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Morphological and Geological Characterization of Mass-Transport Deposits in the Deepwater Fold and Thrust Belt (DWFTB) of Offshore Malaysia

Abstract

Mass transport deposits are prevalent in active DWFTBs and play a variety of roles in the petroleum system, including acting as source rock, reservoir, migration-path, seal, or influencing accommodation for post-emplacement turbidite deposition. However, their role in any specific basin or location in a basin is often difficult to predict, due to the high lateral and vertical heterogeneity exhibited by these enigmatic deposits. The origin-based classification scheme of attached MTDs and detached MTDs can be applied to narrow down the potential host sites for MTD genesis, a fact which has implications for size and nature of the resultant deposits. This study aims to analyze several ancient MTDs utilizing 3D seismic, well logs and biostratigraphic assemblages to enhance recognition criteria of these MTD types. 700 km2 of high-resolution, 3D seismic data and two new exploration wells were used for detailed analysis of Late Miocene-to-recent age, “attached-” and “detached-MTDs” found in the DWFTB of offshore Malaysia. The thick and spatially extensive chaotic seismic reflection packages, interpreted as “attached MTDs” are observed in pre- to syn-kinematic sequence while the relatively smaller-scale, thinner chaotic seismic reflections of “detached MTDs” are developed in the syncline between FTB anticlines in syn-kinematic sequence. Attached MTDs, where well penetrations occur, show a proximal main body that is mud-rich, mounded in shape, and contains rafted blocks. Muds show elevated densities compared to surrounding background muds. Well bore images of the interval show disorganized azimuth/dips, however distal parts show concordant azimuth/dips. Deposits show an abundance of outer neritic fauna supporting the interpretation that these deposits are derived from a near-shelf geomorphologic provenance. Detached MTDs have no well penetration, but seismic geomorphologic interpretations show host scarps along the steepened slopes of mainly anticline forelimbs. They dominantly occur in intervals where uplift exceeds sedimentation, but also can occur when sedimentation exceeds uplift. The timing of occurrence and distribution of detached MTDs records the degradation history of the paleo-slope related to the thrust-cored folding. Recognition of the origin of and timing of MTDs can aid in uncertainty assessment in their capacity to seal or leak hydrocarbons, as well as their extent and lithology, and offer recognition of significant deforming events in DWFTB.