Objective and Methods Under high temperatures, oil and gas produced from the in situ pyrolysis of tar-rich coals will migrate toward and accumulate in the overburden strata after passing through the fracture zones on the coal seam roof, causing potential contamination risks for the underground environment. To understand the release and distribution characteristics of typical contaminants generated from the underground pyrolysis of tar-rich coals, this study investigated the typical tar-rich coals from the Shenfu mining area of the Jurassic coalfield in northern Shaanxi Province. Based on data on the stratigraphic structures and lithology and using the self-developed equipment for similarity simulation experiments on the in-situ pyrolysis of tar-rich coals, this study examined the composition of the pyrolysis products of tar-rich coals, along with the spatiotemporal distributions of typical contaminants like carbolic oil, naphthalene oil, washing oil, anthracene oil, and asphalt in coal tar under different temperatures.

Results and Conclusions The results indicate that the contents of typical contaminants in the overburden strata increased first and then decreased with time under different pyrolysis temperatures, with various components differing greatly in content and enrichment horizon. Under pyrolysis temperatures of 450 ℃ and 650 ℃, the contaminant contents in different overburden strata decreased in the order of asphalt, anthracene oil, naphthalene oil, washing oil, and carbolic oil. At 450 ℃, various components were primarily enriched in the medium-grained sandstone layer due to the relatively minor impacts of the low temperature on the overburden strata. With an increase in temperature, fractures occurred in the overburden strata. Consequently, light components were enriched in the mudstone layer far away from the coal seam at 650 ℃. In contrast, heavy components, featuring high densities and viscosities but a low migration ability, were predominantly distributed in the argillaceous siltstone layers. Temperature was identified as a primary factor influencing the spatiotemporal distributions of typical contaminants in the overburden strata. Specifically, an increase in the temperature would extend the migration range of contaminants. Furthermore, the proportions of carbolic oil, naphthalene oil, and washing oil progressively increased, while those of anthracene oil and asphalt gradually decreased as the temperature rose. The results of this study will provide a theoretical basis for the control of the underground contamination caused by the in-situ pyrolysis of tar-rich coals.