Facies distribution and sedimentary processes on the modern Baram Delta: implications for the reservoir sandstones of NW Borneo

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Abstract

The Baram Delta has been classified as wave-dominated and as a modern analogue for the prolific Tertiary sandstone reservoirs in the Baram Delta Province of NW Borneo. However, the delta's facies distribution and sedimentary processes indicate that sedimentation is presently occurring only on the large mouth bar and that the Baram is more properly categorised as a mixed wave and tide-influenced delta. More than half of the surface area of the mouth bar is dominated by tidal currents and covered with tidally deposited sediments because a relatively large tidal prism controls the nearshore hydrodynamics. Conversely, the widespread Tertiary shoreface reservoir sandstones are clearly wave-dominant and tidal signatures are absent. Consequently, the Baram Delta is not an appropriate analogue for most shoreface reservoir sandstones in NW Borneo.

Introduction

Studies of Tertiary delta deposits commonly use modern deltas as analogues for sandstone distribution and reservoir properties. In some cases, the analogue is the Holocene Delta of the same river system that deposited the Tertiary succession (e.g. the Mississippi and Mahakam deltas; Allen and Chambers, 1998, Coleman, 1981). This approach has been applied in NW Borneo where the Holocene Baram Delta has long been considered a modern analogue for the numerous Tertiary sandstone reservoirs (e.g. Petronas, 1999, Sandal, 1996). An analogue relationship was inferred because plotting wave energy versus tidal range indicates that the modern delta is wave-dominated and because it falls within the wave-dominant field on Galloway's (1975) ternary diagram classification scheme (Schreurs, 1996). However, the facies distribution and reservoir potential of the modern Baram Delta was not described prior to the present study and, although widely cited (e.g. Johnson, Kuud, & Dundang, 1989), the delta's value as an analogue for NW Borneo's reservoir sandstones has been largely speculative.

The present study characterises the sedimentary facies on the modern Baram Delta and reviews their distribution with respect to hydrodynamic processes to determine the relative importance of each process in generating the stratigraphic succession predicted from the facies distribution. Also, the sedimentology, geometry and potential reservoir character of the modern sands are compared to those of producing sandstones in NW Borneo to determine whether the Baram Delta is an appropriate depositional model for those reservoirs.

Section snippets

Morphology and recent history

The subaerial portion of the Baram Delta comprises a broad area (approximately 300 km2) that straddles the border between Brunei Darussalam and the adjacent Malaysian state of Sarawak (Fig. 1). The active delta lobe has a symmetrical, cuspate shoreline geometry that forms a prominent coastal protrusion at the point where its single active distributary channel, the Baram River, enters the South China Sea. The lobe has an estimated thickness of 120 m offshore and is asymmetric in cross-section with

Facies distribution

Five sedimentary facies were identified from the intertidal and shallow subtidal areas adjacent to the shoreline of the delta and on the mouth bar (Abdul Aziz Abdul Rahim, 2000). Facies characterisation was based primarily on lithology, grain size and sedimentary structures from trenching and box coring in the intertidal zone and grab sampling in the subtidal. Degree of bioturbation was noted and benthic foraminifera and palynological assemblages were determined at 47 locations. The five facies

Depositional model

The facies distribution on the Baram Delta indicates that waves and tidal currents are the two most important hydrodynamic processes controlling present-day sedimentation. The occurrence of cohesive mud populated by fully marine calcareous benthic foraminifera in the mouth of the Baram River suggests that the river is not actively contributing significant amounts of sand to the mouth bar. It appears that the fluvial component of sedimentation is limited to supplying the relatively large amount

Predicted stratigraphic succession and the rock record

The uncompacted stratigraphic succession that would be generated by the modern Baram Delta is illustrated in Fig. 6. The basal unit consists of up to 20 m of moderately to highly bioturbated mud. The succession coarsens upward transitionally through approximately 0.4 m of sandy mud and 0.7 m of muddy sand into 0.9 m of clean, parallel laminated fine sand (Fig. 6). This part of the succession is wave-dominant and represents progressively nearshore environments from the pro-delta continental shelf

Hydrodynamics and sediment distribution

The tide-dominant character of the Baram Delta sediments apparently contradicts the delta's classification as wave-dominant on a wave–tide–fluvial ternary diagram and a plot of wave power versus tidal range (Schreurs, 1996; Fig. 7A). However, a closer look suggests that these criteria for wave-dominance may not classify the Baram system accurately, especially when the objective of the classification is to characterise the sedimentary deposits. If the relative abundance of the various

Conclusions

The facies distribution and sedimentary processes of the modern Baram Delta indicate that:

  • 1.

    The Baram is a wave-influenced, tide-dominated delta; its shoreline geometry is destructional and is not indicative of depositional process.

  • 2.

    Modern sedimentation is restricted to the mouth bar and its morphology does reflect the tidal dominance.

  • 3.

    The mouth bar crest separates zones of tide- and wave-deposited sediment and represents the point where tidal and wave energy are approximately equivalent.

  • 4.

    In the

Acknowledgements

The authors wish to thank Brunei Shell Petroleum and Universiti Brunei Darussalam for sponsoring some of the research; Abdul Aziz Abdul Rahim also thanks Esso Production Malaysia Incorporated for financial support. Abdul Aziz Abdul Rahim and Cheong Yaw Peng thank Abdul Razak Damit for valuable field assistance. Sarawak Shell Berhad, the Sarawak Drainage and Irrigation Department, the Meteorological Department of Miri and the Sarawak Natural Resources and Environmental Board provided background

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    1

    Present address: Esso Production Malaysia Inc., Menara Esso, Off Jalan Kia Peng, 50450 Kuala Lumpur, Malaysia.

    2

    Present address: Department of Petroleum Engineering, Curtin University of Technology, P.O. Box U1987, Perth, WA 6001, Australia.

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