Geochemical characterisation and petroleum potential of selected cenozoic coals from Sarawak basin, Malaysia

Previous geochemical analyses of crude oils from offshore Sarawak, Malaysia, suggest a dominant terrestrial organic matter source, likely deposited in peatland environments. To investigate this hypothesis, forty Cenozoic humic coals from Mukah-Balingian and Merit-Pila coalfields in the Sarawak Basin were examined using an integrated suite of bulk and molecular geochemical analytical techniques, including Rock-Eval pyrolysis, pyrolysis-gas chromatography (Py-GC), attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy, and gas chromatography-mass spectrometry (GC-MS). The objectives were to evaluate thermal maturity, organic matter input, and hydrocarbon generation potential. The huminite/vitrinite reflectance (0.27–0.43%Ro), Tmax (383–428 °C), hydrogen index (68–456 mgHC/g TOC), A-factor (0.4–0.6), extract yield (21596–224779 ppm) data, together with the prevalence of hopenes and hopanes, collectively indicate thermal immaturity and an excellent potential for gas and mixed oil/gas generation. Rock-Eval, Py-GC, and source-related aliphatic and aromatic hydrocarbon proxies suggest a dominance of type-III kerogen with varying amounts of type-II kerogen. Notably, the Merit-Pila coals exhibited the highest hydrocarbon-generating potential, while the Mukah coals displayed a greater abundance of type-II kerogen and marine algal organic matter. However, despite variations in organic matter composition, principal component analysis revealed no significant correlation between source input and petroleum potential. These findings underscore the importance of prioritising deeper offshore equivalents of the studied coals and coaly sequences as probable source rocks for ongoing and future exploration efforts in the Sarawak Basin. Additionally, closed-system pyrolysis of the coals is recommended for more accurately modelling of petroleum generation and expulsion.