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
Petrographic analyses show mean vitrinite reflectance (Ro) increasing from background levels of 0.55% to ~4.80%, loss of liptinites, formation of isotropic coke, and fine mosaic anisotropic coke structure at the intrusion contact. Ro indicates temperature at the coal/intrusion contact was above 355°C using the paleogeothermometer model of Barker and Pawlewicz (1994). Volatile matter (VM) decreases and fixed carbon (FC) content and ash increase approaching the intrusion contact. Carbon increases whereas nitrogen, hydrogen, oxygen, and sulfur decrease approaching the intrusion. The presence of carbonate minerals has a major influence on the proximate and ultimate data. Organic carbon (TOC) concentration decreases from 77% in unaltered coals to 35% in the coals close to the intrusion. There are no significant changes in δ13C in the whole coal (-25.28‰ to -24.88 ‰) or in pure vitrinites (-25.33‰ to -24.96 ‰) approaching the intrusion. Changes in the isotopic signatures 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 (12C-rich pyrolytic carbon) close to the intrusion. These geochemical and petrographic data suggest that there is no evidence for large-scale methane generation due the rapid heating of the Springfield (No. 5) coals by the intrusion.