The Interior Department said Monday that it plans to block oil and gas leasing on about 11 million acres on Alaska’s North Slope, or roughly half of a 23-million acre reserve set aside for energy development decades ago.

The action, announced in connection with a federal lawsuit brought by environmentalists, would reverse a Trump administration effort to expand oil production in the National Petroleum Reserve in Alaska.

The reserve had been set aside for oil and gas development in the 1920s. Under former President Barack Obama, the federal government restricted oil and gas development to 11.8 million acres of the reserve.

The Trump administration moved to expand that to 18.6 million acres, saying developing the resources would improve the nation’s energy security and boost the Alaskan economy.