A comparative geochemical study of humic coals from Cenozoic Sarawak basin, Malaysia and cretaceous Benue trough, Nigeria / Asiwaju Lanre Oluwaseun

Asiwaju Lanre , Oluwaseun (2023) A comparative geochemical study of humic coals from Cenozoic Sarawak basin, Malaysia and cretaceous Benue trough, Nigeria / Asiwaju Lanre Oluwaseun. PhD thesis, Universiti Malaya.

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Abstract

Although humic coal is generally described as a gas-prone source rock, a significant number of oil-prone coal-bearing sequences in Australasia and Southeast Asia have expelled commercial volumes of oil. Nevertheless, the mechanism of oil generation and expulsion from humic coals remains poorly understood. Therefore, this thesis seeks to provide new insights into the geochemical controls on the petroleum potential of humic coals. A total of sixty Tertiary and Upper Cretaceous coals from Sarawak Basin, Malaysia, and Benue Trough, Nigeria were investigated using bulk and molecular geochemical techniques such as standard proximate analysis, pyrolysis-gas chromatography (Py-GC), Rock-Eval pyrolysis, Fourier transform infrared spectroscopy (FTIR), gas chromatography-mass spectrometry (GC-MS), elemental analyser isotope ratio mass spectrometry (EA-IRMS), and inductively coupled plasma mass spectrometry (ICP-MS) to determine their thermal maturity, hydrocarbon generation potential, organic matter input, kerogen type, paleovegetation, paleoclimate, and environments of deposition. Furthermore, principal component analysis (PCA) of geochemical parameters was carried out to evaluate the probable controls on the hydrocarbon generation potential of the coals. First, vitrinite reflectance and Tmax data, and maturity-related biomarker parameters all signify low thermal maturity for all studied coals except the Lamja Formation coals of the Benue Trough which are in the early maturity stage. Furthermore, hydrogen index (< 300 mg HC/g TOC), TOC (> 20 wt. %), A-factor (> 0.4) and extract yield (> 10000 ppm) data for most of the coals indicate excellent potential for gas and mixed oil and gas generation. Rock-Eval, Py-GC, and source-related aliphatic and aromatic hydrocarbon parameters indicate that the coals are derived mainly from terrestrial organic matter but with a considerable proportion of marine algal organic matter in the Benue Trough coals. Additionally, δ13C values and the abundance of terpenoids imply the predominant contribution of gymnosperms and angiosperms to the paleoflora of the Benue Trough and Sarawak Basin, respectively. Bimetal proxies (Sr/Ba, Sr/Cu, and C-value), and δD values are generally suggestive of a warm and humid climate during the accumulation of the paleopeats. However, the presence of ≥ 6-ring combustion-derived polycyclic aromatic hydrocarbons (PAHs) in the Gombe Formation coals of the Upper Benue Trough implies relatively drier conditions during the Maastrichtian. Additionally, n-alkane proxies (Pwax, Paq, n-C23/n-C29, etc.) suggest that Liang Formation coals of the Sarawak Basin were deposited under relatively drier and strongly seasonal paleoclimate during the Late Pliocene. When compared with published global average abundances, the investigated coals are mostly depleted in major oxides and trace elements, suggesting peat accumulation in freshwater-influenced environments. Furthermore, the low to moderately high ash content of the Sarawak Basin coals indicates the presence of ombrotrophic and rheotrophic peat deposits, while the relatively higher ash content of the Benue Trough coals is indicative of prevalent rheotrophic conditions. PCA result of selected geochemical proxies suggests that thermal maturity, source input, peat hydrology, paleoflora, and marine incursions do not influence hydrocarbon generation potential. However, climatic, and depositional conditions both appear to slightly influence the hydrocarbon generation potential of the studied humic coals.

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