Geological factors controlling the occurrence of Carboniferous-Permian tar-rich coals in the Ningdong coalfield

Objective The Ningdong Coalfield of the Ordos Basin boasts abundant tar-rich coal resources, of which tar-rich coals in the Carboniferous-Permian coal seams exhibit unique occurrence characteristics due to differences in thermal evolution and sedimentary environments.

Methods This study analyzed the occurrence characteristics and critical material composition of tar-rich coals in the Hongdunzi and Siguquan mining areas. Based on this, it explored the controlling effects of the degree of coalification and sedimentary environment on the tar yield.

Results and Conclusions The results indicate that the Carboniferous-Permian coal seams in the Ningdong Coalfield exhibit average tar yields ranging from 7.15% to 11.07%, which generally vary in different zones, different coal seams, and even different parts of the same coal seam. The degree of coalification is identified as a core geologic factor controlling the occurrence of tar-rich coals in the study area. With an increase in the degree of coalification, the tar yield trended upward initially and then downward, peaking at C_daf=84%. Under the action of coalification, a similar evolutionary trend can be observed between the H/C and O/C ratios and the tar yield. Meanwhile, there were significant positive correlations between active components (i.e., vitrinite and exinite) and the tar yield merely under C_daf <84%. Coal-forming parent materials, climatic conditions, and paleosalinity of water bodies exert minor impacts on the tar yield. In contrast, the reducibility and hydrodynamic conditions of water bodies indirectly affect the discreteness of the tar yield of coals at the same ranks. The vitrinite/inertinite ratio (V/I), gelification index (GI), ash composition index (K), U/Th ratio, and δU values indicate that coal-forming environment features deep water, occlusion, moisture, strong reducibility, and small quantities of woody tissues, which are conducive to the formation of tar-rich coals with a high tar yield. In contrast, indices such as ash yield and SiO₂+Al₂O₃ indicate strong hydrodynamic conditions and substantial quantities of terrigenous clast inputs, which are unfavorable for the formation of tar-rich coals with a high tar yield.