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. Reservoir quality in the Cepu fields appears to change with stratigraphic age with the Miocene having better reservoir quality than the Oligocene. In addition, there are reservoir quality differences between fields of similar age. Core analysis and thin section studies show that these isolated carbonate buildups were all deposited as grain dominated shallow water platforms with similar facies distribution. However, there appears to be variation in the amount of diagenetic leaching that controls reservoir quality. Reservoir quality can be correlated directly to subsidence rates. Subsidence rates control the amount of time the fresh water lens has to leach the carbonates and enhance the reservoir quality. Improved age control and facies analysis coupled with 3D seismic imaging and depth volumes have allowed a more detailed interpretation of the sedimentation rate of these carbonate buildups. Data from three different fields in the Cepu block suggest * Mobil Cepu Ltd that relatively high sedimentation rates occurred during the Oligocene Chattian, much lower sedimentation rates predominated during the Miocene Aquitanian and lower Burdigalian and again higher sedimentation rates prevailed in the upper Burdigalian. Age dates and subsidence rates have been refined and calibrated better by improved large foraminifera discrimination. Strontium isotope analyses have not been useful in this environment due to extensive diagenesis.