Contact Metamorphism of Bituminous Coal by Intruding Dike in the Illinois Basin Causes Short-Range Thermal Alteration

Abstract

Changes in high-volatile bituminous coal (Pennsylvanian) near contacts with volcanic intrusions in Illinois were investigated with respect to coal chemistry, carbon and hydrogen stable isotope ratios, and pore structure. Vitrinite reflectance (Ro) increases from ~0.6% to ~5% within 4.7 m from the dike. Elemental chemistry of the coal shows distinct reduction in hydrogen and nitrogen content approaching the intrusions. No trend was noticed for total sulfur content, but decreases in sulfate and organic sulfur contents towards the dikes indicate thermal sulfur reduction (TSR). Carbon isotopic values did not show significant changes, whereas hydrogen isotopic values showed a distinct trend of becoming more negative toward the dikes. Contact metamorphism has a dramatic effect on coal porosity. The mesopore volume decreases 3 3 from 0.01 cm /g in the unaffected coal to 0.004 cm /g at a distance 3 of 4.5 m away from the contact, then hovers around 0.004 cm /g closer to the contact. In contrast, the micropore volume shows a 3 progressive decrease from 0.04 cm /g in unaffected coal to almost 3 0.01 cm /g at the contact. Strongly decreasing mesopore and micropore volumes in the altered zone, together with frequent cleat and fracture-filling by calcite, indicate deteriorating conditions for both coalbed gas sorption and gas transmissibility.