Publications

The NSO 'A' field is located on the Sunda/Malacca shelf, in the North Sumatra Offshore (NSO) Block. The NSO-A1 discovery well, drilled in 1972, found hydrocarbon gas in Middle Miocene carbonate rocks of reefal origin. Six appraisal wells were subsequently drilled, the most recent in late 1990. Because of drilling problems, mainly lost circulation in the carbonate reservoir, the well data obtained from the appraisal program were generally disappointing.In 1990, an extensive 3D seismic survey was carried out in the NSO area, a portion of which covered the NSO 'A' field. The purpose of the survey was to image more accurately the known reservoirs for determining reserves and optimizing production plans, as well as to identify viable prospects before development of the area begins.Interpretation of the 3D data over the 'A' field identified different seismic facies within the carbonate reservoir. These seismic facies have been integrated with the geological data in order to construct a depositiond model for the field. The seismic facies analysis was critical for developing the model because of the inadequate geological data obtained from the wells.Three distinct facies could be identified on the 3D seismic data and correlated with the wells: reef, nearreef and inter-reef. The main concerns which this facies mapping addressed were reserve determination, areas of severe lost circulation and the distribution of dolomite. The near-reef and inter-reef areas were found to have better reservoir properties than the reef core, thereby influencing the extrapolation of well parameters and impacting reserve calculations. In addition, the reef facies, with zones of vuggy to near cavernous type porosity, was correlatable to wells which had experienced severe lost circulation. Finally, dolomite was found to occur only within the reefal facies, enabling a model to be established for predicting its distribution.In summary, less favorable reservoir quality, severe lost circulation and dolomitization were all associated with the reefal facies identified using the 3D seismic. Based on this interpretation, future development of the NSO 'A' field will try to minimize the number of wells drilled in these reefal areas. The reefal areas probably cannot be avoided completely, however, because they do correspond to the topographic highs of the reservoir, hence they contain the thickest portion of the gross gas column. A geological model, based on the 3D facies mapping, is currently being prepared for input into reservoir simulation, the results of which will quantify the pros and cons of drilling the different facies.