The North Dakota government said there's been a release of material used to produce oil and gas into a creek in the heart of the country's oil developments.

The Health Department said it was notified by oil company Summit Midstream of a release of an unknown amount of brine into a creek downstream from Blacktail Lake in Williams County.

Energy companies inject brine, or salt water, to improve oil and gas production from shale deposits.

"An environmental contractor for the responsible party is on scene for cleanup and remediation," the Health Department said in a statement Thursday.

The Environmental Protection Agency said brine may contain toxic metals and radioactive substances that can be "very damaging" to the environment and public health if released on the surface.

There was no statement on the release from Summit Midstream. The company in July said it reached an agreement with North Dakota Pipeline Co., a subsidiary of Enbridge, to build a new crude oil pipeline structure associated with the Bakken shale oil play in the state.

North Dakota lies at the heart of the shale boom in the United States. Oil production in September, the last full month for which data are available, was 1.18 million barrels per day on average, with nearly of that coming from the Bakken reserve area.