P-T conditions of vein formation and fractions of chemical species. (a) P-T conditions of vein formation estimated from microthermometry for the two-phase inclusions with only CH4 vapor and the one-phase carbonic inclusions. The gray square indicates the P-T condition of fluid inclusions at locality W1 estimated by Lewis and Byrne (2003). The light-gray zone indicates the peak temperature around locality W4 based on vitrinite reflectance (Underwood et al. 1993). (b) The fractions of chemical species in the C-H-O fluids in equilibrium with graphite, buffered by the reaction among quartz, magnetite, and fayalite, along the geothermal gradient of 35°C/km (lithostatic) determined by using the Perple_X computer program package (Connolly 1995). indicates the molar ratio of CO2 / (CO2 + CH4). The profile of with depth under the lithostatic condition (black broken curve) is similar to that under the hydrostatic condition (gray bold curve).