Abstract: Stratigraphic and Lateral Variations of Source Rock Attributes of the Pematang Formation, Central Sumatra
Barry J. Katz
The Pematang Formation of the Central Sumatra basin has been shown to be the primary, if not the only, source for the basin's 10+ billion bbl 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 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 basins' nutrient pool. These controls result in an increase in both the level of organic enrichment and oil-proneness toward the top of the unit. Superimposed on this pattern are changes in organic matter content and character caused by short-term climatic oscillations. The lateral controls on organic facies can be examined at two different scales: basinal and subbasinal. Basinal variations can be r lated to (1) different relative subsidence rates among the various subbasins and (2) variations in lake-water chemistry as a result of the nature of the paleodrainage basin. On a subbasinal scale, organic facies appear to be controlled by (1) hydrodynamic processes and circulation and (2) variations in water depth. These lateral variations are manifested in the level of organic enrichment, the proportions of oil- and gas-prone kerogen, and variations in oil composition.
AAPG Search and Discovery Article #90982©1994 AAPG International Conference and Exhibition, Kuala Lumpur, Malaysia, August 21-24, 1994