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Origin of Hydrocarbons, Kangean Block Northern Platform, Offshore N.E Java Sea
The Pagerungan gas field was discovered by ARBNI in 1985. This discovery, along with the JS53A-1 oil discovery by Cities Service in 1977, confirmed that a mature hydrocarbon-generating kitchen area was present on the Northern Platform province of the Kangean PSC.
Results from an integrated geological and geochemical study of Northern Platform hydrocarbons and source rocks indicate that hydrocarbons in the JS53 oil field and in the Pagerungan gas field were generated by Eocene and pre-Eocene age source rocks in an east-west trending synclinal kitchen area which runs between the two fields. The Late Eocene age Ngimbang Clastics source facies (coals and carbonaceous shales) have been correlated geochemically to the oil at JS53 and to the condensate at Pagerungan. Geohistory modeling indicates that the Paleocene-Middle Eocene age Pre-Ngimbang Formation is the source of the gas at Pagerungan. The Cretaceous-age sediments are shown to be over-mature and non-generative.
Interpretation of seismic (tied to well penetrations) revealed the nature of the structuring of the Northern Platform. The seismic shows an east-west trending synclinal trough in the Cretaceous and Pre-Ngimbang sediments just north of the Pagerungan Field. The north limb of the syncline subcrops beneath the JS53 field and the Igangan-1 well, while the south limb of the syncline subcrops beneath the Pagerungan Field. Geohistory modeling shows that Ngimbang and Pre-Ngimbang sediments (but not Cretaceous) are at maximum burial today in the syncline. Oil and gas generated by these formations migrated up the limbs of the synclines and into the Ngimbang reservoirs at JS53 and Pagerungan structures.
Hydrocarbon charge calculations were performed in order to model the amount of hydrocarbons generated and expelled from the source intervals in the kitchen area, which were then available for migration into JS53 and Pagerungan closures. The calculated volumes compare favorably to the observed volumes of oil and gas in place in the two structures. It is concluded that hydrocarbon generation on this part of the Northern Platform was triggered relatively recently by sedimentation associated with a north-south compressional event, which affected the Kangean PSC in the Late Miocene. As the Pagerungan anticline rose, sediments that were shed to the north filled in and deepened the synclinal trough, triggering hydrocarbon generation and migration which then filled the Pagerungan and JS53 structures.
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