Samantha Joye of the University of Georgia told CNN researchers have taken samples from 2 miles to 80 miles from the well. She said the thickness of the oil ranges from 1/4 inch up to 2 inches.
"I think what we're seeing is oil that was on the surface, that has sedimented down to the bottom," Joye said in a telephone interview.
Joye said the research team will not be able to confirm the substance is oil and that it came from BP's well until it is analyzed in the laboratory.
The samples also showed few signs of the animals normally found on the seabed, including marine worms, Joye said.
Millions of gallons of crude oil spewed into the water after the April explosion and fire on the Deepwater Horizon drilling platform.
Samuel Walker of the National Oceanic and Atmospheric Administration said oil is common on the floor of the Gulf of Mexico, both from manmade sources and from natural seepage.