Germany imposed limits on fracking on June 24, dealing a blow to efforts to develop shale gas. Under legislation passed by its lower house of Parliament, fracking will be banned in clay formations, which typically lie between 1,000 meters (m) and 2,500 m deep.

Scientific test drilling will be allowed, but only with the permission of the relevant state government and under the watch of independent experts.

Fracking for deep-lying or "tight" gas, typically 4,000 m to 5,000 m deep, which has been done for more than 30 years in Germany, will continue, but under more stringent regulation.

In tightening its rules on fracking, Germany follows France, whic banned the practice, and the Netherlands, which last year introduced a moratorium on shale exploration until 2020.

The German ban is indefinite, but Parliament will reassess it in 2021 under a compromise reached between the conservative Christian Democrats (CDU) and the left-wing Social Democrats (SPD).

Fracking involves blasting chemicals and water into rock formations to release trapped gas. Opponents cite the risk of triggering earthquakes and contaminating drinking water.

Germany's gas industry has warned restricting fracking could increase the country's dependence on imported energy, much of which it imports from Russia.

"Shale gas is an important option, where we have practically given away our chances with this legislative package," said Christoph Loewer, managing director of German oil and gas association BVEG in Hanover.

Germany could extract between 320 billion cubic meters (Bcm) and 2.03 Tcm of gas from depths below 1,000 m in northern Germany, according to the Federal Institute for Geosciences (BGR). That potential amount dwarfs the 110 Bcm of conventionally available gas still left, according to estimates by BGR.

Last year, Germany extracted 9% of its domestic gas demand. Companies involved include BEB Erdgas and Erdoel, Mobil Erdgas-Erdoel, GDF Suez E&P Deutschland, Wintershall and Dea.