ExxonMobil's (NYSE: XOM) acquisition of XTO Energy officially ushered in the natural gas era. Two years later, Exxon is back at the bidding table with plans to double down on natural gas. Exxon's management even provided some clues about potential takeover targets.

The strategy
XTO Energy President Jack Williams recently revealed a fondness for "bolt-on transactions," which are the purchase of assets adjacent to Exxon's existing assets. He added about the U.S., "We think the market dynamics are set up to where you can potentially get good value, so we will continue to look." Exxon management has also expressed interest in natural gas liquid acquisitions because NGLs are currently more profitable than shale gas. Natural gas varies in its composition around the world from normally 80% to 95% methane with the rest composed of natural gas liquids propane, butane, and other hydrocarbons.

So those are Exxon's criteria. Now let's look at some companies that make the cut.

Takeover targets?
Exxon could be looking to add assets in the Barnett shale. The Barnett shale was the original shale gas play. However, many in the energy industry believe the Barnett is past its production prime. But Exxon doesn't.

According to company execs, the Barnett is one of Exxon's most profitable gas fields. Management also believes the region has years of significant productivity left. It's no wonder Exxon is committing more resources to the area. A Barnett-related acquisition could be in the offing, and Quicksilver Resources (NYSE: KWK) is a great candidate for two main reasons.

First, Quicksilver owns a considerable amount of productive acreage adjacent to Exxon's assets in the Barnett. This makes Quicksilver's assets a perfect bolt-on transaction for Exxon.   

The second reason is that much like recently acquired Petrohawk Energy, Quicksilver lacks financial footing to weather a prolonged period of depressed natural gas prices. This makes the company a prime takeover target.  

Exxon could be interested in gaining additional exposure to the Haynesville shale. Last year, Exxon increased production in the region fourfold, indicating an increasing interest in exploring the area.

A company of interest in the Haynesville could be EXCO Resources (NYSE: XCO). EXCO, along with Exxon, operates the most rigs in the portion of the Haynesville shale underlying Texas. This makes the company a good bolt-on acquisition.

EXCO also appears to be undervalued; at least that's what the CEO thinks. The CEO recently attempted to buy the company at nearly a 25% premium to its recent share price. However, the deal eventually fell through.

EOG Resources (NYSE: EOG) operates a significant number of rigs in the Texas portion of the Haynesville as well, making it another bolt-on takeover candidate.

EOG also holds a sizable position in the Eagle Ford shale with a varied production mix, which includes natural gas liquids. The company also has operations in China, aka, the sleeping giant of shale gas.  

Provident Energy (NYSE: PVX) could also be on Exxon's shopping list. The company is Canada's largest pure play natural gas liquids. Natural gas liquids closely track the price of crude oil, making them an attractive resource for Exxon. Takeover speculation isn't the only reason to own shares in Provident. The company offers a fat 6.2% yield as well, making it an attractive income investment.

The bottom line
Exxon is shopping for more natural gas, and speculative investors should take note. Many companies may match Exxon's criteria with some more perfectly than others. And it's reasonable to think Exxon will use its massive financial resources to make multiple acquisitions. Take a look at some of these names and others as well for potential investment opportunities.   

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Fool contributor Adam J. Crawford does not own shares in any company mentioned in this article. 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.