Deepwater folding and thrusting offshore NW Borneo, SE Asia
The slope and deepwater portion of the offshore NW Borneo continental margin hosts a number of proven hydrocarbon accumulations. Reprocessed and post-stack depth-migrated regional 2D seismic data reveal the occurrence of an extensive series of deepwater folds located at the leading edges of imbricate thrusts. Typical thrust-top folds include (1) anticlines characterized by large interlimb angles that lack a sea-floor expression; (2) anticlines with medium interlimb angles that show a clear sea-floor expression and normal faulting in the crest; and (3) anticlines with small to medium interlimb angles, a clear sea-floor expression and intensive crestal faulting associated with partial crestal failure. The different fold types occur at specific locations within the fold–thrust system, the widest and youngest anticlines near the present-day thrust front, and the narrowest and oldest folds in the most landward parts of the fold–thrust belt. Geometric restoration of the deepwater fold–thrust system along six regional shelf-to-basin cross sections provides incremental measurements of fault- and fold-related shortening for the time between the Miocene and present day in deepwater NW Borneo. Across the study area, the main thrust and fold activity appears to be largely of Pliocene–Holocene age. An apparent maximum of both incremental and total shortening is located in the central part of the study area. This location coincides with the maximum width of the fold–thrust belt and the preferential location for the development of the most recent deepwater anticlines.
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Onshore fold–thrust belts are commonly perceived as ‘difficult’ places to explore for hydrocarbons and are therefore often avoided. However, these belts host large oil and gas fields and so these barriers to effective exploration mean that substantial unexploited resources may remain. Over time, evaluation techniques have improved. It is possible in certain circumstances to achieve good 3D seismic data. Structural restoration techniques have moved into the 3D domain and increasingly sophisticated palaeo-thermal indicators allow better modelling of burial and uplift evolution of source and reservoirs. Awareness of the influence of pre-thrust structure and stratigraphy and of hybrid thick and thin-skinned deformation styles is augmenting the simplistic geometric models employed in earlier exploration. But progress is a slow, expensive and iterative process. Industry and academia need to collaborate in order to develop and continually improve the necessary understanding of subsurface geometries, reservoir and charge evolution and timing; this publication offers papers on specific techniques, outcrop and field case studies.