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Abstract

A regional study of deepwater northwest Borneo indicates that within the Late Miocene to Recent deepwater thrust belt, up to 50% of the sediments comprise large-scale, remobilized units interpreted as mass-transport deposits (MTDs). More than 6000 km2 of 3D seismic data, 20 exploration and appraisal wells, and more than 600 m of core were examined as part of this integrated study. The MTDs are 10 to 200 m thick, and are composed predominantly of claystone, though subordinate sands and siltstones are present. Intercalated with the MTDs are turbidites, which form sandstone reservoirs of the petroleum discoveries found in the area. Intriguingly, the thickest sands are often found immediately overlying the MTDs, and there is evidence to suggest that the topography on top of the MTD may influence sand distribution. There is also seismic-stratigraphic evidence suggesting that MTDs may be locally erosive and could also form lateral and vertical seals and even contain source rocks. In the study area, core and borehole image logs are the most reliable data types to identify the MTDs, although the overcompaction of the MTDs often means that simple borehole density, sonic, and resistivity logs can also be used. A variety of MTD lithofacies indicate a continuum from debritic claystones with extremely variable, incoherent, dip architectures in the image logs to more simply folded claystones, which have more coherent dips. Overcompaction of the MTDs relative to the overlying and underlying turbidites often creates an impedance contrast that can be mistaken for that of a sandstone-claystone interface in the turbidites. Hence, seismic interpretation in areas with MTDs is more challenging and potentially misleading than in areas without MTDs.

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