3D seismic geomorphology of fluvial systems

Abstract

Fluvial sandstones constitute one of the major clastic petroleum reservoir types in many sedimentary basins around the world. This is especially true in the Tertiary basins of Southeast Asia, which display a wide range of fluvial channel reservoir types. This study is based on the analysis of high-resolution, shallow (seabed to ca. 500 m depth) 3D seismic data which provide exceptional imaging of the geometry, dimension and temporal and spatial distribution of fluvial channels. The Malay Basin comprises a thick (>8 km), rift to post-rift Oligo-Miocene to Pliocene basin-fill. The youngest (Miocene to Pliocene), post-rift succession is dominated by a thick (1-5 km), cyclic succession of coastal plain and coastal deposits, which accumulated in a humidtropical climatic setting. This study focuses on the Pleistocene to Recent (ca. 500 m thick) succession, which comprises a range of seismic facies, mainly reflecting changes in fluvial channel style and gross stratigraphic architecture. The succession has been divided into four seismic units (Unit 1-4), bounded by basin-wide stratal surfaces. Units 3 and 4 have been further divided into two sub-units. Two types of boundaries have been identified: 1) a boundary that is defined by a regionally-extensive erosion surface at the base of a prominent incised valley (e.g. Horizons C.1 and D.1); 2) a sequence boundary that is defined by more weakly-incised, straight and low-sinuosity channels which is interpreted as lowstand alluvial bypass channel systems (e.g. Horizons A, B, C, and D). Each unit displays a predictable vertical change of the channel pattern and scale, with wide low-sinuosity channels at the base passing gradationally upwards into narrow high-sinuosity channels at the top. The wide variation in channel style and size is interpreted to be controlled mainly by the sea-level fluctuations on the widely flat and tectonically-quiescent Sundaland Platform.