Abstract

The NSO ‘A’ gas field is located on the Malacca shelf, in the North Sumatra Offshore (NSO) Block. The field was discovered in Middle Miocene carbonate rocks by drilling the NSO-A1 well in 1972. Six appraisal wells were subsequently drilled, but because of drilling problems, mainly lost circulation in the carbonate reservoir, the well data obtained from the appraisal program was limited. Initially, for volumetric determination, a simple geologic model was generated using a field-wide zonation based on E-log correlation only.

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 reservoir, for determining reserves and optimizing production plans.

Interpretation of the 3D data over the ‘A’ field within the carbonate reservoir was matched with geological information from well logs and cores in order to construct a depositional model for the field. Seismically, three distinct facies could be identified: reef, near reef and inter-reef. Vertically, these three facies could be divided into three distinct zones based on log-derived porosity trends. Porosity is generally higher in the uppermost zone, and decreases with depth. Dolomite was found to occur only in the lowest zone in the reefal facies.

The integration of the 3D seismic interpretation and detailed study of geological data resulted in a new geological model of NSO ‘A’ field. Thickness maps, porosity maps and water saturation maps were constructed for each zone and each facies. Permeability was estimated and mapped using porosity-permeability transforms, which had been generated for each facies and each zone from available core data. All of the maps were contoured guided by the seismic facies distribution.

The results of geological model indicate that the uppermost zone of the reefal facies is the most productive zone, due to common vuggy porosity. By contrast, the dolomite, which has the lowest porosity observed in the reservoir, is still potentially productive through fractures.

This model will be used as input to reservoir simulation, which in turn will help to optimize exploitation of the NSO-A.