Geochemical analysis reveals selective coking and hydrothermal isotopic alterations in magma-heated coals, indicating that mudstone barriers mitigate thermal degradation.
Key Points
To evaluate the thermal alteration of organic matter and determine the protective barrier role of mudstone during rapid heating from magmatic intrusions in coal seams.
Examined coal seam profiles (No. 8 and No. 9 seams) in the southern North China Basin exposed directly or buffered by mudstone against magmatic intrusion.
Characterized maceral evolution, residual organic compounds (including n-alkanes and n-alkylcyclohexanes), stable isotopes (δ13C and δ15N), and total organic carbon to total nitrogen (TOC/TN) ratios across thermal gradients.
Intrusive heating selectively converted vitrinite into pyrolytic carbon and fine/coarse cokes while inertinite remained largely unaffected, causing severe pyrolysis of short-chain n-alkanes at contact zones.
The δ13C values varied within narrow ranges with a slight negative excursion at 50–100 cm, whereas δ15N values and TOC/TN ratios fluctuated widely due to high-temperature hydrothermal fluid interaction.
The mudstone-shielded coal seam retained higher organic molecular diversity and buffered hydrothermal isotopic fractionation compared to the directly intruded seam.