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

Abstract

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. McGilvery et al, 2004). 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. Introduction to Gravity-Driven Processes Poorly consolidated sediments are prone to fail due to changes in pore pressure that result in instability, ultimate failure, and redistribution by gravity-driven processes. These deposits are the result of four primary processes: sliding, slumping, debris/grain flows, and turbid flow. Shanmugan (2006) groups these deposits into two broad categories (1) mass-transport deposits and (2) sediment-flow deposits. Mass-transport deposits are coherent masses comprising slides and slumps. Whereas sediment flow deposits comprise debrites and turbidites. Keeping reservoir potential in perspective, of these processes, turbid flow generally generates the best quality reservoirs. Although slides, slumps, and debrites may be reservoirs they have comparatively low productivity compared reservoirs comprising turbid flow deposits. Note that slope failure may result in a process continuum from slides through to turbidity currents (Fig.1). However the continuum products may not be preserved, for example, slumps may develop without going through a slide stage. Or, under the right conditions, a turbid flow may quickly develop from a slump momentarily passing through the cohesive (debris) flow condition boundaries. All four process described here are observed in offshore Borneo borehole images, three of which are illustrated in Fig. 2.