ABSTRACT: Structural and petroleum systems modelling of the Eocene Ngimbang-sourced petroleum systems of the eastern East Java Sea. Part 2: Petroleum systems model

Although several source rocks and their resulting petroleum systems can be defined in the eastern East Java Sea, our discussion is limited to the Petroleum System based on extensive coals and carbargillites developed within the Eocene Ngimbang clastics. The oil and much of the gas potential of the system is limited to the bedded coals, which have high Hydrogen Index (mean initial HI 333 mg/gC) and are highly wax-prone. Net coal (>50% TOC) thickness generally increases in response to the total Ngimbang clastics isopach, ranging from absent in thin, alluvial-upper delta plain areas, up to 30m thick.

The majority of the correlative petroleum products (waxy oil where low mature, gas-condensate where highly mature) are confined to interbedded fluvial Ngimbang clastics, or immediately overlying Ngimbang carbonates, although some leakage occurs locally into the Neogene.

Large areas of the basin can be shown to be at maximum present day burial and thermal stress, and heat flow and geothermal gradient are high by world standards. However, careful calibration and validation of the coals' generation and expulsion behaviour reveals that they have a high oil expulsion temperature (averaging 140 degrees C at 5 degrees C/Ma) which severely limits the areas of effective kitchen. This is especially true in the deep water areas of the basin where the overburden is typically 500m thinner and the temperature at the sediment-water interface is typically 25 degrees C cooler. Furthermore, due to low oil expulsion efficiency, most (about two thirds) of the coals' potential can only be realized as gas at high temperatures (>160 degrees C). Since the areas of gas kitchen development are even more restricted than the oil kitchen, it seems likely that the true volumetric potential of the Ngimbang petroleum system is likely to be realized only after millions of years of future burial and heating!

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia