A model has been developed to explain the composition of gas withdrawn from the Gasbuggy chimney following an underground nuclear explosion to stimulate the recovery of natural gas. The model assumes that homogeneous, gas-phase reactions occurred during cavity formation among the species formed from the volatile fraction of 1550 tons of formation per kiloton of explosive yield. After chimney collapse, additional heterogeneous reactions occurred involving this gas mixture and natural gas which significantly altered the composition. This work suggests two criteria for choosing the shot points for future explosions: the rock should be free of solid carbon, and relatively rich in carbonates. The application of these criteria is expected to reduce significantly the radioactive burden of T and 14C found in the hydrocarbons subsequently produced.