The state reports that over 7,000 gallons of oil leaked from a North Slope well owned by Caelus Energy.

Tom DeRuyter is overseeing the response to the leak with the Alaska Department of Environmental Conservation (DEC). He says DEC thinks most of the spill was contained to a gravel pad, but about three gallons of oil made it out to the tundra.

“I don’t want to minimize the size of this spill — that was a lot of oil that came out of this well,” said DeRuyter. “It was something that happened without notice and it took a long time to really understand what had happened and for everybody to react accordingly. And that is very unusual.”

Caelus first reported the spill in mid-June. Then, the company thought just 5 gallons had escaped a cellar surrounding the well.

“The spill itself was significantly larger than anybody had thought up front, and that led to the migration outside of the gravel pad,” said DeRuyter.

According to Caelus, the spill came from a well that was drilled and plugged in 2013.

The oil that leaked is not crude. Rather, it’s a mixture of fluids Caelus put in the well, described by the state as mineral oil and “diesel freeze protect fluid.”

The state thinks the incident happened after water filled the cellar and froze. Ice damaged a valve and allowed the oil to escape.

The state also reports that plugs in the bottom of the cellar were not in place, allowing the water to flow in. But DeRuyter said until the investigation is complete, he can’t say if human error was a factor.

Cleanup on the spill is ongoing.

Caelus did not agree to an interview for this story. In a statement, the company said it is working with DEC to minimize damage.

“We appreciate the hard work of our employees, contractors and the Alaska Department of Environmental Conservation on this incident,” spokesman Casey Sullivan said in a text message.

Caelus Energy is a small, private company based in Texas. It’s the company behind the potentially massive Smith Bay oil discovery announced in October.