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Re-Evaluation of the Sedimentology and Evolution of the Kais Carbonate Platform, Salawati Basin, Eastern Indonesia: Exploration Significance
Miocene Kais carbonates of the Salawati Basin have been productive reservoirs since 1936. There have been approximately 160 exploration wells drilled in the basin targeting the carbonates. As many as 30 fields have been discovered which have produced 375 MMBO and 175 BCFG through 2002. Because of extensive exploration, there is a large amount of data available on Kais carbonates and several studies on the carbonates, both of regional and field scales, have been published. Studies from the 1970s to early 1990s resulted in similar regional interpretations, mainly because the studies were conducted by the same operator.
Beginning in the mid-1990s, Santa Fe Energy Resources (later Devon Energy and now PetroChina) assumed operatorship within the Salawati Basin and Kais carbonates were re-evaluated. Regional studies from 1997-2000 have yielded new interpretations on the distribution, paleo-environments, and age of the Kais reefs and carbonates, based on regional seismic re-mapping, newly acquired seismic data, and petrographic-biostratigraphic data from 85 exploration wells within the basin. Studies of basin evolution, geochemistry, and structure have contributed to the understanding of the evolution of the Kais platform.
The revised interpretation concluded that the Kais carbonates of the Salawati Basin had continually developed within the shelfal area since the early - middle Miocene. The lagoonal facies of the carbonates, which are locally proven source rocks, was previous ly interpreted as basinal facies.
Productive reefs of the Kasim complex, previously interpreted as pinnacle reefs growing at the slope of the southern shelf, are actually lagoonal reefs which grew behind the Walio barrier carbonates. Basinal facies of the Kais were deposited to the south and southwest of the study area. These basinal facies were previously interpreted as carbonate shelf deposits. The changes in interpretation mainly resulted from recognition that the Salawati Basin was undergoing inversion (basin's polarity reversal) during Kais sedimentation. This was not taken into account in previous interpretations.
The re-evaluation of the Kais carbonates has provided the rationale for the discoveries in the lagoonal areas previously considered as basinal areas (Matoa, SWO, Matoa-20, Amuk) and opened opportunities for other prospective areas outside the classic Kasim-Walio area, as proven by the offshore Koi discovery in 2000. This study concludes that a new look at an old objective is important for reviewing exploration interest in a mature basin like the Salawati Basin.
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