Coaly source rocks of NW Borneo: role of suberinite and bituminite in oil generation and expulsion

The coaly source rocks currently investigated are of Tertiary age, collected from areas of Bintulu, Sarawak and Labuan Island, East Malaysia, and Brunei-Muara, Brunei Darussalam. Much of the petroleum in the Southeast Asian region was generated from organic matter of higher plant origin deposited within a lower delta plain to prodelta setting. However, the specific nature and origin (other than being 'higher plant') of the organic matter, has not been identified. In this study, the maceral suberinite and, the associated maceral, bituminite have been recognised as the most oil-prone macerals. It was petrographically observed that subsequent to hydrocarbon generation, the suberinite-phlobaphinite framework breaks down, giving rise to liptodetrinite, fluorinite, vitrodetrinite and exsudatinite. Accumulation of disordered or disintegrated suberinitic constituents subsequently formed bituminite. These suberinitic-rich coals appear to be associated with mangrove precursors prominently within paralic and shallow marine depositional settings. It was also noted that these hydrogen-rich constituents may have been transported into relatively deeper marine environments as suggested by the presence of suberinite in turbiditic sediments from Labuan Island.