The Macro-Scale Architecture of Mass-Transport Complexes Offshore Borneo, South China Sea

Abstract

The large-scale (km) architecture of mass-transport complexes (MTC) and deposits offshore Borneo has been previously described using shaded relief maps of the seafloor and shallow subsurface (e.g. . These shallow examples are good analogues for subsurface mid-Miocene MTCs identified as largely transparent or chaotic seismic facies bounded by coherent layered facies and penetrated in offshore Sarawak/Sabah deep-water exploration wells. For the first time, using borehole image logs, the macro-scale architecture of the MTCs penetrated by five wells are analyzed. Detailed analysis of borehole image dips and lithofacies show drilled offshore Borneo MTCs comprise a mélange of stacked metre-to-decimeter scale slides, slumps, debris and grain flows that stack into techno stratigraphic units.Recognition and analysis of reservoir-scale architecture using borehole image logs is important for identifying potential sub-seismic reservoirs (thin-beds), high-grading zones for pressure samples and understanding reservoir emplacement processes. Observed changes in palaeoslope orientation reflect large basinal adjustments due to tectonics and large-scale mass-failures along the developing slope. Each onset of a new tectnostratigraphic regime results in a local reduction in sediment supply. The proportion of intra-MTC facies in each well illustrates the high degree of heterogeneity not generally reflected on the standard open-hole log suite. Intra-MTC heterogeneity may have a significant deleterious impact reservoir properties and reservoir productivity. The presence of MTC's, generally a positive feature on the sea floor, may have an impact on sand fairways.