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Abstract: 3-D Seismic Facies Analysis of a Reefal Buildup, Offshore North Sumatra

Wayne L. Alexander, Maria R. Nellia

The "A" field is located on the Sunda shelf, offshore north Sumatra. The A-1 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 was generally disappointing.

Prior to development of the offshore area, an extensive 3-D seismic survey was shot, a portion of which covered the "A" field. Interpretation of the 3-D 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 depositional 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 3-D seismic data and correlated with the well data: reef, near-reef and inter-reef. The main concerns 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 impacting reserve calculations. In addition, the reef facies, with zones of vuggy to near cavernous type porosity, was correlatable to wells that had experienced severe lost circulation. Finally, dolomite was found to occur only within the reef facies, enabling its distribution to be predicted.

AAPG Search and Discovery Article #90982©1994 AAPG International Conference and Exhibition, Kuala Lumpur, Malaysia, August 21-24, 1994