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Biomarker studies using oil samples and sediment extracts from wells located in the northern part of the Sabah Basin show that the organic matter source for the basin's hydrocarbon to be dominantly terrigenous, indicated by high triterpane/sterane ratios, the presence of compounds diagnostic of land­ derived organic matter such as oleanane, and resins W, T and R, and the general predominance of C29 regular steranes over C27 and C28 components. The consistently similar mass fragmentograms of both mJz 191 and mJz 217 suggests that the oils and sediment extracts originated either from the same source rock or, more likely, from different source rocks with similar chemical compositions. The similarity of biomarker distributions in samples from different environments suggests that detrital plant components from similar higher land plant assemblages were redistributed by sedimentary/transport processes to various depositional settings. Petrographic studies of kerogen concentrates confirm the dominance of land-plant contribution and the general absence of marine organic matter even in the marine sediments. For the oils, the 20S/(20S+20R)-sterane biomarker maturity ratios are lower than the equilibrium value of 0.55 and moretane/hopane ratios > 0.10. This supports the apparent retardation of the isomerisation process previously reported by Grantham (1986) to occur in Far Eastern Tertiary oils. The triterpane 22S/(22S+22R) ratio, on the other hand, appears to reflect the expected maturity levels of the oils. The majority of the extracted sediments are shown to be immature by pyrolysis Tmax, vitrinite reflectance and kerogen elemental composition, indicating that the extracted hydrocarbons have migrated into the sediments from elsewhere. Therefore, biomarker maturity ratios from the extracts cannot be used to determine the true maturity levels of the sediments as the in situ biomarkers will have been mixed with those already present in the migrating hydrocarbons. Variations of biomarker maturity ratios with depth resemble those of the Production Index in all the studied wells, thus confirming the influence of non-indigenous residual hydrocarbons on the biomarker maturity ratios.

Sterane and triterpane biomarker characteristics from oils and sediment extracts of the Middle-Upper Miocene sequences, Northern Sabah basin