Hydrogen gas of organic origin in shales and metapelites

The major residual gas in shales and metapelites changes from CO2, CH4, to H2 in this order with increasing temperature. Drastic decrease of CO2 derived from decarboxylation of sedimentary organic matter is due to the expulsion of pore fluids with dissolved CO2. The CH4 concentration in residual gas subsequently increases with organic maturation and reaches the maximum at paleo-temperature of ca. 250 °C. The CH4 concentration decreases during late metagenesis to metamorphism probably due to the formation of H2 gas and graphite. The H2 starts to increase at paleo-temperature of ca. 200 °C. The H2 gas is the most abundant gas in residual gas of metapelite. The δ2Η value of H2 in shales and metapelites is quite low to be in the range of ca. -850 to − 650‰. The change of major residual gas from CH4 to H2 with increasing paleo-temperature and the significantly low δ2H values of H2 suggest that H2 gas in shales and metapelites is largely derived from liberation of hydrogen in organic matter.