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The Arun gas and gas distillate field (estimated 13.7 tcf hydrocarbon gas in place) is a large Miocene coral-algal reef complex located on an intrabasin high in the North Sumatra basin. It was discovered in 1971 by Pertamina/Mobil Oil Indonesia following definition of a reflection seismic anomaly. The field is a large, asymmetric stratigraphic trap 18.5 km (11 mi) long and 5 km (3 mi) wide. The Arun Limestone which forms the reservoir is overlain, underlain, and possibly surrounded by shale. The limestone ranges in thickness from zero west of the field to about 1,200 ft (365 m) in well A-10. Closure on the Arun Limestone is at least 1,200 ft (365 m).
The reservoir rock is made up of several carbonate rock types, including coral-algal boundstones, foraminiferal packstones and wackestones, mixed-skeletal wackestones and packstones, and dolomite. Interstitial fill of the reef consists of lime and reef detritus (i.e., skeletal wackestones and packstones); grainstone fabrics are notably absent in the Arun reef and related facies.
Diagenesis has had a strong effect on the original sediments; (1) the lower part of the reservoir is completely dolomitized; (2) patches of limestone throughout the Arun are recrystalized to sparry calcite; and (3) much, if not most, of the reservoir is strongly micritized. The most pronounced effect of diagenesis has been the formation of secondary moldic and vuggy porosity formed by the dissolution of aragonite fossils (mainly branching corals, mollusks, and foraminifera). Had there been no diagenesis of the original limestone, it is doubtful that a hydrocarbon reservoir would be present on the Arun structure.
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