Hydrogen gas of organic origin in shales and metapelites

The changes in inorganic and organic gases retained in shales and metapelites with increasing heating temperature are not fully understood. The compositional and isotopic changes of residual gases such as H-2, CH4, and CO2 in marine shales and metapelites during burial diagenesis and metamorphism were investigated in the present study. Shale rocks and metapelites which experienced paleo-temperatures in the range of 100-600 degrees C were collected from the borehole cores in Niigata sedimentary basin and outcrops exposed in Kochi district, Japan. Gases released from shale or metapelite fragments during pulverization in the laboratory were analyzed as the residual gas. The major residual gas in shales and metapelites changes from CO2, CH4, to H-2 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 degrees C. The CH4 concentration decreases during late metagenesis to metamorphism probably due to the formation of H-2 gas and graphite. The H2 starts to increase at paleo-temperature of ca. 200 degrees C. The H-2 gas is the most abundant gas in residual gas of metapelite. The delta H-2 value of H-2 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 H-2 with increasing paleo-temperature and the significantly low delta H-2 values of H-2 suggest that H-2 gas in shales and metapelites is largely derived from liberation of hydrogen in organic matter. (C) 2017 Elsevier B.V. All rights reserved.