EFFECTS OF THERMAL ALTERATION ON COAL GEOCHEMISTRY: IMPLICATIONS OF GEOCHEMICAL AND PETROGRAPHIC SIGNATURES FROM THE SPRINGFIELD (NO. 5) COAL, ILLINOIS BASIN

Igneous intrusion can change the geochemical properties of coal including vitrinite reflectance, maceral petrography, mineralogy, isotope composition, and bulk geochemistry. It is important to explain the evolution of different geochemical and petrographic signatures in coal close to an intrusion. This study evaluates the hypothesis that thermally altered coals show different geochemical trends compared with coals that have gone through normal burial maturation. The current study reports density-gradient centrifugation data with geochemical and physical properties including petrography, proximate, ultimate, and Rock-Eval data from the Pennsylvanian-age, thermally altered Springfield (No. 5) coal in the Illinois Basin.

Petrographic analyses show vitrinite reflectance increasing from background levels of 0.5% to 4.8%, loss of liptinites, formation of isotropic coke, and development of very fine mosaic anisotropic coke structure at the intrusion contact. The density of vitrinite macerals in the unaltered coals is about 1.27 gm/mL whereas close to the intrusion contact vitrinite density increases to 1.52 gm/mL. There is a strong correlation between vitrinite density and vitrinite reflectance (r = 0.99). Volatile matter (VM) decreases and fixed carbon (FC) content, ash, and mineral matter increase approaching the intrusion contact. Nitrogen and hydrogen decrease whereas carbon increases approaching the intrusion. The presence of carbonate minerals has a major influence on the proximate and ultimate data. Even after removal of carbonates, the decrease in volatile matter and data plotted on van Krevelen and Seyler diagrams do not follow typical trends associated with normal burial coalification. Rock-Eval total organic carbon (TOC), free oil content (S₁), remaining hydrocarbon potential (S₂), carbon dioxide from pyrolysis of the organic matter (S₃), and hydrogen (HI) and oxygen (OI) indices decrease whereas thermal maturity (T_max, ⁰C) increases close to the contact. T_max (⁰C) and vitrinite reflectance, hydrogen index and dry, ash-free VM correlations and HI and OI plots also do not follow normal burial coalification trend. These geochemical data suggest that these coals may have followed a different maturation track because of the geologically rapid heating associated with the intrusion.