Bulletin of the Geological Society of Malaysia, Volume 37, July 1995, pp. 13 – 31
Texaco Inc., 3901 Briarpark, Houston, TX 77042 USA
Abstract: The Pematang Formation of the Central Sumatra basin has been shown to be the primary, if not the sole, source for the basin‘s 10+ billion barrels of recoverable oil. This lacustrine unit, which is restricted to a series of Paleogene half-grabens, typifies the variability present in many rift source rock systems.
Differences in organic facies (i.e., level of organic enrichment, kerogen composition, bitumen and product character) occur both stratigraphically and laterally as a result of a series of complex interactions. The stratigraphic controls on organic facies within the Pematang appear to be: (1) the changing relationship between subsidence and sedimentation; (2) long-term climate trends influencing both lake level and regional vegetation patterns; and (3) the evolution of the lake basin‘s nutrient pool. These controls result in an increase in the level of organic enrichment and oil-proneness toward the top of the unit. Superimposed on this pattern are changes in organic matter content (carbon content) and character (palynofacies, biomarker and isotopic compositions) caused by short-term climatic oscillations. Lateral controls on organic facies can be examined at two different scales – basinal and sub-basinal. Basinal variations can be related to: (1) different relative subsidence rates among the various sub-basins; and (2) variations in lake water chemistry as a result of the nature of the paleodrainage basin. On a sub-basinal scale organic facies appear to be controlled by: (1) hydrodynamic processes and basin circulation; and (2) variations in water depth. These lateral variations are manifested in the level of organic enrichment, the relative proportions of oil- and gas-prone kerogen, and variations in oil composition.