FLUVIAL SYSTEM SEQUENCE STRATGRAPHY IN A RIFT BASIN: PERSPECTIVE FROM OLIGOCENE PEMATANG FORMATION OF THE BENGKALIS TROUGH, MALACCA STRAIT, INDONESIA

摘要

A Rift Basin's stratigraphic framework is commonly linked to its rapid mechanical subsidence, where episodic depositional successions are primarily controlled by the tectonic subsidence with short periods of rapid accommodation, in response to extension and fault reactivation. The Bengkalis Trough is part of a rift basin in the Malacca Strait, and is comprised of Paleogene syn-rift strata, which has a very promising hydrocarbon system. These include the Pematang Formation, comprised of thick conglomeratic sandstone layers overlain by organic-rich brown shale deposits. This study aims to better understand the complexities of sequence stratigraphy and depositional processes of fluvial systems within a rift-basin. Numerous data sources were incorporated in this study, such as cores, borehole images, wireline logs and biostratigraphic data, in order to formulate a comprehensive interpretation and model. It is hoped that the analytical description and stratigraphic approach may contribute insight into syn-rift sequence stratigraphy and sedimentary processes during deposition of the Pematang Formation. This interpretation indicates that the Pematang Formation was deposited during a series of transitional progressions in a fluvial environment. Major sedimentary features were identified: (1) massive conglomerate (Gm) facies; (2) planar cross- bedded sandstone (Sp) facies; (3) laminated sandstone (Sh) facies; and (4) massive claystone (Fm) facies. Three depositional sequences were observed based on stratigraphic evidence, with each sequence bounded by flooding surfaces (SB1, SB2, SB3, and SB4). In general, major sequences are: Lowstand System Tract (LST) ― stacked-channel deposits in braided system, and Highstand Systems Tract (HST) ― individual channel deposits in meandering system. By utilizing integrated resources to define stratigraphic sequences and sedimentation processes the depositional history of the rift-related sedimentary basin is revealed.