Several major integrated oil and gas companies are in a rough spot -- they just can't seem to meet production goals. In 2013, ExxonMobil (NYSE:XOM) expects total production to decline by about 1%, and it isn't the only one hurting. The one distinct advantage these big oil companies have is a mountain of cash to buy a company or two. It is the fastest way to get a boost in production without rolling the dice on a new speculative play.

The one risk a company has with an acquisition is that it could overpay for the asset, and then the production gains would be offset by the price tag. Let's take a look at a simple calculation that can help evaluate the value of a company, and then see what natural gas companies could be had for a deep discount.

While there are certainly some very complicated methods for evaluating an energy company, a quick and dirty method is to see how the enterprise value of the company (all equity and debt minus cash) compares to the total proved reserves on the company's books. For example, when BHP Billiton (NYSE:BHP) bought independent gas company Petrohawk back in 2011, it paid \$12.1 billion for a company that had 3.4 trillion cubic feet of natural gas in proven reserves. This means that the company paid about \$3.55 per thousand cubic feet of natural gas for the company's reserves. Based on an S&P Capital IQ screen of exploration and production companies with a total enterprise value of \$1 billion-\$15 billion, an average company in this space would have an enterprise value per thousand cubic feet equivalent of \$6.65. So based on this metric, its seems as though BHP got a pretty good deal.

Since oil and gas price spreads have deviated so far from a BTU-equivalency basis, its not as effective to use this metric when evaluating oil-heavy companies. If you want to do your own calculations, be sure to use per-barrel oil equivalency for oil-heavy companies. Also, If you want to see a couple liquids-heavy assets that could be acquisition targets, click here. Based on this calculation, here are three natural gas companies trading at a deep discount:

Ultra Petroleum
Ultra certainly hasn't seen any love lately. Natural gas prices have fallen, and so has the share price of Ultra. Despite being one of the low-cost producers in the space, Ultra has an enterprise value per thousand cubic feet equivalent of about \$1.67. Not only is the company valued much lower than its peers, but it's value is less than half of what a thousand cubic feet of natural gas trades for at the Henry Hub spot price.

What is even more surprising about this low price tag is that the company may be sitting on much, much more gas than what is on its proven reserves. The company has just barely begun to tap its 260,000 acres in the Marcellus Shale, and its runway for the Jonah field in Wyoming is proving to be quite long. With a total enterprise value of \$4.9 billion, it could be a ideal pickup for a company like Chevron, which has stated that it hopes to boost production by 20% between now and 2017 through a large push in unconventional shale plays like the Marcellus.

Perhaps a company like Advantage doesn't get its due because all of its operations take place in Canada, but it doesn't preclude the fact that this company is valued at a bargain basement price of \$1.20 per thousand cubic feet of gas. This modest \$1 billion natural gas company is located right in the heart of the Montney Shale play in Alberta, and although the company touts just under 1 trillion cubic feet equivalent in proved reserves, the company believes its assets in the Montney hold over 10 trillion cubic feet equivalent.

A company like Advantage should be right in the wheelhouse of integrated oil and gas giants. It is an undeveloped producer with favorable assets in a shale play that could contain enough gas to supply North America for over a century. A company like Exxon, which just completed a deal for Celtic Exploration, a company similar to Advantage, should be familiar with the potential of companies in this region.

WPX Energy (NYSE:WPX)
Of all the independent oil and gas companies out there, you will be hard pressed to find a company that is valued as low as WPX. Right now, the company is valued at exactly \$1.00 per thousand cubic feet equivalent. Of all the companies that fit the parameters mentioned above, it has the lowest price. What also sets this company apart from the other two on this list is that it has a decent liquids profile as well: 27% of its reserves are in oil and natural gas liquids, and the location of these assets are pretty attractive as well.

Despite oil only being 12% of the company's proved reserves, those reserves come in one of the premier unconventional shale plays in the U.S.: the Bakken. Its 80 million barrels of proved reserves are equal to the reserves of Kodiak Oil & Gas, yet WPX is valued at one-sixth the value of Kodiak. Granted, the lower price tag is mostly from the gas assets on the books, but WPX has strong positions in the Marcellus, Niobrara, and the Piceance Basin, possibly part of the largest source of oil in the world.

What a Fool believes
Using the enterprise value per thousand cubic feet equivalent is a shortcut method for evaluating a company. While it can be effective for quickly pointing out potentially undervalued companies, it should not replace a more thorough analysis.

Fool contributor Tyler Crowe has no position in any stocks mentioned. You can follow him at Fool.com under the handle TMFDirtyBird, on Google +, or on Twitter, @TylerCroweFool.

The Motley Fool recommends Chevron and Ultra Petroleum. The Motley Fool owns shares of Ultra Petroleum and has the following options: Long Jan 2014 \$30 Calls on Ultra Petroleum, Long Jan 2014 \$40 Calls on Ultra Petroleum, and Long Jan 2014 \$50 Calls on Ultra Petroleum. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.