Source: Flickr user Seth Sawyers.

Natural gas has often seemed like an afterthought within the energy industry. However, as the cleaner-burning fuel has gained prominence over the past decade, so has the deal volume and value of natural gas mergers.

There have been several deals of note in recent years as energy companies rushed in to grab a piece of the shale gas action. While the enthusiasm for shale gas has died down as the price of gas has fallen, that has not stopped gas-related mergers from happening, nor should it stop future deals. Here's a look at some of the biggest deals of the past, and what deals could happen in the years ahead. 

Let's make a deal
As it does with oil, ExxonMobil (NYSE:XOM) holds the record for the largest natural gas merger in history. The company's $41 billion deal, including the assumption of debt, to acquire XTO Energy in 2009 was a big bet on the future of shale gas in America, as it made Exxon the top natural gas producer in the country. One of the reasons the deal was so large is because Exxon paid a huge 25% premium to acquire XTO Energy. However, Exxon's CEO Rex Tillerson justified the cost at the time by saying that the deal wasn't made for a near-term boost but was about the "next 10, 20, 30 years."

Prior to that, the largest natural gas-focused deal was ConocoPhillips' (NYSE:COP) $35.6 billion acquisition of Burlington Resources in 2005. At the time, the deal doubled the company's global natural gas production and turned ConocoPhillips into the nation's largest natural gas producer. While ConocoPhillips has since fallen down the leaderboard, the deal has provided the company with a lot of optionality to grow its natural gas production in the years ahead.

Other notable natural gas deals include Anadarko Petroleum's (NYSE:APC) combined $21.1 billion purchase of both Kerr-McGee and Western Gas Resources in 2006, which gave the company large natural gas resources in the U.S., and BHP Billiton's (NYSE:BHP) 2011 acquisition of Petrohawk for $12.1 billion. 

Another deal worth noting is Devon Energy's (NYSE:DVN) 2001 purchase of Mitchell Energy & Development for $3.1 billion. Mitchell was an early pioneer of hydraulic fracturing and, at the time of the deal, helped push Devon Energy up to the nation's No. 2 natural gas producer. Thanks to the knowledge and technology it gained from that deal, Devon helped launch the shale gas revolution, and it steadily grew its gas production over the next decade before shifting its focus to oil a couple of years ago.

Source: Flickr user Roy Luck

More wheeling and dealing on the way?
In more recent years, we've seen a number of smaller natural gas acquisitions as big oil giants have joined Exxon in gaining a foothold in promising U.S. shale gas plays by acquiring many of the small independents that pioneered these plays. In 2010, Chevron (NYSE:CVX) spent $4.3 billion to acquire Marcellus shale-focused Atlas Energy, while Royal Dutch Shell (NYSE:RDS.A) joined it in that play, spending $4.7 billion to buy East Resources.

Meanwhile, other smaller natural gas-focused companies like Southwestern Energy (NYSE:SWN) and Chesapeake Energy (OTC:CHKA.Q) steadily acquired their way up the natural gas leaderboard through leasing lots of acreage and complimenting that with smaller deals. However, last year, Southwestern made its biggest deal to date, paying over $5 billion to buy Chesapeake's southern Marcellus shale business. 

More deals could be in the works, especially as oil and gas prices are depressed, leading to a potential consolidation boom.

One name that always seems to top the list of potential merger candidates is Chesapeake Energy. Earlier this year, there were market rumors that Indian oil giant Oil and Natural Gas Corporation was looking at acquiring Chesapeake. A deal for Chesapeake might not challenge the Exxon/XTO record, as the company's current enterprise value is just over $20 billion, but it would instantly make any acquirer the second largest gas producer in the U.S.

Other large pure-play gas producers that could potentially make a compelling acquisition target for a larger rival are Southwestern at a $15 billion enterprise value, Ultra Petroleum (OTC:UPLM.Q) at $14 billion, and Range Resources (NYSE:RRC) at $12 billion. 

Investor takeaway
Natural gas' importance to the energy industry has grown in recent years thanks to the discovery of huge shale gas reserves in the U.S. Most of the early growth in shale gas was captured by smaller independents, many of which were subsequently scooped up by larger rivals. That trend could continue in the years ahead as another wave of consolidation could hit the industry thanks to currently weak oil and gas prices that are hampering returns and growth in the industry.