Effects of maturity on the pyrolytic fingerprint of coals from North Borneo

Pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) and thermally assisted hydrolysis and methylation (THM-GC-MS) were applied to 27 coal samples of lignite to high volatile bituminous rank (vitrinite reflectance %Ro = 0.28-0.66) from the Malaysian states of Sabah and Sarawak on Borneo, to investigate rank effects on molecular properties. The molecular fingerprints of these coals appeared to be controlled by diagenetic and, to a smaller extent, early catagenetic transformations (%Ro 0.28-0.50). The molecular assemblages from immature coals consist mainly of lignin-derived material and long-chain (≥ C24) ester-bound polymethylene chains, whereas mature coals are enriched in short (C10-C25) free and/or trapped (isoprenoid) hydrocarbons, alkylated phenols and resinite aromatic derivatives. By comparing a large suite of potential molecular proxies of maturation (indices of pristane formation, resin aromatization, lignin alteration stages, etc.) and other maturity proxies (%Ro, Tmax), rank effects on different coal constituents are illustrated, as well as the suitability of the proxies for specific rank ranges. The results highlight the importance of decarboxylation of long-chain fatty acids (%Ro 0.35-0.40) and the progressive accumulation of resistant aryl-O (probably diaryl ether) bonds at advanced stages of maturity. The coals from Pinangah in Sabah were more mature than those from Sarawak (Mukah and Balingian coalfields), whereas the molecular signature of other coals from northeastern Sabah (Sandakan area) and southeastern Sabah (Silimpopon and Maliau areas) varied across the rank range studied.