New Report by Agency Lowers Estimates of Natural Gas in U.S.

WASHINGTON — Just how much natural gas is trapped underground in the United States?

The difficulty and uncertainty in predicting natural gas resources was underscored last week when the Energy Information Administration released a report containing sharply lower estimates.

The agency estimated that there are 482 trillion cubic feet of shale gas in the United States, down from the 2011 estimate of 827 trillion cubic feet — a drop of more than 40 percent. The report also said the Marcellus region, a rock formation under parts of New York, Ohio, Pennsylvania and West Virginia, contained 141 trillion cubic feet of gas. That represents a 66 percent drop from the 410 trillion cubic feet estimate offered in the agency’s last report.

The Energy Information Administration said the sharp downward revisions to its estimates were informed by more data. “Drilling in the Marcellus accelerated rapidly in 2010 and 2011, so that there is far more information available today than a year ago,” its report said. Jonathan Cogan, a spokesman for the agency, added that Pennsylvania had made far more data available than in previous years.

Under the agency’s new estimates, the Marcellus shale, which was previously thought to hold enough gas to meet the entire nation’s demand for 17 years at current consumption rates, contains instead a six-year supply. The report comes just five months after the United States Geological Survey released its own estimate of 84 trillion cubic feet for the Marcellus shale.

The estimates are important because they underpin policy decisions on energy subsidies and exports. Market analysts look to these estimates in making investment decisions. Historically, they have varied widely based on assumptions about the future of technology, coming regulations on drilling and the long-term price of gas.

Previously inaccessible, shale gas has been unlocked in recent years by advances in a drilling technology known as hydraulic fracturing, or hydrofracking. These advances have prompted a drilling frenzy in states like Louisiana, Pennsylvania and Texas, which has helped create tens of thousands of jobs, lowered energy prices for consumers and offered the promise of lessening American dependence on foreign energy.

Despite the lower estimates, the agency’s report noted that shale gas would continue to have a growing impact on the broader energy market. The share of natural gas produced by drilling in shale formations is projected to more than double, from 23 percent in 2010 to 49 percent in 2035, the report said. The United States will also become a net exporter of liquefied natural gas by 2016, while natural gas prices are expected to remain low for more than a decade, according to the report.

Energy companies are also likely to be undaunted by the new lower estimates because they are confident that whatever the total amount of available gas, technology will improve over time so that they can access the gas more efficiently and profitably. This assumption depends on the price of gas rising soon from the rock-bottom levels where it has lingered since late 2008.

That prediction includes gas from shale wells, offshore wells and Alaska’s North Slope. But many energy experts question these types of projections because they include substantial amounts of natural gas that many scientists and engineers say may never be tapped.

Drilling proponents, including investors and many politicians, tend to embrace optimistic projections, even though estimating resources is an inexact science.

Some of the earliest and most optimistic estimates of gas resources have come from academia. In 2009, Terry Engelder, a geosciences professor at Pennsylvania State University, helped accelerate the rush to drill for natural gas in Pennsylvania and surrounding states by projecting that more than 500 trillion cubic feet of natural gas could be produced from the Marcellus.

This estimate is more than three times as high as the estimate for the Marcellus region from Energy Information Administration, and it is higher than what federal energy officials now say can be found in the entire country.

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Mr. Engelder said last week that he stood by his estimates, citing assumptions that he believed remained reasonable, and he questioned whether federal officials were being too conservative.

“I don’t know what E.I.A. did other than cave into peer pressure from the U.S.G.S.,” he said, referring to the United States Geological Survey, which released its new estimates in August.

Energy companies, which use different formulas for calculating their numbers, tend to present even higher estimates.

For example, more than three dozen companies have leased land in the Marcellus region, and each company provides its own estimates to investors for how much gas it believes can be found under the acreage they possess. The combined resource estimates of just two of those companies, Range Resources and Chesapeake Energy, whose acreage holdings represent a small fraction of the Marcellus shale, is roughly equal to the amount federal energy officials now say can be found in the entire region.

Many experts note that it is hard to predict how difficult or easy it will be to extract gas in different parts of the country because the geology varies drastically.

Since the hydrofracking boom is fairly recent, there is also a shortage of data to indicate how much gas wells will produce over the long term.

In private discussions, some federal energy officials have raised questions about the way oil and gas companies may be inflating estimates of the amount of recoverable gas.

“The variability of shale gas well performance is crucial to any assessment of the resource potential of a shale play,” Philip Budzik, an Energy Information Administration research analyst, wrote in an e-mail to an industry analyst last April.

“Companies highlight their highly productive and profitable wells,” Mr. Budzik wrote, “while ignoring their ‘dogs,’ thereby giving the public the impression that every well is a ‘gold mine.’ ” The information administration declined to comment about the e-mail.

Last summer, the New York attorney general and the Securities and Exchange Commission sent subpoenas to several companies to see whether they were accurately portraying the amount of recoverable gas to investors. The offices declined to comment about the subpoenas.

A version of this news analysis appears in print on January 29, 2012, on Page A16 of the New York edition with the headline: New Report by Agency Lowers Estimates Of Natural Gas in U.S. Order Reprints|Today's Paper|Subscribe