Geochemical study on nitrogen isotope composition, speciation distribution, and influencing factors of vitrinite-rich coal seams during the Late Carboniferous

Abstract

This study focuses on Coals 8 and 9 of the Late Carboniferous Taiyuan Formation in the eastern part of the north China Craton Basin. Through comprehensive industrial analysis, XPS, and nitrogen isotope techniques applied to 16 collected coal samples, the distribution of nitrogen in coal and the differentiation mechanism of δ¹⁵N values were revealed. The results indicate that most coal samples exhibit high ash content, high volatile matter, and ultra-low moisture. Nitrogen exists in an organic state, with N-5 dominating and N-6 being secondary. The δ¹⁵N values range from 0.72 to 3.15‰ for Coal 8 and - 1.90 ~ - 0.38‰ for Coal 9, deviating from China's typical range, with positive and negative differences between Coal 8 and 9. These isotopic differences are influenced by factors such as depositional environments and diagenesis. δ¹⁵N shows a negative correlation with total nitrogen content and a close association with macerals (negative correlation with vitrinite and positive correlation with inertinite). The deep-water reducing environment in the east and the shallow marine environment at the southern margin play regulatory roles in nitrogen isotope fractionation. This study provides crucial geochemical evidence for the reconstruction of ancient depositional environments and the nitrogen cycling mechanisms during coalification processes.

Keywords: Elemental analyses; Nitrogen form distribution; Nitrogen isotope analyses; X-ray photoelectron spectroscopy.

Fig. 1

Results of N 1 s XPS analysis of coal samples.

Fig. 2

Schematic representation of the functionality of the main nitrogen forms in coal.

Fig. 3

Ratio of N-5 and N-6 to C_daf content in the studied coals.

Fig. 4

Ratio of C_daf and N_daf to δ¹⁵N content in the studied coals.

Fig. 5

Ratio of microcomposition to δ¹⁵N values for the studied coals.

Fig. 6

Ratio of N-5, N-6 and N-6 + N-Q to δ¹⁵N values for the studied coals.

Fig. 7

Carboniferous Permian paleogeography of northern China. (1- Palaeolands; 2- Intermountain basins; 3- Uplands and alluvial plains; 4- Riparian and coastal plains; 5- Riparian; 6- Shallow sea and riparian; 7- Direction of sea incursion; 8- Late Early Permian to Early Late Permian arid climate-controlled areas; 9- Humid climate control area; 10-δ¹⁵N content).

Fig. 8

Ash content changes in coal from different regions of the North China Craton Basin.

Fig. 9

Plot of mirror inert ratio content and trend of change in coal in different areas of the North China Craton Basin.

Fig. 10

Heat map of correlation between elements and δ¹⁵N of coal samples from different areas of the North China Craton Basin.

Fig. 11

Morphological share of elemental N in different areas of the North China Craton Basin.

Fig. 12

Geographic location map (A), tectonic outline map (B), sampling location map (C) and sampling bar chart (D) of the study area.