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Secondary gas emissions during coal desorption, Marathon Grassim Oskolkoff-1 Well, Cook Inlet Basin, Alaska: implications for resource assessment

United States Geological Survey, Box 25046, Denver, CO 80225 USA
Pinnacle Gas Resources, Sheridan, WY 82801 USA

Cuttings samples of sub-bituminous humic coals from the Oligocene to Pliocene Tyonek Formation, Cook Inlet Basin, Alaska show secondary gas emissions whose geochemistry is consistent with renewed microbial methanogenesis during canister desorption. The renewed methanogenesis was noted after initial desorption measurements had ceased and a canister had an air and desorbed gas mixture backflow into the canister during a measurement. About a week after this event, a secondary emission of gas began and continued for over two years. The desorbed gas volume reached a new maximum, increasing the total from 3.3 to 4.9 litres, some 48% above the pre-contamination total volume.

The gases released during desorption show a shift in the isotopic signature over time of methane from ¹³C_CH4 of –53.60 and D_CH4 of –312.60 at the first day to ¹³C_CH4 of –57.06 and D_CH4 of –375.80 after 809 days, when the experiment was arbitrarily stopped and the canister opened to study the coal. These isotopic data, interpreted using a Bernard Diagram, indicate a shift from a mixed thermogenic and biogenic source typical of natural gases in the coals and conventional gas reservoirs of the Cook Inlet Basin to a likely biogenic acetate-fermentation methane source. However, the appearance of CO₂ during the renewed gas emissions with a ¹³C_CO2 of +26.08 to +21.72 , interpreted using the carbon isotope fractions found for acetate fermentation and CO₂ reduction between CO₂ and CH₄ by Jenden and Kaplan (1986), indicates a biogenic CO₂-reduction pathway may also be operative during renewed gas emission.

Adding nutrients to the coal cuttings and canister water and culturing the microbial consortia under anaerobic conditions led to additional methane-rich gas generation in the laboratory. After this anaerobic culturing, ultraviolet microscopy showed that canister water contained common, fluorescent, rod-like microbes comparable to Methanobacterium sp. Scanning electron microscope investigations of the coal matrix showed several morphological types of microbes, including rod, cocci and spherical forms attached to the coal surface. These microbes apparently represent at least a portion of the microbial consortia needed to depolymerize coal, as well as to generate the observed secondary methane emission from the canister.

The introduction of 48% more methane from secondary sources has a major impact on coal-bed methane resource assessments and also in determining the true, in-situ degree of methane saturation in coal-beds using isotherms. Canister and isotherm measurements that show "supersaturation" of methane may actually be the result of additional gases generated during secondary methanogenesis.