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

Abstract

Igneous intrusion into organic-rich sedimentary rocks and coals has been suggested as a factor in the large-scale release of 13C–depleted thermogenic CH4, which may have led to global warming and mass extinction events in the geologic past. If a significant release of 13C–depleted thermogenic CH4 results from the intrusion of coal or organic-rich rocks, then it should produce 13C–enriched residual coal and dispersed organics in rocks adjacent to the intrusion due to the release of isotopically lighter CH4 gas. A review of the literature suggests only minor changes in the δ13Corg of coals adjacent to intrusions; however, a few studies have shown that changes in δ13Corg in intruded shales may be slightly more pronounced. The current study further evaluates the geochemical, isotopic, and petrographic changes that result from contact metamorphism and specifically compares the intrusion of coal to that of an organic-rich shale collected from the same general vicinity. Data for two different transects of intruded Pennsylvanian coal (Danville (No. 7) Coal) and an intruded organic-rich shale in the southern part of the Illinois Basin are presented.Both transects show similar increases in mean vitrinite reflectance (Rr); reflectance increases from background levels of 0.66% to 4.40% in the Danville (No. 7) Coal and 0.71% to 4.78% in the organic-rich shale. In addition, both transects show the formation of isotropic coke, and even development of fine circular mosaic anisotropic coke structure at and near the contact with the intrusion, along with the visual loss of liptinites at higher reflectances. In the Danville Coal transect, volatile matter, N, H, S, and O decrease whereas fixed carbon, C, and ash increase approaching the intrusion. The coal shows a marked decrease in remaining hydrocarbon potential (S2) and hydrogen index (HI) and an increase in Tmax (°C). Trends in most of the Rock-Eval parameters for the organic-rich shale are less clear due to variations in the amount of organic matter present, but a significant increase in thermal maturity (Tmax, 0C) is observed.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 2m 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.