* Vast area divided between energy development, conservation
* Reserve home to huge caribou herds, migratory birds
* Republican senator says plan locks up gas reserves
By Ros Krasny
WASHINGTON, Dec 19 (Reuters) - The U.S. federal government on Wednesday announced a plan to manage energy drilling on part of Alaska's North Slope, with the 23-million-acre National Petroleum Reserve to be divided between areas available for oil and gas leases and those that are protected from development.
The announcement by U.S. Interior Secretary Ken Salazar followed the completion of an environmental impact study, which recommended development of areas that contain about 72 percent of the estimated "economically recoverable" oil in the reserve.
The move drew criticism from Alaska Sen. Lisa Murkowski, who said the Obama administration had not gone far enough to open up oil and natural gas resources in the area.
Salazar said the plan as conceived would allow for the potential construction of pipelines carrying oil or gas from operations in the Chukchi and Beaufort Seas through the NPR region, also known as the Western Arctic Reserve.
The "balanced approach" would help protect "significant caribou herds, migratory bird habitat and sensitive coastal resources that are critically important to the culture and subsistence lifestyle of Alaska Natives," Salazar said.
The administration has authorized 177 oil and gas leases in the reserve since May 2011, covering some 1.4 million acres. So far only exploratory drilling has occurred.
Under Wednesday's blueprint 11.8 million acres would be open for development, which are estimated to hold 549 million barrels of economically recoverable oil and 8.7 trillion cubic feet of economically recoverable natural gas.
Murkowski, the top Republican on the Senate Energy Committee, takes the view that the reserve's legal purpose is to provide petroleum to the United States to ensure the nation's energy security.
The administration's plan "locks up 83.5 percent of the likely natural gas in the reserve. That is totally unacceptable," she said.
The reserve is the largest single tract of public land in the country, roughly the size of Indiana, and is managed by the Bureau of Land Management.
Several environmentalists said they liked the plan, which protects the area around the North Slope's biggest lake.
"This is a nice holiday present for America's waterfowl hunters and bird watchers," said Adam Kolton, executive director of the National Wildlife Federation's national advocacy center, who praised the administration's "balanced approach."
But Brendan Cummings, a senior attorney for the Center for Biological Diversity, decried the decision as "unfortunate".
"The Obama administration seems set on turning most of America's Arctic into a sacrifice zone for the benefit of oil companies," he said in an email.