animal numbers.