The impact of rapid heating by intrusion on the geochemistry and petrography of coals and organic-rich shales in the Illinois Basin

No systematic changes in δ13C occur in the No. 7 Coal transect as the intrusion is approached, with δ13C varying between − 25.4‰ and − 24.8‰. The organic-rich shale transect shows a minor 1.2‰ enrichment in δ13C (from − 25.2‰ to − 24.0‰) within 2 m of the intrusion. These isotopic shifts are not of a magnitude that would be expected if associated with a large-scale release of thermogenic CH4. In addition, no evidence exists in either transect for 13C-depleted condensed gas or pyrolytic carbon at the intrusion contact that could have moderated the isotopic signature. These data agree with those reported previously that indicate no clear isotopic evidence for large-scale CH4 generation due to rapid heating by igneous intrusion into coals or sedimentary rocks.