Isotopic composition and content of organic nitrogen in the coals of Qinshui Coalfield, North China

•Ndmmf of Qinshui coal decreases significantly with coalification.•δ15Ndmmf increases significantly with increasing maturity.•Brackish influenced coal has higher Ndmmf and δ15Ndmmf than the freshwater influenced one.•Permian coal has lower Ndmmf and δ15Ndmmf than the Carboniferous one.

Coal nitrogen has two modes of occurrence, bound to organic matter and fixed into ammonian illite (fixed-NH4), with the organic nitrogen mode being dominant. Organic nitrogen geochemical characteristics (isotopic ratio and content) can serve as indicators of certain geological processes, and therefore, understanding the geochemical behavior of organic nitrogen in coal is important. A total of 25 coal samples were collected from Qinshui Coalfield, North China, and Kjeldahl and Rittenberg analyses were used to determine the organic nitrogen content (Ndmmf) and isotopic ratio (δ15Ndmmf) of these coals, respectively. Results indicate that the organic nitrogen content in the Permo–Carboniferous coals of Qinshui Coalfield averages 0.99%, consistent with Chinese coals. The δ15Ndmmf averages + 3.85‰, which is higher than that of Chinese coals. Major geological factors influencing the organic nitrogen geochemistry of Qinshui coals include coalification, depositional environment, and coal-forming age. A multiple linear regression model was used to quantitatively examine the relationship of Ndmmf and δ15Ndmmf to these geological factors and their degree of influence on the organic nitrogen geochemistry. The Ndmmf shows a significantly decreasing trend with coalification and is influenced by depositional environment and coal-forming age to a relatively smaller extent. The δ15Ndmmf is mainly influenced by coalification and coal-forming age, followed by depositional environment. As maturity increases, the δ15Ndmmf increases significantly. Carboniferous coals have a higher δ15Ndmmf than the Permian ones due to mantle outgassing and differences in the climatic conditions during their formation.