Methane generation from low-maturity coals and shale source rocks at low temperatures (80–120 °C) over 14–38 months

Abstract

This study contributes long-term (14 to 38 months) experimental evidence for geocatalytically mediated methanogenesis in immature to early mature shale and coal source rocks at temperatures from 80 to 120 °C. Borosilicate glass tubes with pre-outgassed coal and shale source rock chips and water were sealed under vacuum, sterilized, heated isothermally, and finally opened in connection with a vacuum line where headspace gases methane (CH4) and carbon dioxide (CO2) were collected. The possibility of admitting pre-existing CH4 from closed pores into product gases during heating experiments was evaluated by comparative ball milling of original and heated rock chips, followed by quantitation of released gases with a novel closed-circuit laser-based SARAD RTM2200 gas detector system with sub-µmol sensitivity. The yields of produced CH4 from individual source rocks were corrected by subtracting the amounts of pre-existing CH4 from formerly closed pores in original source rocks that had leaked during long-term heating. Different shales and coals express contrasting propensities to geocatalytically generate CH4 and CO2, with CH4 yields ranging from 0.1 to 5.5 µmol g−1 total organic carbon (TOC). CH4 yields from two petrographically different samples of Springfield Coal with comparable thermal maturity suggest that liptinite expresses a far higher propensity for methanogenesis, but liberates less CO2 than vitrinite. Shale from the Second White Specks Formation generated approximately 10 times more CH4 than New Albany Shale per g of TOC, further suggesting complex controls on CH4 generation during catalytic methanogenesis. Higher temperature can enhance the activities of catalytic methanogenesis. The extrapolation of laboratory-based reaction rates to natural conditions in organic-rich buried sediments suggests that geocatalytic methanogenesis can be fast enough in some source rocks to generate economically sizeable gas plays from immature to early mature source rocks over geologic time.