Thermal influence of diabase intrusions into coal beds and sedimentary host rocks of the Santa Terezinha coalfield (rio bonito formation, Paraná Basin), Brazil

Abstract

Coal seams in contact with igneous intrusions can be thermally affected, causing localized increase in rank, and making the coal attractive for exploration. The aim of this study is to evaluate the thermal influence of diabase intrusions on coals and host rocks of the Rio Bonito Formation in the Santa Terezinha coalfield, Paraná Basin. It was collected 39 samples from five wells and submitted to: (1) electron microprobe analysis of clinopyroxene, using these minerals as a geothermometer; (2) X- -ray diffraction of clay minerals of the host rocks at different distances from the intrusion, to assess the mineralogical changes in clay minerals caused by the effect of the intrusions and (3) 1D modeling of well CBM-001-ST-RS. Clinopyroxene geothermometry indicates a magma crystallization temperature of around 1136 °C. The results of X-ray diffraction showed chlorite as the main clayish product of the mineralogical changes by the thermal effects on the host rocks. Chlorite appearance is usually accompanied by a drastic reduction of kaolinite in the host rocks (siltstones and sandstones) at distances shorter than 1 meter from the intrusive contacts. Based on vitrinite reflectance data in samples from well CBM-001-ST-RS, a temperature peak of ~ 213 °C was calculated for the coal layers in the top of the section, affected by intrusions. Meanwhile, in the lower section (2 m from the intrusion) a peak temperature of ~ 120 °C was reached, related to burial diagenesis. In the vicinity of diabase intrusions, the vitrinite values increased, observing that the thermal affected zone extends up to an approximate distance, from the intrusive contact, of 0.3 times the thickness of the intrusion.