The Hydrocarbon Property Variation in the West Baram Delta Petroleum System: Unravelling the Respective Effects of Biodegradation and Source Facies

Azlina Anuar and M. Jamaal Hoesni
Subsurface Technology Group, PETRONAS Research Sdn Bhd, Kajang, Malaysia

In contrast with the majority of the world’s deltas which are mostly built upon passive margins, the West Baram Delta is deposited on an actively subducting margin located offshore NW Borneo. Rapid clastic influx, due to the Middle Miocene uplift and subsequent erosion of the Sabah and Sarawak landmass, with simultaneous space creation led to the accumulation of the West Baram Delta’s 8-9km thick depositional sequence of predominantly terrigenous origin. The source of the West Baram Delta hydrocarbons was intuitively subscribed to Type III land-derived kerogen. Biomarker evidence conclusively determined that these oils were generated from land-plant materials preserved in oxidising environments (Pr/Ph ratios 3.5-7.0). Wide variations in API gravities (19.4° to 45.3°), wax contents (1.8-14.8wt%) and pour point temperatures (-25° to 24°C) are attributed to biodegradational effects as the result of in-reservoir bacterial alterations and not due to source facies differences. The petroleum system of the West Baram Delta has never been truly defined despite prolific hydrocarbon occurrence, often hampered by the inability to identify discrete source intervals. A generally organically lean nature of the sedimentary sequences (TOC 0.1-17wt%, average = 0.85wt%, std deviation = 1.1wt%) in the delta is believed to be compensated by its sheer volume and the closely interbedded sand and shale intervals. Preliminary oil-source rock correlations indicate a Late Miocene coastal-fluvial shale interval as a potential source rock. Using these insights as boundary conditions, the hydrocarbon generation, migration and accumulation is modeled using 2D seismic data. The effect of overpressure, which may be caused by disequilibrium compaction and/or gas generation, on the distribution of hydrocarbon accumulations in the West Baram Delta is investigated.

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