The influence of heating on the carbon isotope composition, organic geochemistry and petrology of coal from the Upper Silesian Coal Basin (Poland): An experimental and field study

The impact of natural intra-deposit heating on the delta C-13 signature, organic geochemistry, and petrology of coal and coal-bearing rocks characterised by various degrees of coalification and palaeoenvironments in the Upper Silesian Coal Basin, Poland, is elaborated. Reconstruction of palaeofire performed by heating experiments up to 400 degrees C in open and semi-closed systems with different heating regimes confirms the crucial significance of temperature and oxygen access. In open-system heating, released C-13-depleted gases enrich residue coke in C-13 compared to raw coal. Petrological examinations did not show the impact of palaeofires on the maceral properties of coal. However, the carried-out experiment caused the formation of devolatilisation pores, rounded edges, cracks, pale rims, as well as higher reflectance and paler colour that was what was expected. Extractable compounds become highly depleted, and low-weight organic compounds nearly absent. Relatively high contents of combustion-formed PAHs are an indicator of open-system heating. In semi-closed systems, the final total isotopic composition was almost unchanged as no components are carried away though changes in petrography and geochemistry occur. Increased extract yields reflect the release of bitumen from closed pores and partial pyrolysis of organic matter. Depletion of lighter pi-alkane compounds, total carbon TC and volatiles decrease, and variable values of various alkyl aromatic hydrocarbon ratios are also indicative of semi-closed heating. Coal seams suspected of intra-deposit heating show geochemical and isotopic features similar to semi-closed- rather than open-system heating, and their delta C-13 signatures and organic geochemistry did not respond strongly during laboratory re-heating.

Automated summary

This study elaborates on the influence of natural intra-deposit heating on the delta C-13 signature, organic geochemistry, and petrology of coal and coal-bearing rocks in the Upper Silesian Coal Basin, Poland. Heating experiments conducted in open and semi-closed systems demonstrated the importance of temperature and oxygen access in palaeofire reconstruction. The impact of palaeofires on coal properties was observed in terms of devolatilisation pores, reflectance, and extractable compounds. The isotopic and geochemical features of coal seams suspected of intra-deposit heating were more similar to semi-closed-system heating, with delta C-13 signatures and organic geochemistry showing minimal response to laboratory re-heating.