WASHINGTON -- About half the oil and more than a quarter of the natural gas beneath 99 million acres of federal land is off-limits to drilling, the Bush administration says in a report that industry sought to highlight environmental and other hurdles to development.

Just 3 percent of the oil and 13 percent of the gas under federal land is accessible under standard lease terms that require only basic protections for the environment and cultural resources, according to the survey, which was ordered last year by Congress.

An additional 46 percent of the oil and 60 percent of the gas "may be developed subject to additional restrictions" such as bans to protect winter rangeland for foraging antelope, nesting areas for bald eagles and jagged slopes from erosion during parts of the year.

The revised inventory, released Tuesday by the Interior Department's Bureau of Land Management, is starkly different from a study done three years ago. That version, which covered 59 million acres in the Rocky Mountains, estimated more than 80 percent of oil and gas was accessible, although in some cases subject to restrictions. Environmentalists often cited that figure in arguing that a wealth of energy resources is available for developing without going into pristine areas now off limits.

Energy companies disliked the earlier inventory because, they argued, it suggested more oil and gas was accessible, even when restrictions made drilling difficult or impossible. They lobbied Congress to force a change in the way the inventory is taken.

Language was inserted with little fanfare into energy legislation last year.

The BLM was required to include a much larger amount of land, while excluding from the inventory lease areas where drilling already was under way. The agency also was directed to take into account seasonal drilling bans to protect animal habitat, and consider such leases as significantly restricted.

All these changes reduced the percentage of oil and gas resources that can be viewed as open for energy development.

For example, the 2003 report found that 26 percent of the oil and 24 percent of the natural gas in the Rocky Mountains was subject to some restriction. The new survey says access in the Rockies is limited to 69 percent of the oil and 65 percent of the gas.

The new inventory covers 99 million acres -- 40 million more than the 2003 report -- from Alaska to the Appalachian Mountains. The area is estimated to hold 187 trillion cubic feet of natural gas and 21 billion barrels of oil, or 76 percent of onshore federal oil and gas resources.

The amount of oil considered accessible without limits declined by about two-thirds from 2.2 billion barrels to 743 million barrels under the new inventory. Accessible natural gas was cut by about the same proportion, from 87 trillion cubic feet to 25 trillion cubic feet.

To put those numbers in perspective, the United States imports about 12.4 million barrels of oil a day and uses about 21 million. It produces about 20 trillion cubic feet of natural gas annually.

The Arctic National Wildlife Refuge in Alaska (ANWR), which is off-limits to drilling and was included in the survey, is believed to have about 10.4 billion barrels of oil, while nearby Prudhoe Bay, the country's largest oil field, has produced more than 10 billion barrels since 1977.

Examined for the first time are three oil and gas basins in the Southeast and the Appalachian Mountains and a basin in northern Alaska, including ANWR, which is believed to have the largest untapped U.S. oil field -- one where leasing is not allowed.

By omitting federal land already being drilled without access restrictions, the inventory reduced the amount of resources considered accessible by 5 percent.

BLM Director Kathleen Clarke called the new inventory "a more complete and accurate picture" and a "helpful tool as we consider the road forward and the needs of the growing nation."

Environmentalists, who had used the 2003 study to argue against drilling in sensitive wild areas, said the new study gives an impression that more land is closed to energy development than really is the case. They said often the BLM waives the seasonal restrictions that the inventory cites as evidence of an area not being accessible.

"We are already seeing water quality diminished and key habitat in the West lost to oil and gas drilling and associated infrastructure," said Brian O'Donnell, public lands director for Trout Unlimited. The new inventory "equates clean water, wildlife habitat and the needs of hunters and anglers with red tape."

Industry officials said the new survey gives a more accurate view of the hurdles they face in tapping domestic energy resources.

"It will absolutely help in giving policymakers a better understanding of what (drillers) are facing," said Dan Naatz, a vice president of the Independent Petroleum Association of America.

---

On the Net:

Bureau of Land Management: www.blm.gov