Abstract

Air Serdang and Mandala fields both have reservoir rocks composed of skeletal packstones in the Lower Miocene Baturaja Formation at a depth of about 1500 meters. Air Serdang Field covers approximately 2900 acres; Mandala Field located 5 km to the south, is smaller and covers only about 1800 acres. Air Serdang has a 60-meter-thick oil column with a 45-meter gas cap; the structurally higher Mandala Field also has a thick gas cap, but over an oil column only 8 meters thick.

Reservoir rocks are characterized by common fragments of branching corals with molluscs, and benthonic foraminifers in a micritic and locally quite porous matrix. This same fauna occurs in the more shaly and tighter intervals along with platy corals and planktonic foraminifers. The porous reservoir rocks in both Air Serdang and Mandala fields were deposited in a complex of carbonate mudbanks draped over a basement paleohigh during the Early Miocene marine transgression. Low-relief channels separate the carbonate mudbanks.

Continuous cores through the upper 44 meters of the 56-meter thick Baturaja Formation in the ASDJ-3 well in Air Serdang Field can be divided into five reservoir units on the basis of porosity and lithology, particularly the amount of detrital clay. Clay-free packstones in Reservoir Units 1 and 3 have average porosities of 15% whereas the slightly argillaceous wackestones in Reservoir Units 2 and 4 average about 5% porosity and lack sufficient permeability to contribute much to production. Reservoir Unit 5 in the lower Baturaja is highly variable with both shaly and clean carbonates, but generally lacks significant porosity. Similar reservoir units can be recognized in the Mandala-2 cores. These cores show that low-relief carbonate mudbanks rich in micrite can be important reservoirs in the Miocene section of Indonesia.