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

摘要

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 δ~(13)C_(CH4) of -53.60 per thousand and δD_(CH4) of -312.60 per thousand at the first day to δ~(13)C_(CH4) of -57.06 per thousand and δD_(CH4) of -375.80 per thousand 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 CO_2 during the renewed gas emissions with a δ~(13)C_(CO2) of +26.08 to +21.72 per thousand, interpreted using the carbon isotope fractions found for acetate fermentation and CO_2 reduction between CO_2 and CH_4 by Jenden and Kaplan (1986), indicates a biogenic CO_2-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.%Des échantillons de découpes de déblais de houilles subbitumineux humiques, provenant de la Formation Tyonek de l'Oligocène au Pliocène, dans le Bassin de Cook Inlet, Alaska, indiquent des émissions de gaz secondaire dont la géochimie demeure compatible avec le renouvellement de la méthanogénése microbienne durant la désorption de la cartouche filtrante. Le renouvellement de la méthanogénése a été noté après l'arrêt des mesures de désorption initiale, à la suite du refoulement d'un mélange d'air et de gaz dans la cartouche filtrante durant cette prise de mesure. Environ une semaine après cette manifestation, une émission de gaz secondaire s'est produite et a continué au delà de deux années. Le volume du gaz émis a atteint un nouveau maximum, augmentant le total de 3.3 à 4.9 litres, environ 48% au-dessus du volume total de pré contamination. Les gaz relâchés au cours de cette émission indiquent un changement dans la signature isotopique de méthane depuis δ~(13)C_(CH4) de -53.60 per thousand et depuis δD_(CH4) de -312.60 per thousand le premier jour à δ~(13)C_(CH4) de -57.06 per thousand et δD_(CH4) de -375.80 per thousand après 809 jours, quand l'expérience a été arrêtée arbitrairement, et que la cartouche a été ouverte afin d'étudier la houille.