Helkogen was a highly toxic gas which was found beneath the surface of Hell. It was the planet's only natural resource and a means by which the natives Helkans could engage in trade. Because of its abundance, it was very cheap.
During the Second Dalek War, as part of Operation Genocide, the Daleks enslaved the Helkans to mine pockets of helkogen for the Death Wheel. The Daleks planned to release the gas above Hell and wipe out the entire Helkan population. The Seventh Doctor and Abslom Daak helped destroy the Death Wheel before the gas was released. (COMIC: Nemesis of the Daleks)