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Sample mounting for organic petrology: no thermal effects from transient exposure to elevated temperatures (2019)

December 17, 2021

For sample mounting, organic petrology laboratories typically use cold-setting epoxy-resin (e.g., 40 degrees C, used by Oklahoma Geological Survey, OGS) or heat-setting plastic (e.g., 180 degrees C, used by U.S. Geological Survey, USGS). Previous workers have suggested a systematic vitrinite reflectance (VRo) increase was associated with the thermoplastic preparation process, relative to epoxy mounting, which was attributed to moisture loss from the transient high temperatures of plastic mounting. In this study, we evaluated thermal effects to low maturity organic matter from transient exposure to elevated temperatures. A subbituminous coal sample was subjected to long-term (4 to 38 weeks) exposure to temperatures of 85 to 120 degrees C and afterward evaluated by multiple approaches to test thermal advance [elemental analyses, Rock-Eval pyrolysis, Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), pyrolysis gas chromatography, and petrographic analyses, including vitrinite reflectance and spectral fluorescence], all of which showed no detectable systematic changes between the original sample and its heat-treated products. We also compared vitrinite reflectance of six low maturity samples (those most likely to react to transient heating) mounted via both cold-setting epoxyresin and heat-setting thermoplastic. Results indicate measured VRo of a sample prepared by one mounting process was within the standard deviation of reflectance for the same sample prepared via the other process. Moreover, VRo results were not systematically higher in thermoset mounts. Contrary to previous work, these results suggest thermoplastic mounting or other transient exposure to elevated temperatures does not impact thermal maturity estimates from reflectance measurement for low maturity organic samples. Furthermore, the average interlaboratory difference in measured VRo (between OGS and USGS) for the same sample prepared by either epoxy-resin or thermoset mounting was 0.038%, about double the average difference between VRo for the same sample prepared via epoxy-resin versus thermoset in a single laboratory (0.024%). This result indicates interlaboratory variability impacts interlaboratory VRo measurement reproducibility to the extent that systematic differences could not be observed between thermoplastic and cold-setting sample preparation approaches, even if such differences were present.