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Eocene Submarine Fan Sedimentation in Southwest Java
Submarine fan deposits can be important exploration targets but have yet to be widely exploited as potential oil and gas reservoirs in the Indonesian region. Very few have been actively drilled or even recognized in the subsurface, even though they should be relatively frequent given the active tectonic setting of the area. There are many reasons why these deposits have received so little attention, including the lack of a well described ancient example within the region.
Well exposed coastal outcrops of the Middle to Late Eocene Ciletuh Formation located in the Ciletuh Area, Southwest Java, have been described on the basis of field study and laboratory analysis, and interpreted as a sand-dominated submarine fan complex. The outcrops consist of laterally continuous, fine to very coarse grained sandstones and sandy conglomerates. A number of classic sediment gravity flow features are present including turbidites with partial Bouma sequences, debris flow deposits and fluidized slump deposits. The sediments are believed to have possibly been deposited in a series of parallel slope grabens oriented perpendicular to slope.
Two separate lithofacies are recognized in the Ciletuh Formation; a quartzose lithofacies composed of mostly quartz (58-84%) and a wide variety of lithic rock fragments; and a less pervasive volcanic lithofacies composed almost entirely of volcaniclastic sediments. Mesozoic granitic continental crust and Late Cretaceous subduction complex areas lying to the north are interpreted to have supplied the majority of quartz and lithic fragments, while a possible Eocene local volcanic arc is believed to have sourced the volcanics.
The reservoir quality of the quartzose sandstones is poor due to near complete destruction of originally high primary porosity by a combination of compaction and carbonate cementation. Primary intergranular porosity values are estimated to have ranged from 25-40% prior to burial. Tectonic compaction associated with subduction compression is believed responsible for destruction of a large percentage of the porosity.
Even though the Ciletuh Formation deposits examined in this study have very low reservoir potential, they present a useful example of a sand-rich submarine fan in the region, and indicate that similar sandstones elsewhere in Indonesia could provide a viable petroleum reservoir under more favorable tectonic or diagenetic conditions.
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