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Carbonate reservoirs are the main hydrocarbon producers in Western Indonesia and has been successfully developed by Medco Energi. Hydrocarbons have been produced from the Baturaja, Peutu, Berai, Mundu and Kujung Formations. This publication summarizes the diversity in these carbonates, leading to varying reservoir properties, production performance and some lessons learned for future exploration.

The Miocene Baturaja Formation (BRF) in South Sumatra Basin (SSB) was developed as a series of mounded reef complexes and platform carbonates. Those carbonates are characterized by extensive low-relief build-ups and fringing carbonates. Rapid changes of sea level during deposition resulted in heterogeneity of the rock properties. To date, the cumulative BRF production is 273 MMBO and 500 BCF from the Musi and Palembang Highs.

Strong meteoric diagenesis and fracturing resulted in porosity and permeability enhancement of the Lower Miocene Peutu carbonate in Alur Siwah Field - North Sumatra Basin. After the field reactivation project, production from the old Alur Siwah Field commenced in 2018 with cumulative production of ~100 BCF.

The Oligocene Berai Formation is the only carbonate gas producer in the Kutai Basin. In the Kerendan Field, Berai is characterized by a robust, table-top shape and tight gas reservoir deposited in a semi-restricted ocean. The Upper Berai carbonate has estimated gas reserves greater than 1 TCF. Despite the tight reservoir and high pressure high temperature (HPHT) conditions, it has been produced since 2016.

The Pliocene Mundu Formation in the offshore NE Java Basin is a unique reservoir that consists of the hollow tests of Globigerina foraminifera fossils. Mundu is a layered reworked carbonate deposited in a deep-water environment and has excellent reservoir quality. By 2024, 710 BCF had been produced from Mundu in Medco's operated blocks.

The Lower Miocene Kujung oil discovery was made on a complex coral patch reef characterized by high-relief morphology of the karst system. Despite the Kujung having generally low porosity, karstification and dolomitization provide enhanced porosity-permeability intervals. Identifying karst and dolomite zones are critical for future well placement and field development plans.

Even though carbonate reservoirs have been produced for a long time, some surprises are still encountered during exploration. Good understanding and ability to accurately predict the facies and the corresponding reservoir quality distribution are critical part for successful exploration and development, yet those aspects remain challenging.