Embedded in a 54,000 square mile patch of Alberta are an estimated 170 billion barrels of recoverable bitumen, a semi-solid form of crude oil. The Athabasca Oil Sands make Canada the world No. 2 in terms of crude oil reserves, trailing only Saudi Arabia.

The Athabasca deposit is the largest reservoir of crude bitumen in the world and the largest of three major oil sands deposits in Alberta, along with the nearby Peace River and Cold Lake deposits. It’s the only large oil sands reservoir in the world suitable for large-scale surface mining, though most of it can only be produced using more recently developed in-situ technology. These are highly cost-intensive processes.

But changes in the global demand profile over the last decade and the depletion of the easy-to-exploit major fields discovered in the 20th century have pushed oil prices to a new, permanently higher range. The downside for the per-barrel price oil, in other words, is a lot higher than it was five years ago, at a level that will likely leave high-cost oil sands production economic while we search for scalable alternatives.

Meanwhile, ExxonMobil’s (NYSE: XOM) USD41 billion takeover of XTO Energy (NYSE: XTO) and Total’s (NYSE: TOT) USD2.25 billion joint venture with Chesapeake Energy (NYSE: CHK) are the latest high-profile events in what’s likely to be a period of renewed excitement for unconventional natural gas.

XTO is one of the leaders in the business of extracting gas from shale and other dense geological formations. The existence of these potential resources has been known to geologists and producers for years. The problems were of economics and technology. But gas, spiked, too, along with oil, and that spurred new investment in development of horizontal drilling and hydraulic fracturing.

Demonstrating the importance of unconventional plays, in 2009 the US overtook Russia as the world’s largest producer of natural gas. According to Energy Dept data, America’s overall production for 2009 rose approximately 3.7 percent to an estimated 624 billion cubic meters of natural gas. According to the DoE, “Minimal hurricane disruptions and significant growth in production from onshore shale basins have contributed to the increase in domestic supply.”

Many of the massive reservoirs of natural gas held within shale formations were inaccessible until new advancements in extraction techniques such as hydrofracking allowed companies to drill deeper and reach farther.

According to some measures, the US alone is sitting on more than a century’s worth of natural gas at current rates of consumption. And because it emits half the greenhouse gases of coal, new unconventional reserves present the country–and the continent, for that matter–a unique opportunity to address concerns about energy security and climate change.

The hope in some quarters is that natural gas can serve as a bridge fuel to a low-carbon, sustainable energy future.

Here’s a look at several prominent unconventional gas plays.

According to a 2009 DoE report, the Marcellus Shale is the largest unconventional natural gas play in North America. The Marcellus covers approximately 95,000 square miles over six states in the US Northeast. With its proximity to the large northeast US natural gas market and expanding pipeline take-away capacity, Marcellus gas receives a premium price and has one of the lowest breakeven production costs in North America.

Longtime CE Portfolio holding Enerplus Resources (TSX: ERF-U, NYSE: ERF) entered the US shale gas fray in August 2009 with the purchase of a 21.5 percent working interest in approximately 540,000 gross acres of the Marcellus. Management’s best estimate places total reserves of approximately 1.4 trillion cubic feet net to Enerplus, nearly double the company’s existing booked reserves.

Enerplus plans to drill approximately 750 gross wells over the next five years and expects production volumes from its Marcellus assets to grow to 100 million cubic feet of natural gas by 2014.

The Barnett Shale was the focus of the first major operation to extract gas from shale. This formation deep below Texas was discovered by wildcatters in the 1950s. Geologists estimate the Barnett could hold around 30 trillion cubic feet. In 2007, the field produced 1.1 trillion cubic feet of gas, making it the second-largest source in the US. The Barnett now accounts for more than 6 percent of US natural gas production.

Future development depends on the ability of industry technicians to affect another technological advance: The problem is to extract gas in the highly populated Dallas/Fort Worth area without harming homes, schools, businesses and airports.

Researchers estimate that the Fayetteville Shale could hold as much as 20 trillion cubic feet of natural gas. The Fayetteville covers about 4,000 square miles on the Arkansas side of the Arkoma Basin.  The Fayetteville Shale Play Deposit is about 60 to 575 feet thick, and the depth ranges from 1,450 to 6,700 feet deep. 

Chesapeake Energy estimates that the Haynesville Shale could hold as much as 245 trillion cubic feet of natural gas. The Haynesville extends over 3 million acres, from East Texas to Northwest Louisiana and into Arkansas.

In additional to the pressure being abnormally high, the Haynesville is very deep and thick, which essentially “packs” more natural gas into the formation. Producing natural gas from the Haynesville involves drilling wells from 10,000 feet and to 13,000 feet deep. The Haynesville has been said by some to hold an estimated 250 trillion cubic feet of recoverable gas.

The Woodford Shale covers approximately 1,500 square miles in Southeastern Oklahoma. The shale is found at depths from 6,000 to 12,000 feet, and is generally 120 to 200 feet thick. There’s an estimated 4 trillion cubic feet of recoverable gas in the play.

Experts estimate that there’s about 250 trillion cubic feet of natural gas in Northeast British Columbia, of which 10 to 20 percent is recoverable. The Montney Shale, a deposit located deep below British Columbia and extending into Alberta, holds up to 50 trillion cubic feet of natural gas. The Horn River Shale–potentially one of the biggest unconventional plays on the continent–holds an estimated 200 trillion cubic feet.