Hilcorp Alaska on Saturday discovered a crude oil leak from an undersea pipeline connecting two production platforms in western Cook Inlet.

The oil leak is on the west side of the inlet and is unrelated to the company's ongoing natural gas leak on the eastern side of the basin near Nikiski.

The oil leak – southwest of Tyonek near Granite Point — was discovered at 11:20 a.m. Saturday after workers on the Anna Platform "felt an impact" and spotted an oil sheen, according to the Alaska Department of Environmental Conservation.

"In looking over the side of the platform to see what was going on, personnel observed sheen and bubbles coming up from near one of the platform legs where the 8-inch line is located," the state agency said in a situation report issued Saturday night. "Personnel confirm that the bubbles or sheen were not observed the day prior."

What caused the impact is not known.

"We are not sure if it was ice, or something else," said Kristin Ryan, director of DEC's Spill Prevention and Response division.

An overflight at 12:30 p.m. spotted "six sheens, with the furthest sheen being 3.5 miles south of the Anna Platform," the agency said.

The largest sheen was 10 feet by 12 feet, the state agency said. Two other sheens were about 3.5 feet by 22.5 feet.

The size of the leak is unknown, Ryan said.

The oil capacity in the pipeline is 461 barrels, and the line is at full capacity, the agency said.

The crude oil pipeline runs between the Anna and Bruce platforms. Production at the platforms was shut down Saturday to stop the flow of oil through the pipeline.

"Upon discovering the sheen, Hilcorp activated their response contractor, shut in the Anna and Bruce platforms, and reduced the line pressure from 70 psi to 5 psi," the agency said.

The reduction in pressure should lead to a "much reduced" leak rate, Ryan said. The line is 75 feet underwater.

"During the 1:30 p.m. overflight no sheen was observed," the agency said.

"The oil spill response vessel Perseverance arrived at the Anna Platform around 12:45 p.m. and scouted the area for sheens with no success," DEC said.

The oil leak is unrelated to the natural gas leak occurring on the east side of the inlet, about 3 1/2 miles northwest of Nikiski. That gas leak was discovered on Feb. 7, on an 8-inch pipeline running between an onshore facility and the A Platform built in the mid-1960s.

Hilcorp has delayed repairs on that leaking gas line, saying it's too dangerous with sea ice choking the inlet. The company was hoping to send divers down next week to fix the gas leak, if the sea ice clears enough.

A repair by divers also won't happen immediately for the oil leak.

"Hilcorp has hired a diving contractor to investigate the line and conduct repairs. It is anticipated that this work can be conducted late next week," DEC said.

Hilcorp reported the leak to DEC at 12:05 p.m. today.

"At this time, the cause of the release is unknown," the company said in a statement sent by Lori Nelson, of Hilcorp Alaska. "Emergency response protocols have been initiated. Further updates will be submitted when they become available."

The company is the dominant producer in Cook Inlet, after arriving in Alaska in 2011 and rapidly acquiring facilities, many of them old. The Alaska Oil and Gas Conservation Commission has hit the Houston, Texas, company with multiple fines and warnings for its practices in Cook Inlet and on the North Slope, saying repeatedly it has a "history of noncompliance" in Alaska.

The DEC said Saturday that "Hilcorp believes that the leak source may be from a flange located within the first 50 feet of the 8-inch line coming off from the Anna Platform."

On Sunday, Hilcorp plans to send a foam pig through the line past the suspect flange — or joint. The pig would act like a squeegee, with the goal of pushing the oil past the trouble spot and cutting off the leak site, said Ryan.

After that first step is taken, the company will "reassess if driving the pig further down the pipe is warranted," the DEC said.

The DEC and the U.S. Coast Guard will send personnel to the Anna Platform Sunday to monitor that effort, Ryan said. Cook Inlet Spill Response Inc., an oil spill response contractor, will also have vessels in the water, she said.

An Incident Command Post is being established at CISPRI offices in Nikiski, the agency said.

Bob Shavelson with watchdog group Cook Inletkeeper said potentially many thousand gallons of oil could have leaked into waters occupied by endangered Cook Inlet beluga whales, salmon and other animals.

"It's unbelievable," he said.