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Abstract

Abstract: Offshore of northwest Borneo, the occurrence of distinct submarine mass failures on the upper continental slope poses a substantial challenge to deepwater operations for the energy industry. These features are part of a complex of mass-transport deposits (MTDs) that occur in the near-surface interval across most of the upper continental slope, including a large area undergoing field development for hydrocarbon production. In the study area, the shallowest and most prominent feature discernible on conventional 3D seismic data is MTD 1, which has a profound influence on the present-day seafloor topography. This feature has a distinctive fan-like outline in plan view, a maximum strike dimension of approximately 6 mi (10 km), a dip extent up to 24 mi (40 km), and a maximum thickness up to 570 ft (176 m). The fan-like external form and the presence of a dip-oriented erosional keel suggest that the depositional process was a less coherent debris flow, with little to no original internal stratigraphy preserved. The less coherent nature of this feature is further supported by a key observation that this MTD overran an area of substantial high bathymetric relief, which is located in the area considered for a field development.

Locally overlying MTD 1 are a series of younger near-seafloor features, termed “canyon-to-fairway” corridors that display a confined updip to less confined downdip plan-view morphology. These unique features locally erode and smooth the rugose top surface of the near-surface MTD 1 and can be interbedded with the lower intervals of the usually overlying draped sediments. Development of these late Pleistocene canyon-to-fairway corridors suggests that these features probably formed during a period of sea-level fall or at a lowstand. A blanket of three distinct intervals of draped sediments cap this entire sequence, composed mostly of muddy turbidites grading upward into hemipelagic deposits. The present hummocky seafloor topography mimics the rugose top surface of the shallowly buried MTD 1, except along its northeast lateral margin and where smoothed by canyon-to-fairway corridors. Internally within MTD 1, physical properties probably vary substantially both laterally and vertically, because draped sediments, turbidites, and occasional channelized sediments were incorporated in the failed matrix of this feature.

Some of the geohazards, potentially affecting a field development, are a direct a result of the ubiquitous occurrence of MTD 1 in the study area. These potential geohazards include local steep slopes, seafloor scarps, and variable near-seafloor soil conditions. Understanding the impact of each of these potential geohazards, caused primarily by the presence of MTD 1, on a field development is vital input for selection of production well-site locations and placement of subsea infrastructure.

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