Critical insights from Rock-Eval analysis of vitrains

Abstract

Rock-Eval S2 and S4 curves are commonly used to interpret active source rocks. In this work we report the formation of unusual, uneven, spiky S2 pyrograms from Rock-Eval of manually isolated vitrain bands from Mvb and Lvb coals. The spikes and unevenness in the S2 curves for coking-vitrains are interpreted to form from the release of gases or bubbles bursting from the melt during the pyrolysis stage. Crucible swelling numbers measured for these vitrains were observed to be 3 to 4 times higher than the parent coal from which they were separated. Field Emission Scanning Electron Microscope (FE-SEM) images also document formation of three dimensional bubbles due to bursting of gaseous bubbles, in the pyrolysis-residue (using Rock-Eval) of coking-vitrains. For bulk coal samples and vitrains isolated from HvbA coals, the S2 pyrograms were observed to be smooth. At higher sample weights, the formation of spikes for the vitrains from Mvb and Lvb was observed to increase, irrespective of the heating-rates used, owing to the release of greater amounts of gases or bubbles bursting from the melt. The results indicate the importance of cross-examining S2 pyrograms as it offers clues towards the behavior of coals during pyrolysis, and for better interpretation of results.