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ABSTRACT: Tertiary Reservoirs Offshore Southeast Sumatra--Sunda Giants Revisited

Robert K. Park, I. Asjhari, K. I. Martin, S. Mitrodihardjo

The Sunda basin is one of several depocenters occurring within the Sumatra-Java back-arc province, one of the world's major hydrocarbon fairways. Basin development began in the late Eocene, when high sedimentation rates matched the rapid subsidence, resulting in a continental fluviatile and lacustrine clastic dominated sequence. This sequence includes the Talang Akar Formation, a primary exploration target in the basin. Sedimentation rates decreased toward the end of Oligocene when transition facies were deposited prior to the onset of the marine conditions. The lower to middle Miocene Batu Raja carbonates form the other major play in the basin.

Two fields, Cinta and Krisna, which have been on production for more than 10 yr, have recently been reexamined and the data are being used to improve reservoir modeling, field development, and management.

Cinta is supported by a strong water drive with good vertical communication through the multiple stack of braided channels ensuring pressure support in the crestal region. Interruption by irregularly developed shale stringers and numerous small faults introduce an element of heterogeneity, which combines with an adverse mobility ratio to cause poorer than expected sweep efficiency. Despite early water breakthrough, subsequent slow increases in water cut presage a long productive life.

Poor sweep efficiency also is a characteristic of Krisna field. This carbonate reservoir is undersaturated and exhibits a typical harmonic decline. The mobility ratio of oil is good, but the carbonate reef-to-slope facies is more heterogeneous on a small scale and is supported by only a moderate water drive. Fracture stimulation can be helpful in some instances but generally is not appropriate in this highly argillaceous sequence.

Despite these insights, production histories from the numerous fields within the basin suggest that reservoir typecasting may be ill advised.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990