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NSB 'A' Field: Construction of Geological Model Incorporating Data from the Anomalous Well, NSB-A7
The NSB "A" field lies on the Malacca Platform of the Sunda / Malacca Shelf in the North Sumatra Basin, near the northwest edge of the North Sumatra Offshore block.
The NSB A-1 discovery well found gas in the Lower-Middle Miocene Malacca reefal limestone. Five subsequent appraisal wells confirmed the discovery but data recovery was poor and a final appraisal well, NSB A-7, was spudded in 1990 near the center of the field with the objective of completing the pre-development data base. Plans included a thorough drillstem test program, coring the entire reservoir section, and penetrating basement for base - Malacca seismic control. However, the quantity of data obtained was disappointing and some results were surprising.
Repeated heavy losses of drilling fluid forced curtailment of the coring program and drilling had to be halted well short of basement. Furthermore, log responses suggested generally lower porosities than in surrounding wells. With the nearest wells only approximately 1 km away it was suspected that the apparent low porosities could be due to non-correlatable causes such as the effects of loss prevention materials. However, a lithologic check revealed an anomalously high dolomitic content in the NSB A-7 well; this was confirmed by running detailed calcimetry on surrounding wells and was supported by mixed lithology log analysis. Therefore, the apparent low porosities were due at least in part to lithological changes.
With this information, a field geological model was then constructed by zonation of the reservoir using porosity trends correlated among wells. A diagenetic model was invoked to estimate areal distribution of dolomite away from well control. Risk analysis was used to accommodate the range of volumetric uncertainties.
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