Identification of tar-rich coal based on the true density and natural gamma ray response of coals

[Objective] Tar yield, derived from the Gray-King assay, serves as the sole metric for evaluating tar-rich coal. However, insufficient exploration data hinder its effective application in the fine-scale evaluation of extensive tar-rich coal. [Methods] Based on binary classification, this study proposed the optimal thresholds for tar-rich coal identification using true density (ft) and natural gamma-ray (GR) log. Moreover, this study elucidated the intrinsic mechanisms behind the significant petrophysical parameter responses of tar-rich coal. [Results and Conclusions] Key findings are as follows: (1) Coals with various metamorphic grades differ in identification threshold. For instance, long-flame tar-rich coal, the optimal identification thresholds consist of raw coal's true density of 3 and GR responses daf) corresponds to richer aliphatic structures in coals, creating more favorable pyrolysis conditions for tar production. Conversely, a higher true density corresponds to higher aromatic structure content, which is more unfavorable for tar production through pyrolysis. Overall, the coupling effects of multiple factors contribute to a strongly negative correlation between the true density and tar yield of coals. These findings can serve as a theoretical guide for evaluating tar-rich coal based on geophysical log parameters. © 2024 Science Press. All rights reserved.