Abstract

In 1996, P.T. Exspan Sumatera, an affiliate of Medco Energi Corp., discovered a substantial amount of oil in the Baturaja Formation (Early Miocene) by drilling three consecutive wildcats in the Semoga, Kaji and Sembada structures in its Rimau Block, South Sumatera. The area around Rimau Block has produced from the Batu Raja for many years and has been considered mature with little remaining potential. The new discoveries have stratigraphic trapping components which reveals significant exploration potential over the South Sumatera area. In early 1997, Exspan drilled the Tiki-1 wildcat just off the block and struck gas with a thin oil column in the same formation. These successes represent the first Baturaja discoveries on a structural element called the Palembang High in Rimau Block, where previously only the Talang Akar Formation (Oligo-Miocene) had been productive.

Appraisal wells and the ensuing development program in the Kaji area have provided a wealth of information about the petroleum system in the block. The new data show that on the Palembang High, the Baturaja Formation is represented by a good quality reef-related carbonate reservoir. Geochemical analysis suggests that the hydrocarbon in the Kaji-Semoga structures was generated from shallow lacustrine shales of the Talang Akar and the Lemat Formations in nearby kitchens. The hydrocarbon was trapped in current structures by a combination of structural and stratigraphic controls, after undergoing migration processes into the paleo accumulation traps and then remigrating into the present traps. The Telisa shales acts as an effective top seal for the trapped hydrocarbon whilst the facies change of the Baturaja carbonates acts as the lateral seal.