Tunggul Maung is an onshore field located around 170km east of Jakarta, Indonesia. It is mainly a gas reservoir producing from the Upper Cibulakan Formation (Early to Mid Miocene), consisting of shallow-marine limestones interbedded with sandstones and shales. Physiographically, this field lies on the southern part of the Northwest Java basin, which is one of most prolitic hydrocarbon-bearing back-arc basins in Indonesia.
Initial exploration was primarily based on the structure interpretation of 2-D seismic survey and simple geological correlations. TA-1, the first exploration well was drilled in 1996 to prove for the presence of hydrocarbons in the anticlinal pool within the X-14 Zone of the Upper Cibulakan Formation. It is a discovery well, tested around 20 MMSCFG of gas from five DST intervals. Well TA-2, about 2.5km northwest of TMG-1, was then drilled in end 2001 to delineate the structure. It encountered around 17 MMSCFG of gas from four DST intervals. Additionally one interval below the X-16 Zone, just above a “new” unconformity surface, produced around 1600BOPD with 0.11 MMSCFG of gas. This oil was considered as a new discovery, because it was actually not the target of the field delineation.
Immediately following these promising results, a detailed G&G study was performed. We have been forced by this study to conclude that subtle stratigraphic and paleogeomorphic traps exist at subsurface of the field. This discovery marks the opening of a new realm for further exploration of not only gas in the known pools, but also oil in the hidden pools.
The Geology of Gas Reservoirs: Conventional and Unconventional