A log-based method for evaluating the tar yield of tar-rich coal

[Objective] As a kind of coal-based oil and gas resource, tar-rich coal is significant for the supply of oil and gas resources, as well as the clean and efficient utilization of coals. Presently, the identification of tar-rich coals primarily depends on the Gray-King assay. In contrast, there is a lack of studies on the accurate identification and evaluation of tar-rich coal using geophysical methods. [Methods] This study investigated coals in the Jurassic Xishanyao and Badaowan Formations in Santanghu Basin. Xinjiang. Based on petrophysical experiments, as well as the typical log response characteristics of tar-rich coal, this study developed a new log-based method for quantitatively evaluating the tar yield of tar-rich coal. [Results and Conclusions] Key findings are as follows: (1) Compared to tar-bearing coal, tar-rich coal features a high content of materials with hydrogen-rich structures and unfavorable pore structures, which contribute to the log response characteristics of low density, high compensated neutron logging, and high resistivity. (2) The multistate 2D NMR experiments reveal that tar-rich coal is characterized by double peaks of the T1 spectrum and strong signals in zones with T21/T2>10, while oil-rich coal shows insignificant singles in these zones. This contrast is related to the high content of materials with hydrogen-rich structures in tar-rich coal. (3) Based on the log response characteristics of tar-rich coal, this study established indicator Z for tar-rich coal using resistivity and neutron log curves, with a higher Z value indicating higher tar yield of the coal. Building on tar yield calibration, this study determined the linear relationship between the coal tar yield and Z achieving the accurate, convenient, and practical calculation of the tar yield of tar-rich coal. The above understanding offers theoretical guidance for identifying and evaluating tar-rich coal based on geophysical logs. © 2024 Science Press. All rights reserved.