From MSNBC:

It's unclear if the platform was producing when the explosion occurred, but the Coast Guard said that a mile-long sheen is now visible.

Mariner's platform is in 340 feet of water, which would make any spill response much easier than the response to BP's blown-out well. The BP well is about 200 miles east of the Mariner platform and lies at a depth of 5,000 feet. Crews were forced to rely on remote-operated vehicles to tend to the underwater equipment. Experts say shallow-water wells are much easier to handle.