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Abstract

The present-day slope in the licensed part of the deep-water acreage in Brunei (NW Borneo) has been modified into a stepped slope profile by mobile shale ridges, and is characterized by the dominance of “slope accommodation space.” Canyon systems, which cut across the shale ridges, are partially in-filled with channel-levee complexes that display various degrees of channel sinuosity. The presence of high amplitude reflection packages at the base of the channel-levee complexes indicates the existence of subtle ponding within the canyon systems prior to infill by channel-levee systems.

A generic model for stepped slopes having a slow-moving substrate is described based on the structural, erosional, and depositional elements that are observed on images of the present-day sea floor and the shallow seismic data on the Brunei slope. Turbidite deposition on these slopes is dominated by shale prone “bypass facies assemblages,” but prospective reservoirs may be present in the form of channel-levee deposits and fan lobes. The fan lobes are deposited in subtle “ponded accommodation space” and at local breaks on the slope induced by mobile shale ridges.

Two deeper subsurface seismic examples are presented where the shallow observations and the model have been used to interpret the most likely depositional setting in the absence of well control. A third example shows how the increased understanding of the slope processes has helped to interpret the complex depositional setting of the Merpati and Meragi gas and condensate discoveries.

Seismic facies analysis calibrated with the sparse well data indicate that despite close similarities with the bypass and slope facies assemblages in the Gulf of Mexico, the Brunei facies assemblages contain less net sand, reflecting the mud-dominated sediment provenance and the limited amount of available accommodation space.

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