Incised Valley Sandstone Reservoirs: Kotabatak Field, Central Sumatra Basin, Indonesia—Case Example
Authors: Dawson William C. , Tankersley Terrell H. 
Published: October 2019

This study of Kotabatak field (835 MMSTB OOIP) in the Central Sumatra Basin, Indonesia provides a case example of the application of sequence stratigraphy, based on the integration of core, wire-line log, and biostratigraphic data, as a predictive reservoir characterization tool for a proposed EOR (pattern waterflood) project. A peripheral waterflood (started 1981) has not performed as anticipated because previous studies of Kotabatak field failed to recognize the presence and significance of highly permeable incised valley-filling (IVF) sandstones. IVF sandstones typically have excellent reservoir characteristics, and thus, the recognition of IVF features has important implications for reservoir modeling studies. Lithofacies maps of Bekasap strata in Kotabatak field provide geologic explanations for field-wide variations in: oil production rates, remaining oil-in-place, water injection rates, and produced water.

The Bekasap Formation is subdivided into three lithostratigraphic units (A-, B-, and C-Sands); the Bekasap A-Sand has accounted for most (80%) of the cumulative production (180 MMSTB) from Kotabatak Field. Four lithostratigraphic units (A-1, A-2, A-3, and A-4 sandstones) are recognized locally within the Bekasap A-Sand stratal package. Chronostratigraphic correlations reveal that IVF reservoirs are restricted to the Bekasap A-3 Sand. The A-3 Sand is underlain by the 21 ma sequence boundary. Bekasap A-3 incised valleys record the entrenchment of an estuarine channel complex into underlying offshore marine strata. This entrenchment of the estuarine depositional system has resulted in an abnormal vertical association of lithofacies wherein marginal marine (estuarine), and locally nonmarine (fluvial), strata directly overlie offshore marine lithofacies. Apparently, the intervening shallow marine lithofacies have been eroded during the basinward shift of estuarine processes.

Bekasap A-3 (IVF) reservoirs consist mainly of tidally-influenced estuarine channel lithofacies; these sandstones have an average porosity of &gt; 20% and an average horizontal permeability of 800 md (maximum 7.5 darcies). Characteristically, channelized Bekasap A-3 Sand reservoirs exhibit a strongly directional permeability parallel to the channel axis. In contrast, underlying Bekasap A-4 sandstones are fine-grained, glauconitic, profusely bioturbated lower shoreface lithofacies, representing a highstand systems tract (HST). The A-4 Sand has an average porosity of 15% and an average permeability of &lt; 100 md. Because these IVF and HST sandstones have markedly different reservoir properties, Bekasap reservoirs within Kotabatak field are compartmentalized. Bekasap A-3 IVF sandstones occur preferentially in the northwestern area of Kotabatak field which, historically, has exhibited excellent reservoir performance. Low-permeability Bekasap A-4 Sand reservoirs are predominant in the southeastern area of Kotabatak field and are the target of a pilot pattern waterflood.

Bekasap A-3 estuarine sandstones are overlain conformably by Bekasap A-2 / A-1 tidal lithofacies and comprise the lower part of a transgressive systems tract (TST). Uppermost Bekasap strata are overlain by the Telisa Formation which represents middle- to outer-shelf paleoenvironments of deposition. Telisa strata record the maximum mid-Miocene transgression and form the regional top seal for the giant oil accumulations within the Central Sumatra Basin. The study of Kotabatak field reservoirs demonstrates clearly that sequence stratigraphy is a powerful geologic tool for improving the understanding of stratal architecture and consequently field development strategies. The integration of sedimentologic, chronostratigraphic, and production-engineering data sets, at the field scale, is essential to the understanding of sandstone reservoir continuity and connectivity prior to reservoir simulation studies and the designing of EOR projects.