Even though U.S. natural gas prices have bounced away from the decade lows of last April, the country's Securities and Exchange Commission requires companies to calculate and report year-end oil and gas reserves using 12-month average prices.

Last year, the average price for natural gas at delivery point Henry Hub was $2.77 per million British thermal units, 30 percent below a year before, as big supplies from shale gas fields continued to weigh, according to Reuters data.

There is some dark irony here, since it was the industry itself that lobbied to change the SEC rule so that, from 2009, they could use a 12-month average instead of the year-end price.

This sharp price markdown will translate into big cuts to estimates of proved reserves. Reserves are important because they are used in determining the value of a company and are used in pricing loans.

"This is going to be a real issue for the companies with a lot of exposure to natural gas," said Neal Dingmann, oil and gas analyst with Suntrust Robinson Humphrey.

U.S. exploration and production companies likely to see large revisions include Ultra Petroleum Corp (UPL.N), Cabot Oil and Gas Corp (COG.N), Southwestern Energy Co (SWN.N) and Chesapeake Energy Corp (CHK.N), analysts said.

Chesapeake and its Oklahoma City rival, Devon Energy Corp (DVN.N), have already taken gas reserves off the books, in a preview of what to expect when companies in the sector issue annual reports over the next six weeks.

Raymond James analyst Andrew Coleman said the write-offs meant few attentive oil and gas investors would be hugely surprised by the reserves impact.

The bigger question is when a solid recovery in natural gas prices could take hold. "We don't think it's going to happen any time soon," said Coleman, whose firm anticipates a larger glut of gas than most.

A Thomson Reuters StarMine analysis supports this view, with analysts' 2013 profit estimates for major U.S. gas producers cut sharply in the past month. While analysts give no predictions for reserves, their pessimistic outlook for earnings indicates no dramatic recovery in natural gas prices in the near term.

The data also show the various challenges for companies based on their gas dependence and size, with larger players enjoying more flexibility and suffering less of a hit to share prices. Southwestern remains an anomaly in terms of its financial performance, since it has no significant debt and consistently produces gas at the lowest cost in the country.

LOSING MONEY

Production from shale fields such as Marcellus in Pennsylvania and West Virginia, Haynesville in Louisiana, Fayetteville in Arkansas, and new fields in Ohio and elsewhere, have kept gas priced well below the $4 mark that the companies often cite as a benchmark for healthy returns.

At least two chief executives of big oil companies with significant shale gas investments have already complained of losing money. Rex Tillerson, CEO of the largest U.S. natural gas producer, Exxon Mobil Corp (XOM.N), said: "We are all losing our shirts today," when talking about natural gas prices.

About 20 percent of Exxon's proved natural gas reserves are in the United States, according to its annual SEC filing.

Christophe de Margerie, CEO of France's Total SA (TOTF.PA) told French newspaper Le Monde that its investment in the Barnett Shale in Texas "shows a serious loss," ominous words that may spell trouble for operators in North Texas gas fields.

In its third-quarter earnings report, Chesapeake said it had already removed some of its reserves in the Barnett and Haynesville Shale due to low prices in the first nine months of the year. The company is due to report its full-year earnings on February 21.

(Reporting By Anna Driver and Braden Reddall; Editing by Patricia Kranz and Sofina Mirza-Reid)

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