Publications

In Indonesia and much of SE Asia, some of the largest and most prolific hydrocarbon reservoirs are derived from isolated, shallow-water Tertiary carbonate buildups, including Natuna, the fields of north Sumatra, and those of the Cepu block, East Java Basin. ExxonMobil has been exploring and developing gas and oil fields at Cepu since 1999 with the discoveries of Sukowati, Banyu Urip, Cendana and Jambaran fields. The depositional history and controls on carbonate growth of the Cepu block has been of great interest to industry and academia due to the importance of the carbonate buildup play in SE Asia. In this study, improved age control and facies analysis coupled with 3D seismic imaging and depth volumes have allowed a more detailed interpretation of growth, expansion, backstepping and final drowning of the buildups. Data from the Jambaran field indicates an abrupt change from a low accommodation ramp with prograding shoals in the Late Eocene to more aggradational carbonate growth in the Late Oligocene and into the Early Miocene. In addition, several unconformities have been recognized on well logs and seismic data. This study allows the accurate calibration of these encountered sequence boundaries, i.e. the Eocene-Oligocene, the Rupelian 1-Rupelian 2, the Rupelian-Chattian and the Chattian 1 - Chattian 2 boundaries have been identified. As these events coincide with global sea level fluctuations, subaerial exposure and subsequent meteoric diagenisis, recognizing these unconformities not only helps regional correlation but is also important in terms of reservoir quality distribution. This paper utilized the most recent data acquisition and interpretation at Jambaran in order to demonstrate that global eustasy in general and the high amplitude Oligocene and Miocene sea level fluctuations in particular, may have played a larger role in the depositional history of shallow-water carbonate buildups than previously assumed.