BP oil spill from Deepwater Horizon spread over huge area of ocean floor

About two million barrels of oil from the BP spill off the US Gulf Coast in 2010 are believed to have settled on the ocean floor, according to a study.

What happened to two million of the nearly five million barrels from the Deepwater Horizon rig that gushed into the open waters was a mystery until now, said the findings on Monday in the Proceedings of the National Academy of Sciences journal.

Researchers analysed samples collected at more than 500 locations around the Macondo Well, where the leaked oil emerged, and found it had spread across the sea floor.

The oil was found to have moved over an area of 3,200 sqkm from the site, and might have gone even further.

"Our analysis suggests the oil initially was suspended in deep waters and then settled to the underlying sea floor," said the study by the University of California and the Woods Hole Oceanographic Institution in Massachusetts. Researchers came to this conclusion by studying sea-floor sediment cores for residual hopane, a hydrocarbon that comes from crude oil.

BP took issue with the findings and the method researchers used, saying the affected area was overestimated.

"Instead of using rigorous chemical fingerprinting to identify the oil, the authors used a single compound found in every natural seep in the Gulf of Mexico, causing them to find false positives all over the sea floor," BP spokesman Jason Ryan said.

According to the National Science Foundation, which funded the study, "hopane was concentrated in a thin layer at the sea floor within [40km] of the ruptured well, clearly implicating Deepwater Horizon as the source."

Study author David Valentine said the process possibly damaged deep-sea corals.

"The pattern of contamination we observe is fully consistent with the Deepwater Horizon event but not with natural seeps, the suggested alternative."

The National Wildlife Federation has said that studies showed long-lasting harm to dolphins, sea turtles, tuna, loons and other animals.

In pleading guilty to the spill, BP agreed to pay the government US$4.5 billion to settle criminal charges. It also agreed settle damage claims by businesses and individuals for about US$7.8 billion.

Last month, a judge in New Orleans ruled that BP acted with "gross negligence" ahead of the spill, meaning BP may face billions of dollars in new fines.