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 δ13C 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 °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 13C-depleted gases enrich residue coke in 13C 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 n-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 δ13C signatures and organic geochemistry did not respond strongly during laboratory re-heating.