Abstract

The Serang Field is considered a major discovery for Unocal Indonesia during the 1990s. The fringe of the field was drilled in 1973 but the true field extent was unknown until the mid-90s when 3D seismic was shot and interpreted. Delineation drilling confirmed the new interpretation. The Serang Field time structure is partially occluded by a classic fault shadow. A velocity model was used to predict the magnitude and extent of this rather large anomaly resulting in a depth interpretation with dips opposite those observed on the time sections. Significant field extensions both north and south were made based on this early model. Development wells were targeted to avoid lost circulation in carbonates found on the footwall of the bounding fault and to optimize oil recovery.

The main reservoir interval is Upper-Miocene, stacked, coalesced, fluvial-deltaic channel sands. The sand/shale ratio is approximately 60/40, so that almost half of the oil column is usually shaled out. Geologic mapping based on the modern analogue of the Mahakam Delta has helped optimize well targeting and knowledge of the reservoir.

Recognition of the fault shadow effect, structural delineation, and stratigraphic interpretation have had a major economic impact through greatly increased field reserves, and decreased drilling costs. Risk reduction during field delineation is achieved simply by adequately bracketing the range of possible solutions before drilling and being able to respond quickly to changes while drilling. Using these methods can lead to more successful delineation in other areas where depth conversion is uncertain.