Abstract

Alur Siwah Field illustrates problems in characterizing reserves potential early in the life of a carbonate gas reservoir. This field showed promise of substantial gas accumulation upon discovery in 1972. The first few wells confirmed a gas column in excess of 110 meters in a reefal build up of the Lower Miocene Peutu Limestone. Original gas in place (OGIP) was estimated at 727 BCFG. Two later wells were successful, but reservoir quality and thickness fell short of expectations and OGIP estimates plummeted to 195 BCFG. Development plans were postponed indefinitely.

Alur Siwah was reevaluated later as a part of a major gas project to develop reserves in Aceh. Following reprocessing of seismic data, stratigraphic interpretation of subsurface and geophysical data was combined with additional petrophysical work to characterize the reservoir. The OGIP rebounded to 568 BCFG, with substantial upside potential. 3D seismic acquisition and two infill wells completed pre-project evaluation of the field. The 3D data volume showed steeply dipping margins bounding the field, initially shrinking the OGIP to 565 BCFG. Infill drilling more than offset this by finding thicker and more porous gas pay, and reservoir simulation now indicates an OGIP volume of 717 BCFG.

New drilling, 3D seismic, and reservoir simulation thus ultimately confirmed general estimates of field sizes that were made shortly after discovery. Much lower OGIP figures generated between discovery and final delineation underestimated reservoir thickness and quality. This highlights the importance of incorporating long term visions of field size into strategic planning, rather than strict adherence to reserves estimates based on incomplete data.