Abstract

The Mahakam delta, located on the east coast of Kalimantan, Indonesia, is an active delta system which has formed in a humid tropical environment under conditions of relatively high tides (up to 3 m), low wave energy (H~0.5 m), and large fluvial input. Tidal processes control the sediment distribution patterns and are responsible for the flaring estuarine type inlets and numerous tidal flats. A number of cored borings and surface sediment samples have enabled the study of the facies distribution and lithological sequences occurring in the delta, and in particular those of the potential hydrocarbon reservoirs associated with major sand accumulations.

Three major environmental systems comprise the delta: the delta plain, the delta front and the prodelta.

The delta plain consists of an intertidal swamp with Nipah and mangrove vegetation and is incised by a branching network of fluvial distributaries and tidal channels. The distributary channels exhibit low sinuosity, however the channel thalweg show a well developed meander pattern between the banks. The sediments accumulating in these channels are predominatingly sandy, and form elongate lateral accretion bar accumulations associated with the thalweg meanders. In the distributary inlets, channel bars are common and consist of sand and clayey sand. In contrast to the distributaries, the tidal channels exhibit high sinuosity and only silt and clay accumulates in these channels.

The delta front is a shallow, seaward sloping inter to subtidal platform fringing the delta plain. In the zones of distributary inlets, active progradation occurs and sand accumulates in the form of stream mouth bars, tidal ridges and channel margin shoals. These accumulations form elongate bodies trending perpendicular to the shore. Silt and clay, with abundant plant debris and sparse marine fauna, accumulate between these sand bodies.

In the interdistributary zones, the sediments are mainly silt and clay with a more abundant marine fauna than in the distributary zones. Localised coastal erosion and sediment reworking occurs, accompanied by the accumulation of extensive beach ridges of detrital lignite.

The prodelta forms an inclined foreset slope, linking the delta front to the continental shelf, and is a zone of clay accumulation.

Reservoirs are found in the zones of fluvial distributaries, both in the delta plain and the delta front. Analysis of the geometry of these sand bodies indicates areal extents up to 30 sq. km, with maximum thickness ranging up to 8 – 10 m.

A stratigraphic model is proposed for the progradation development of the delta. This model consists of an offlapping superposition of prodelta, delta front and delta plain environments. It appears that as the delta builds out, the delta front sequence can contain repetitive cycles of offlap and onlap, related to distributary occupation and abandonment.