Publications

The Sabah deepwater fold-thrust belt is one of the most actively explored areas in Malaysia, with significant oil and gas discoveries proving a world-class working petroleum system. However, the mature source rock kitchens in deepwater Sabah are not well-defined and the hydrocarbon charge mechanism remains uncertain. Recently, the Bestari oil discovery in offshore Sabah “Block X”, found multiple stacked oil accumulations in Miocene turbiditic reservoirs. To better understand the hydrocarbon charge and migration mechanisms, detailed geochemical analyses were conducted on reservoir oil samples, together with mud-gas samples. Whole oil gas chromatography (WOGC) analysis revealed that the shallower Kamunsu oils are biodegraded to a moderate level, characterised by the substantial depletion of long chain n-alkanes, whereas deeper Kinarut oils exhibit high n-alkanes values and are only partially biodegraded. The API gravity of the oil samples can be correlated to the degree of biodegradation, providing a reason for the observed variation in reservoir fluid compositions. The oil samples show a strong terrigenous organic matter affinity, as evidenced by a high abundance of long chain n-alkanes, high oleanane index (>50 %) and higher C29 over C27 and C28 regular steranes. Biomarker GC-MS finger-printing shows remarkable similarity between samples and biomarker ratios based on aromatic steranes, which are identical for the oils from all reservoirs. The indication therefore is that the oils have a common origin and that the Kinarut oils have probably leaked upwards into the Kamunsu where biodegradation has occurred. Mud-gas isotope data suggests that isotopic reservoir separation between Kamunsu and Kinarut intervals occurred transitionally through thick intervening Mass Transport Complexes (MTCs) and seems to be related to diffusion during oil migration from the deeper to shallower reservoirs. These geochemical analyses suggest a vertically leaking charge model, which is validated by an in-house basin modelling study. Oil migrating into the shallower reservoir accumulations was affected by biodegradation resulting in lower API gravity.