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Syn-Kinematic Sediment and Facies Distribution Across a Growth-Fault in the Wave-Dominated Deltaic Succession of the Neogene Champion System, Northwest Offshore Brunei Darussalam: An Example of Structural Switching


Lap, Lai Quoc, John K. Warren, University Brunei Darussalam, Brunei


Champion delta hosts multi-million barrel oil and gas fields in shallow shelf sediments. At the scale of Mahrajah Lela (ML) field, the depopod that makes up the regional “delta” prism is actually a complex of smaller inter-related depositional systems (shelf, shoreface and minor tidal). Time variant sediment extent was controlled by eustacy and was driven largely by fault activity at both the basin and local scales. Rapid deposition and progradation of shelf sediment initiated a seaward-steeping growth-fault FML in ML field, which then interplayed with counter-regional Perdana growth-fault, to create a variably-subsiding depocentre in the area between these two faults.

If subsidence induced by the FML fault was greater than that of the Perdana, then sub­sidence at early stage of each active phase created substantial accommodation space across both sides of the fault in the main ML field. Depending on rate of sediment influx into this depression, infilling sediments prograded or aggraded to fill the available space. However, when FML growth was slow, the regional Perdana growth-faults exerted greater influence, resulting in retrogradational sedimentation in the ML field.

Interpretations of core, borehole image, conventional wireline and seismic data in foot­wall area indicate wave-dominated sandy facies are more locally distributed and tidal deposits occur more frequent there. In contrast, layers in the hanging-wall are much thick­er with a greater degree of lateral continuity across the field. Shoreface sandstones in the hanging wall are also cleaner and more petrophysically homogenous, while tidal occur­rences are diminished or absent. When supply was sub-parallel to axis of FML fault, a much a greater volume of sediment accumulated in the down-thrown block compared with the up­thrown block.