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Geochemistry and Petrology of Thermally Altered Coals: 13C Signatures of Whole Coal and Vitrinite Concentrates Suggest No Evidence of Methane Generation from the Springfield (No. 5) Coal, Illinois Basin

Sue M. Rimmer1, Harold D. Rowe2, and Mohammad W. Rahman3
1Department of Geology, Southern Illinois University Carbondale, Carbondale, IL 62901
2Bureau of Economic Geology, University of Texas at Austin, Austin, TX 78713
3ERP program (Geosciences), Southern Illinois University Carbondale, Carbondale, IL 62901

Igneous intrusions into coals or organic-rich rocks, causing the release of greenhouse gases may have led to global warming in the geologic past. This study evaluates the hypothesis that if a large-scale release of δ13C-depleted thermogenic methane resulted from intrusion of the coal, then it should have produced 13C-enriched coal and vitrinite macerals adjacent to the intrusion due to the release of light gases. The study reports geochemical and petrographic data from whole coal and vitrinite macerals, from a transect of thermally altered Pennsylvanian Springfield (No. 5) coal in the Illinois Basin.

Vitrinite reflectance (Ro) increasing from 0.55% to ~4.80%, loss of liptinites, formation of isotropic coke, and fine mosaic anisotropic coke structure at the intrusion contact. Volatile matter decreases; fixed carbon and ash increase approaching the intrusion contact. Carbon increases whereas nitrogen, hydrogen, oxygen, and sulfur decrease approaching the intrusion. Organic carbon (TOC) decreases from a background level of 77% to 35% close to the intrusion. There are no significant changes in δ13C in the whole coal (-25.28‰ to -24.88‰) or in vitrinites (-25.33‰ to -24.96‰) approaching the intrusion. Changes in δ13C are not of a magnitude that would be expected as the result of large-scale thermogenic CH4 generation. Moreover, there is no petrographic evidence of condensed or immobilized thermal products due to rapid pyrolysis. These results suggest no evidence for large-scale methane generation due the rapid heating of the Springfield coal or organic matter by the intrusion.

AAPG Search and Discovery Article #90182©2013 AAPG/SEG Student Expo, Houston, Texas, September 16-17, 2013