DOVER, Del. (AP) — State environmental officials on Wednesday were investigating an oil spill at a refinery that left a sheen floating on the Delaware River.

The refinery in Delaware City, operated by PBF Energy, reported the release about 1:30 a.m. Wednesday.

PBF spokesman Colin Murray said the refinery had deployed booms, absorbents and skimmers to contain the sheen.

"We're not sure exactly what it is," Murray said, adding that it was too early to say whether the material was coming from the refinery.

But David Small, deputy secretary of the Department of Natural Resources and Environmental Control, said the incident appeared to involve a leak in a heat exchanger that is part of the refinery's crude oil processing unit.

Small said an unknown quantity of petroleum got into the refinery's stormwater or wastewater collection system and through the wastewater treatment plant, which discharges into the river.

"Earlier, there was a bit of a sheen on the river that was observed," Small said. "They have boomed the spill, which is good. They have contained what they could see."

Small said he didn't know whether the spill involved crude oil or refined product. He said a wastewater official was on his way to the refinery Wednesday.

"There's a lot we don't know yet. We don't know quantities yet," Small said.

Department of natural resources spokesman Michael Globetti said the leak of what the refinery reported as "gas oil" caused a sheen measuring about 500 feet by 50 feet on the river.

A telephone message left for the refinery shift superintendent was not immediately returned Wednesday.

PBF bought the refinery in 2010 for $220 million after Valero Energy said it would halt operations there and lay off 550 workers.

Gov. Jack Markell's administration provided $20 million in incentives to PBF to restart the refinery and has supported the new crude oil transfer operation.

But state environmental groups have recently raised concerns about the potential harm a new crude oil transfer station at the refinery could cause.

However, Delaware's Coastal Zone Industrial Control Board last month dismissed a challenge filed by the Sierra Club and the Delaware Audubon Society to an air quality permit granted to PBF Energy in May.

The permit granted by state environmental secretary Collin O'Mara involves loading unrefined crude oil onto barges at Delaware City for shipment to another PBF refinery in Paulsboro, N.J. The barge loading operation is part of a larger transfer operation in which PBF brings crude oil by train from various sources, including tar sands oil from Canada, to a new rail loop it built near the refinery.

PBF has spent millions of dollars on the new rail facility and has said the crude oil transfer operation is critical to its long-term business strategy.

Last month, O'Mara issued more than half a million dollars in penalties against the Delaware City refinery for air pollution violations.

The order covers more than a dozen air quality violations since the 2011 restart of the refinery through January of this year.