THE IMPACT OF RAPID HEATING BY INTRUSION ON THE GEOCHEMISTRY AND PETROGRAPHY OF COALS AND ORGANIC-RICH SHALES IN THE ILLINOIS BASIN

If a significant release of ¹³C-depleted thermogenic CH₄ results from the intrusion of organic-rich rocks (OM), then the law of mass balance dictates that a ¹³C-enriched OM residue should result adjacent to the intrusion. However, a review of the literature suggests only minor changes in the δ¹³C_org of OM occur adjacent to intrusions. This study further evaluates the geochemical, isotopic, and petrographic changes that result from intrusion by comparing the intrusion of coal to that of shale. Data are evaluated for two different transects of intruded Pennsylvanian coal (Danville (No. 7) Coal) and organic-rich shale in the southern part of the Illinois Basin.

The two transects show significant increases in mean vitrinite reflectance (R_m); reflectance increases from background levels of 0.66% to 4.40% in the Danville (No. 7) Coal and 0.71% to 4.78% for organic-rich shale. For both transects, there is an increase in vitrinite reflectance, a loss of liptinites, formation of isotropic coke, and even development of fine mosaic anisotropic texture at the contact. In the Danville Coal transect, volatile matter (VM), N, H, S, and O decrease whereas fixed carbon (FC), C, and ash increase approaching the intrusion. For the coal, there is a marked decrease in remaining hydrocarbon potential (S₂) and hydrogen index (HI) and an increase in T_max (⁰C). Trends in most of the Rock-Eval parameters in the organic-rich shale are less clear due to the degree of variation in OM content, but a definitive increase in thermal maturity (T_max, ⁰C) is observed.

No significant changes in the bulk δ¹³C composition of the No. 7 Coal transect occur as the intrusion is approached (-25.37‰ to -24.76‰). However, the organic-rich shale transect shows a 1.31‰ positive shift in δ¹³C approaching the intrusion (from -25.18‰ to -23.87‰). Despite these changes, the isotopic shifts are not of a magnitude that would be expected as the result of any large-scale thermogenic CH₄ generation. In addition, no evidence exists for ¹³C-depleted condensed gas or pyrolytic carbon at the intrusion contact that could have moderated the isotopic signature. These data agree with previous studies that indicate no clear evidence for large-scale CH₄ generation due to the rapid heating or igneous intrusion in coals or sedimentary rocks.