CATALYTIC GENERATION OF METHANE AT 60 TO 100 °C AND 0.1 TO 300 MPA FROM SOURCE ROCKS CONTAINING KEROGEN TYPES I, II, AND III

Low temperature (60 and 100 °C) and long-term (6 months to 5 years) heating of pre-evacuated and sterilized shales and coals containing kerogen Types I (Mahogany Shale), II (Mowry Shale and New Albany Shale), and III (Springfield Coal and Wilcox Lignite) with low initial maturities (vitrinite reflectance R_o0.39 to 0.62%) demonstrates that catalytically generated hydrocarbons may explain the occurrence of some non-biogenic natural gas plays where insufficient thermal maturity contradicts the conventional thermal cracking paradigm.

Our laboratory experiments utilized source rock chips sealed in gold and Pyrex^® glass tubes in the presence of hydrogen-isotopically contrasting waters. Parallel heating experiments applied hydrostatic pressures from 0.1 to 300 MPa. Control experiments constrained the influence of pre-existing and residual methane in closed pores of rock chips that was unrelated to newly generated methane.

This study’s experimental methane yields at 60 and 100 °C are 5 to 11 orders of magnitude higher than the theoretically predicted yields from kinetic models of thermogenic methanogenesis, which strongly suggests a contribution of catalytic methanogenesis. Higher temperature, longer heating time, and lower hydrostatic pressure enhanced catalytic methanogenesis. No clear relationships were observed between kerogen type or total organic carbon content and methane yields via catalysis. Catalytic methanogenesis was strongest in Mowry Shale where methane yields at 60 °C amounted to ~2.5 μmol per gram of organic carbon after one year of hydrous heating at ambient pressure.

In stark contrast to the earlier findings of hydrogen isotopic exchange between water and thermogenic methane in hydrous pyrolysis experiments above 300 °C, the hydrogen isotopic composition of added water exerted limited influence on the δ²H value of methane generated catalytically at low temperatures. The δ¹³C_CH4 values of methane generated catalytically at 60 to 100 °C range from ~-57.6 to -41.4‰ and are thus similar to typical thermogenic methane (δ¹³C_CH4 > -50‰) and microbially generated methane (< -55‰).