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The Internal Architecture of Kumunsu and Kinurat Formation Mass-Transport Complexes Offshore Sarawak/Sabah, South China Sea

Abstract

Using borehole image logs, the internal (macro-scale) architecture of mass-transport complexes (MTC) penetrated by 7 offshore Sarawak/Sabah deep-water exploration wells are described and analyzed. Detailed analysis of lithofacies and dips from the “Kumunsu” and “Kinurat” formations show the MTCs comprise a mélange of stacked metre-to-decimeter scale slides, slumps, debris and grain flows that stack into techno stratigraphic units. The results are interpreted in light of available data, incorporating core, mobility data, outcrop analogues, and seismic-scale features. This macro-scale detail complements the existing excellent description and analysis of large-scale (km) MTC architecture identified as largely transparent or chaotic seismic facies bounded by coherent layered seismic facies (McGilvery et al 2004, Hongbo Lu et al 2011, and Algar, et al 2011). The studied intervals comprise 67% of the undisturbed sediments and 33% that are remobilized (MTC). The MTCs are 50 to 150+ m thick: comprising mudstones, heterolithics (thin beds), and sandstones (5m+ thick deformed channel sandstones and in-channel debrites). This study suggests that the internal MTC anatomy is dominated by coherent soft-sediment slump folds (76%). Slumps are further analyzed by plotting the poles to deformed bedding on equal area stereonets. The fold-axial trends of multiple slumps are plotted to help identify techno-stratigraphic units. Event recognition, and varying axial trends, implies each event comes from a different direction. The slumps are stacked and are often interbedded with slides and debris flows that together attain up to 150+m in thickness. The high proportion of MTCs within the study wells indicate the Miocene slopes were relatively unstable and subject to frequent remobilization. Furthermore, the relatively high proportion of MTC facies in each well show a high degree of reservoir heterogeneity not generally recognized using standard open-hole logging suites. This heterogeneity is important to recognize, and differentiate from good-quality reservoir, since MTCs have comparatively low productivity indices, impacting investment decisions. Sedimentological analysis of image log features provides information for understanding reservoir emplacement, identifies potential sub-seismic reservoirs, and high-grades zones for pressure samples. The results presented may be used for prospect risk assessment and reservoir modelling.