A study says about two million barrels of oil from the 2010 spill at a BP oil rig in the Gulf of Mexico has settled on the ocean floor, a claim BP rejects.
Around 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.
The fate of two million of the nearly five million barrels that gushed into the open waters has remained a mystery until now, said the findings in the Proceedings of the National Academy of Sciences, a peer-reviewed US journal.
Researchers analysed samples collected at more than 500 locations around the Macondo Well, where the leaked oil emerged.
The oil was found to have spread as far as 3200 square kilometres from the site, and may have gone even further, the report said.
"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, Santa Barbara, the Woods Hole Oceanographic Institution in Massachusetts, and the University of California, Irvine.
The study was released on Monday.
Researchers came to this conclusion by studying seafloor 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 impacted area was overestimated.
"The authors failed to identify the source of the oil, leading them to grossly overstate the amount of residual Macondo oil on the sea floor and the geographic area in which it is found," said a statement from BP spokesman Jason Ryan.
"Instead of using rigorous chemical fingerprinting to identify the oil, the authors used a single compound that is also found in every natural oil seep in the Gulf of Mexico, causing them to find false positives all over the sea floor."
According to the National Science Foundation, which funded the study, "hopane was concentrated in a thin layer at the sea floor within 25 miles (40km) of the ruptured well, clearly implicating Deepwater Horizon as the source."
Study author David Valentine of the University of California, Santa Barbara, said the process likely led to the damage of 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 said earlier this year that scientific studies on 14 different types of creatures affected by the spill show that long-lasting harm was done to dolphins, sea turtles, tuna, loons and other animals in the region.
In pleading guilty to the spill, BP agreed to pay the government $US4.5 billion ($A4.87 billion) to settle criminal charges in the case.
It also agreed in 2012 to settle damage claims by businesses and individuals for about $7.8 billion.
Last month, a federal court judge in New Orleans concluded that BP acted with "gross negligence" ahead of the massive spill, meaning it could face billions of dollars in new fines.
The April 20, 2010 Deepwater Horizon drilling rig blowout, which killed 11 people, happened because BP's US subsidiaries, along with oil-services company Halliburton and rig owner Transocean, did not take adequate care in drilling a risky well, the court found.