In situ high pressure-temperature (P-T) measurements on C-H-O fluids at lower crust and upper mantle conditions have been performed to assess the extent and limit of methane generation within the Earth. Experiments were conducted using diamond-anvil cells at temperatures from 250 to > 1500 degrees C and pressures between 1 and 8 GPa, assuming conditions along a subduction-setting geotherm. In many experiments, methanogenesis is observed, in particular, in those experiments in which elemental carbon or reduced transition metals (e.g., Fe-0) were present. The direct reduction of CO2 to methane was not observed under any set of conditions. Methanogenesis is sensitive to C-H-O fluid composition and, specifically, the activity of H-2. Kinetic barriers to direct hydrogenation of CO persists over the range of conditions explored, and hydrogenated transition-metal carbonyl species likely play a critical role in methane formation. Surprisingly, the direct hydrogenation of graphite yields considerable methane. The observation of both CO2 and H-2 generation during high T and P reactions of ferrous oxide, calcium carbonate, and water indicates that methanogenesis in the lower crust and tipper mantle is plausible; however, the absolute yield of methane is strongly controlled by the activity of H-2.