Further complicating matters, Shell was also negotiating to trade half of its 55% stake in Sakhalin II with state-owned gas monopoly Gazprom for half of a giant natural gas field called Zapolyarnoye.

FORBES: Shell Shocked

The fate of that deal is now on hold, and Russia has since excluded Shell from the international consortium developing the giant Shtokman gas field in the Barents Sea.

FORBES: Companies, People, Ideas

The rest comes from spots like Azerbaijan, Angola, Algeria and Iran, where it weathered a bribery scandal in 2004 and is developing the giant South Pars gas field.

FORBES: The Government Oil Play

The oil giant BP says a gas field it runs in partnership with Norwegians and Algerians was attacked.

BBC: Map of Algeria showing In Amenas

Rhodri Thomas, an analyst at Wood Mackenzie, an energy consultancy, says the geology of the Polish shale is similar to that of the Barnett Shale, a giant field in Texas that now supplies about 7% of America's gas.

ECONOMIST: The hunt for shale gas in Europe