Understanding Oil Compositional Variations Across West Baram Delta Through Petroleum System Modelling

Jamaal M. Hoesni¹, Juhana Mishan², and Azlina Anuar^1
1Subsurface Technology Group, PETRONAS Research, Kajang, Malaysia
²Petroleum Geoscience, Universiti Teknologi PETRONAS, Tronoh, Malaysia

The West Baram Delta is one of the most prolific hydrocarbon-bearing basins in Malaysia. In fact, the first oil discovery in Malaysia was made here in 1910. Since then, large oil and gas discoveries have been made, including nine producing oil fields. The Baram Delta complex began prograding onto the Sabah-Sarawak continental margin during the Middle Miocene, overlying the deformed deepwater sediments of the Crocker Formation (Oligocene-Late Miocene) and the Rajang Group (Paleocene-Oligocene). Growth faults downthrown basinwards is the dominant feature in the West Baram Delta Province. Wrench compressive tectonism during the early Late Miocene resulted in a series of NE trending folds and some reverse faults. The intersections between growth faults and compressional anticlines provide excellent hydrocarbon traps. The area recorded low geothermal gradients ranging between 25 to 30 ^oC/km. No single source rock horizon can be identified but the oil fingerprints suggest land-plant Type III as the precursor to the hydrocarbons. It has been a common belief that the source rock becomes more gas prone with depth and distance from the coast. This is partially supported by the oil and gas fields distribution in the area, whereby oil is generally encountered at depths up to 2 km, while gas at around 2-2.5 km. The hydrocarbon charge in the area has been investigated through calibrated 2D migration models. Results from these models assume a source rock sequence within the oil window, which occurs at about 4-5 km. Oil compositional variations can be explained by thermal stress variations and differential retention of oil and gas below thick overpressured shales. Although there is a possibility of source facies variation, this is not the primary controlling factor on the geographic distribution of oil and gas in the area.

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