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The AAPG/Datapages Combined Publications Database

Indonesian Petroleum Association


16th Annual Convention Proceedings (Volume 1), 1987
Pages 339-361

Peusangan–C1 Reef Interpretation with Top Reef Transparent to Seismic

L. B. Hinton, W. S. Atmadja, Suryo Suwito P.


The Peusangan-XF structure, offshore North Sumatra, is located about 70 kilometers northwest of the giant Arun gas field and about 3 kilometers north of the coastline. The structure is a large carbonate build-up within the Early to Middle Miocene Peutu Formation and is located on a large paleotopographic feature called the Western High which is sparated from the Arun-Cunda High to the east by the Jawa Deep. Structural interpretation of seismic data prior to drilling was based on a high amplitude reflector which was interpreted as being an Early Miocene/Late Oligocene carbonate complex overlying a basement high. The Peusangan-C1 wildcat well tested this structure in late 1986 and encountered 669 feet of the Miocene Peutu limestone at 11,685 feet subsea. The upper 486 feet was very porous reefal limestone while the lower 183 feet was tight and dense dolomitic limestone. Poor to fair hydrocarbon shows were observed in the top 100 feet of the porous limestone but flowed mainly water on test.

A post-drilling study of the seismic data has produced interesting results. The high amplitude seismic reflector previously mapped as the top of a carbonate buildup actually corresponds to the lower, tight and dense dolomitic limestone interval while the upper good reservoir quality limestone produces only a very weak Previous HitreflectionNext Hit. The acoustic impedance contrast between the denser, low velocity shales and the less dense, higher velocity reef is slight. Vertical Seismic Profile data and synthetic seismograms confirm the lack of Previous HitreflectionNext Hit response at the top of the limestone.

The C1 well results were compared to strong seismic definition of reef character at the Arun field. In addition, velocity changes with gas-water saturations for the Arun limestone were examined.

In an effort to delineate this good reservoir, modeling and reprocessing of key lines were initiated. Field records were also examined and showed little or no seismic definition of the reef limestone.

In carbonate sedimentation areas, Previous HitreflectionTop seismic may not always define "reef character". Areas of possible occurrence of reefs largely transparent to conventional seismic may be defined by geological interpretation combined with seismic modeling.

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