One or more specific embodiments herein includes a method for recovering hydrocarbon gas from a shale formation comprising injecting oxidizer through a horizontal wellbore into a fracture in the shale formation comprising pores containing hydrocarbon gas, and recovering some of the hydrocarbon gas from the shale formation. Some of the injected oxidizer combusts and increases the temperature of a portion of the shale formation and of the hydrocarbon gas contained in some of the pores in the shale formation, the pressure of some of the hydrocarbon gas increases to a point sufficient to cause formation of new fractures, and some of the hydrocarbon gas passes from the pores through some of the new fractures and is recovered. Preferably, one or more specific embodiments further comprises injecting hydraulic fracturing fluid into the first fracture after the one or more second fractures is formed, sometimes referred to as re-fracing.