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Assessment of Structurally Compatible Interpretation in Fold-Thrust Belts: An Example From Offshore Northwest Borneo


Fold-Thrust Belts have long been spotlighted in petroleum exploration. Many section validation techniques, devised for thrust-related folds based on field analogues and seismic images, have led to significant advances in the prediction and the verification of structural geometry of such structures. Nevertheless, structural interpretation of a fold-thrust structure in three dimensions remains challenging and carries significant uncertainty. Contractional structures exhibit complex, structural variations along strike, which are not addressed by section validation techniques. Producing structurally compatible interpretation with consideration of along-strike variations of fold-thrust structures is a key for reducing exploration and development risks in Fold-Thrust Belts. We demonstrate a workflow for determining structural compatibility of a three-dimensional interpretation of fold-thrust structures with a focus on structural variation along strike. We use a high-resolution 3D seismic data acquired in the slope of offshore Northwest Borneo, Malaysia, to characterize fold-thrust arrays. Within the seismic data coverage, Late Miocene-to-recent deepwater sediments have been deformed into three fold-thrust structures largely trending NNE-SSW and verging NW-ward. These fold-thrusts commonly have complex structures in their central sections, such as imbricate thrusts, erosion of the fold crest and forelimbs, and gas clouds – all of which degrade the seismic signal and obscure features that cannot be imaged in the seismic. Structural interpretation has increased uncertainty in these areas. We plot fault heaves in serial sections along strike to ensure displacements of thrust faults are valid with respect to the overall tendency of fault heave. Deficits in fault heave are examined, with respect to interpreted fault linkages and matched against the relationships between fault segments and fold strains. Minor modifications of fault and horizon interpretations are made, if necessary, while checking consistency with seismic images. Through these steps, structurally reasonable interpretation of the fold-thrust structures is accomplished even where seismic image interpretation is rather ambiguous. The point is this workflow does not require seismic time-to-depth conversion; fault heave is less affected by depth-dependent vertical exaggeration which seismic time profiles inherently display. The workflow can thus be used for a quick check of structural interpretations.