High-velocity basement rocks, comprised of Proterozoic granulites, and Mesozoic metamorphic, andesitic and marine sedimentary rocks have been thrust to surface along low-angle northwest- and southeast-vergent thrust faults in the Acevedo block of Colombia's Suaza Valley in the Upper Magdalena region. Beneath the basement thrust sheets lie oilprospective Cretaceous and Tertiary fluvial-deltaic reservoir sandstones. The complex geometries, nonreflective character and high internal velocities of these basement thrust sheets make it difficult to clearly resolve and accurately position seismic reflections from the stratigraphy beneath them. However, application of high-effort acquisition methods and modern processing streams has allowed sub-thrust imaging and prospect definition. Imaging success is attributed to swath-2D seismic geometries, which were used when shooting above the basement hangingwall. Swath-2D configuration involved four receiver lines and a central 'zig-zag' shotline. Additionally, tomographic statics, pre-stack time migration and anisotropic pre-stack depth migration were all used to obtain optimal imaging and positioning of reflections. Final 2D pre-stack migrated time sections image sub-thrust truncations of reflections from the target Cretaceous Caballos sandstone. Depth mapping delineates a large fault-dependent sub-thrust trap. A 3,000 m sub-thrust well test is scheduled for 2003.


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