Could hot fluids be the cause of natural pyrolysis at the ragged edge of Herrin coal, Millport 7 ½’ quadrangle, Hopkins County, Kentucky?

Abstract The Herrin coal (Western Kentucky No. 11) “ragged edge” is composed of peculiar organic materials, including voids, which defy conventional maceral classification. These materials seem to have originated from natural pyrolysis caused by hot fluids migrating through low-rank coal. However, the observed petrographic patterns and the reflectance analysis were not enough to confirm this hypothesis. In order to obtain for the first time an approximate composition and temperature value of the fluids that may have thermally affected the Herrin coal, the origin of the “ragged edge” of the Herrin coal seam was studied via a combination of analytical techniques mainly focused on the carbonates including optical microscopy, cathodoluminescence, X-ray fluorescence, X-ray diffraction, scanning electron microscopy with energy dispersive spectroscopy, electron microprobe, and Raman microspectroscopy. The sum of indicators is collectively considered evidence that ascending hot fluids thermally affected the coal. These indicators include the ambient reflectance increase due to forced coalification; liptinite fading and “cryptic liptinite” formation at the bottom of the coal seam; the carbonate stable isotopes model temperature that indicates a range of 150 °C to 200 °C, and the Raman spectra proxy of the thermally affected particles indicating a temperature between 160 and 220 °C.