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ABSTRACT: Stratigraphic architecture of multiple incised valley-fill successions in the Menggala and Bekasap formations (Miocene), Bekasap Field, Central Sumatra Basin, Indonesia

Denison, Christopher N.1, Timothy J. Galvin2, none Pujiarko2, and James M. Randle2
(1) Chevron Overseas Petroleum, San Ramon, CA
(2) Caltex Pacific Indonesia, Duri, Indonesia

The Bekasap field is currently producing 23,130 BOPD from 72 wells, mainly from the Bangko and Bekasap formations. As part of a recent reservoir characterization project, five incised valley-fill successions were recognized in a revision of the stratigraphic architecture. Core descriptions, including four essentially complete cores through the reservoir interval, with environmental data from biostratigraphy, provided a detailed facies framework. This was extended by wireline log correlation through 93 wells, enhanced by iterations with 3D seismic.

Above each sequence boundary, incised valley-fill consists of a lower fluvial/tidal, coarse grained, pebbly cross-bedded channel sandstone and an upper, fine to medium grained, highly bioturbated estuarine sandstone succession. Above a marine floodback surface, delta front siltstone/shale parasequences have been variably incised by the subsequent sequence boundary.

Stratigraphic sequences are asynchronous with the established lithostratigraphy. In the Menggala Formation, the older M-1 sequence, known only from logs, has an unconformable base on fluvial/lacustrine Pematang Formation (Paleocene-Eocene). Incision at the base of sequence M-2 locally produces sand-on-sand contacts with M-1. The Menggala elsewhere is considered to be fluvial, but cores in sequence M-2 are mainly highly bioturbated estuarine sandstones. The upper part of M-2 includes delta-front siltstones attributed to the Bangko Formation. Sequence B-1 comprises most of the Bangko (incised valley-fill sandstones), together with the lowest part of the Bekasap Formation (delta-front siltstones). Sequence B-2 is within the Bekasap. Sequence B-3 includes the upper part of the Bekasap, a major floodback surface at the Bekasap/Duri formation boundary, and shelfal parasequences in the Duri Formation.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia