The influence of tectonic deformation on some geochemical properties of coals—a possible indicator of outburst potential

Abstract

Nearly all outburst-prone coal is known to have been tectonically deformed in fault and fold zones which underwent one or more episodes of strong structural deformation. The physical properties of deformed coal, such as strength coefficient and rate of gas desorption, have been broadly studied and used to predict outbursts in the past. However, it can be difficult to reliably determine these physical coefficients from core sampling operations because the coal can be inadvertently pulverized during drilling. The development of a new index which predicts the outburst potential of coal seams and is not influenced by sampling procedures could be beneficial to mining safety technology. Tectonic deformation may not only affect the original physical structure of a coal but also may bring about changes in chemical properties. Any chemical property with the potential to identify outburst-prone coal must also be stable and independent of the physical changes imposed by drilling. Most of the chemical characteristics of coal display no obvious difference between normal undeformed coal (which is not subject to outbursts) and deformed coal. However, recent studies have shown that the chloroform extract yield is a very good geochemical index for differentiating these coals. Nineteen coal samples from five mines experiencing outbursts in the Pingdingshan coalfield, Henan, China, were analyzed. Their mean maximum vitrinite reflectance in oil ranges from 0.97% to 1.28%. All of the samples were prepared to −180 μm (−80 mesh US) and subsamples were extensively extracted with chloroform in a Soxhlet apparatus at 75 °C. The mean extract yield of the 10 deformed coals is 1.45% whereas that of the nine normal coals is 0.44%. We conclude that deformed coal has a higher chloroform extract yield than equivalent coal which has not been tectonically disturbed. Chloroform extraction, therefore, can be used to recognize deformed coal and the potential to undergo further deformation. Chloroform extract yield is an accepted measure of the hydrocarbon-generating potential of coals and other rocks. The results presented here, therefore, suggest that tectonic deformation may modify the hydrocarbon generating potential of coals. The adsorption of soluble organic matter expelled during and following tectonic deformation onto enhanced surface areas may have contributed to the higher extract yields of the deformed coal.