The world's largest publicly traded company by market capitalization said last week it has acquired about 2 million acres of coalbed-methane resources in Germany, its first foray into the exploration of gas trapped in coal seams in Europe.

For centuries, coalbed methane was a safety hazard for coal miners, causing explosions and deaths from asphyxiation. Now, the prospects for traditional fossil-fuel reserves are diminishing, intensifying the majors' hunt for new, unconventional resources worldwide, including coalbed methane.

Coalbed methane, or CBM, already accounts for about 10% of the natural gas produced in the U.S., where independent producers--which, unlike oil majors, don't have refining operations--have been at the forefront of CBM production and technology development. Smaller companies also have been exploiting CBM in Europe, Japan and Australia.

Now, large, international energy companies are rapidly adding CBM and other unconventional natural gas resources, such as shale and tight gas, which are harder and more expensive to extract than conventional gas, because these resources usually have a longer lifespan than normal gas reservoirs. Major oil companies are also trying to be the first to export the expertise and technology developed in recent years in North America to international markets.