Print this pageSecondary Gas Emissions during Coal Desorption, Marathon Grassim Oskolkoff-1 Well, Cook Inlet Basin, Alaska
Charles E. Barker1 and Todd A. Dallegge2
1 U.S. Geological Survey, Denver, CO
2 ConocoPhillips, Houston, TX
Secondary methanogenesis is interpreted as the cause of a late gas pulse that occurred during canister desorption of fresh coal cuttings. This secondary pulse of gas, commencing about 200 days into desorption, increased the total desorbed gas from 3.3 to 4.6 liters, 40% above the pre-contaminated total volume and extended desorption to 625 days.
The gases emitted over time show an isotopic shift from d13CCH4 of -53.60‰ and dDCH4 of -312.60‰ the first day of desorption to d13CCH4 of -57.06‰ and dDCH4 of -375.80‰ after 625 days of desorption. These isotopic data indicate a shift from a mixed thermogenic and solely biogenic source typical of natural gases in this basin to methane sourced from a biogenic acetate fermentation pathway. CO2 appearing later in the desorption experiment with a d13CCO2 of +26.08 to +21.72‰ indicates a biogenic CO2 reduction pathway may also be operative.
Nutrient media culturing of the coal and canister water under anaerobic conditions led to additional methane-rich gas generation. Subsequent ultraviolet microscopy showed that the canister water contained fluorescent, rod-like microbes comparable to Methanobacterium sp. SEM investigations show four microbe morphological types attached to the coal surface representing a portion of the microbial consortium needed to depolymerize coal and generate the observed secondary methane emission.
The 40% methane increase from secondary sources has major impacts on coalbed methane resource assessments and determination of the degree of methane saturation in coal beds using isotherms. Canister and isotherm measurements that show “supersaturation” of methane may be the result of secondary methanogenesis.
AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005