The catalytic effect of manganese carbonate on kerogen pyrolysis in the absence and presence of water

Abstract

To investigate the effect of Mn ore on the chemical components and isotopic compositions of gas products during kerogen pyrolysis in the absence or presence of water, a series of pyrolysis experiments on Type-I kerogen with manganese carbonate and/or water were conducted in closed gold capsules at 325–600 °C for 72 h with 300 bar. The heavy hydrocarbon gas yields increased significantly over 450 °C due to the participation of manganese. As for isotopic compositions, the manganese catalysis caused a decrease in the 13CCH4 and an increase in the 13CC2H6 over 475 °C; the δD values of methane generated during anhydrous pyrolysis with manganese carbonate are much lower than those produced by the pyrolysis of kerogen alone. Furthermore, the δD values are the most negative during hydrous pyrolysis with manganese ore. In addition to the thermal decomposition of manganese carbonate to form carbon dioxide, water provides sources of hydrogen and oxygen to form H2 and CO2. Manganese carbonate inhibits the association of sulfur-free radicals to hydrogen to form H2S. The i-C4/n-C4 ratio for gas products was evidently higher due to the existence of manganese and water, they can enhance the carbocation mechanism.