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

Igneous intrusions into coals or organic-rich rocks, causing the release of greenhouse gases may have led to global warming in the geologic past. However, thermally altered coal/shale typically shows negative shifts and a few ambiguous or positive trends in the δ¹³C of organic matter toward intrusions. It is important to explain the trends in δ¹³C of sedimentary organic matter close to an intrusion in response to thermal alteration. 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.

Petrographic analyses show mean vitrinite reflectance (R_o) 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. R_o 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.