An underwater oil leak has prompted Chevron and its rig contractor Transocean to halt drilling a deep-water appraisal well about 75 miles off the coast of Brazil, the companies said Tuesday.

Satellite images show oil spreading about 100 miles south and southeast of Chevron’s Frade project. a field that is producing 79,000 barrels a day of crude oil. Chevron estimated the size of the spill at 400 to 650 barrels, but an environmental Web site called Sky Truth said satellite images suggest that nearly 15,000 barrels have spread on the ocean surface.

Chevron blamed the sheen on “oil seeps,” but it acknowledged that its well “was suspected to be contributing to oil being expressed through seep lines located on the ocean floor.”

How it might be contributing remained unclear. Transocean spokesman Guy Cantwell said there was “absolutely no” blowout and that its rig workers are plugging the well. Chevron spokesman Kurt Glaubitz said that the leak was “related to the drilling of the well” but would add only that “we continue to investigate the cause of the seeps.”

Some analysts speculated that there might have been a failure of the well casing, the steel pipe that lines the well and is supposed to be able to withstand high pressures. Neither company would comment on that.

Chevron’s description of “seep lines” suggested that there might be fault lines in the rock near the well, possibly caused by the drilling, analysts said. Reuters reported that Floriano Carvalho, director of the regulatory Brazilian National Agency of Petroleum, said Chevron’s drilling “raised pressure,” creating a crack in the rock from which oil began leaking to the surface.

John Amos, who writes the Sky Truth blog and who was one of the first analysts to realize the scope of the BP Gulf of Mexico spill in 2010, said that the leak “raises questions once again about deep-water drilling risk.This is Chevron, and Transocean. Major global players who are very active in U.S. waters”

Amos said that satellite images from NASA taken three days ago show an apparent oil slick originating from the Chevron drilling location and extending across 918 square miles. At a thickness of one micron, that works out to a spill volume of 628,000 gallons, or 14,954 barrels, Amos said. Chevron said a fleet of 17 vessels was working to control the leaks. Eight vessels were working at the sheen, with six vessels working in pairs to contain and recover the oil near source and two boats working on “mechanical dispersion” toward the tail of the sheen.

The spill appears to have started Nov. 8, suggesting a daily rate of 3,738 barrels, he added.