Brunei Bay northwest Borneo : depositional system
Brunei Bay in Northwest Borneo is a semi circular marine embayment formed during the last marine transgression. It represent a complex tropical depositional system in a micro-mesotidal setting in which the following elements can be recognised - an elongate bird's foot delta with abandoned lobes, drowned river valleys, cuspate delta and classic tidal estuary - all within 50 krn of each other. Understanding the control on the morphology of the sand bodies produced within the overall depositional system is of great importance because it is believed that many of the subsurface hydrocarbon reservoirs in Northwest Borneo were deposited in a depositional setting similar to that of modern-day Brunei Bay. This study therefore has major implications for defining the architecture and connectivity of subsurface reservoirs. A detailed bathymetry survey of western Brunei Bay has been undertaken which acted as guide for subsequent hydrodynamic measurements and coring programme. The results of this study indicate that the localised depositional setting plays a key role in understanding sandbody geometry because the orientation of sandbodies and facies distribution varies considerably. Tidal processes playa significant role in shaping the sediment distribution and facies characteristic of the area although locally wave processes can play a significant role. Tidal sandbodies parallel to the direction of drowned rivers, form a large arcuate-shaped complex less constrained by the structural trend. This study suggests that within Brunei Bay, morphology and tidal range is not a good indicator of the dominant process. Mud, exceptionally rich in plant debris derived from the mangrove swamps, accumulates in low energy environments peripheral to the distributary channel, reinterpretation of many coal horizons in the Miocene of the region which have largely been ascribed to a raised setting. This study indicates that the present day Brunei Bay is an excellent analog for the tidally influenced succession of Northwest Borneo. Conversely, it also indicates that the Baram Delta located approximately 100 krn west of Brunei Bay is not a suitable analog for the interpreted wave influenced Miocene succession of Northwest Borneo.