Publications

In 2001, Mobil Cepu Ltd. acquired a very large, high quality onshore 3D seismic survey in the Cepu Block, East Java Basin, which provides a unique opportunity to understand the depositional and burial history of an extensive carbonate platform. Figure 1 shows the location of the Cepu Block and 3D seismic survey.Oligo-Miocene age isolated carbonate build-ups are the major proven reservoirs that have recently been discovered in the Cepu area. Several significant oil and gas discoveries have been found in these reservoirs including the Banyu Urip oil field which is one of the largest oil fields found in Indonesia since the 1970's (Figure 2).Carbonate build-ups in the Cepu Block have diverse morphologies ranging from steep-sided, narrow pinnacles to broad platform deposits. These buildups developed on a single larger isolated platform that began to be deposited during the Lower Oligocene. Through the Upper Oligocene and Lower Miocene, carbonate deposition ceased over significant areas of this platform while other areas continued to grow as carbonate deposition kept up with relative sea level rise. This process eventually resulted in several separate and isolated carbonate buildups that drowned at different times and have distinct morphologies, mostly related to the underlying extensional faults and relative subsidence rates across the platform.The thickest of these carbonate buildups are up to 2 km thick. The thicker buildups drown in Lower Miocene time and are covered by Middle Miocene clastics that are low quality seals. However in certain cases, these result in large oil accumulations because trapped gas is held in overlying clastic reservoirs or is leaked due to a top seal with low capillary-entry pressure. Other areas of the Cepu platform drowned earlier during the Oligocene. These carbonate deposits have different morphology, lower reservoir quality and more clayrich seals with high capillary-entry pressure, and commonly contain large gas columns.Documenting the variations in Oligo-Miocene carbonate sedimentation has been instrumental in understanding these hydrocarbon accumulations and should have relevance for similar reservoirs in other Indonesian basins.