The Deepwater Horizon drilling rig, operated by BP Oil and owned by Transocean Ltd., exploded and started burning April 20. Eleven rig workers were never found and are presumed to have died.
Oil from the area is called light sweet crude, but Edward Overton, a professor emeritus of environmental science at Louisiana State University, said the name is deceptive. It contains heavy compounds, called asphaltenes, that do not burn easily or evaporate, even in the warm climate off Louisiana.
"When you've got a spill like this," said Overton, "there are three things you can do. You can burn it, scoop it up out of the water, or use chemical dispersants to break it up. This oil is not particularly good with any of those three."
"With light crude," he said, "you could burn most of it -- 70 or 80 percent. With heavy crude, I don't know. I'm not optimistic."
ABC News' Matt Gutman and Jake Tapper provided additional information. ABC News Radio and The Associated Press contributed to this report.