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 ther-mal 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 control-ling 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 Cdaf=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 compon-ents(i.e.,vitrinite and exinite)and the tar yield merely under Cdaf<84%.Coal-forming parent materials,climatic condi-tions,and paleosalinity of water bodies exert minor impacts on the tar yield.In contrast,the reducibility and hydro-dynamic conditions of water bodies indirectly affect the discreteness of the tar yield of coals at the same ranks.The vit-rinite/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.
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