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Lowside (footwall) closures sealed by reverse-slip faults and convergent strike-slip faults offer opportunities for significant field extension and new field prospects in basins deformed by contraction. The faults have reverse separation in cross section and transverse closure (in the direction of reservoir dip) is often provided by dip of beds away from the fault at structural upturns. The upturns are common and form at the edge of the footwall block as a consequence of block-edge folding, fault drag, and shortening transverse to fault strike. Effective fault seal and longitudinal closure (parallel to reservoir strike) are the most uncertain trap controls. Fault seal may be provided by the juxtaposition of older, less permeable rocks against the down-dropped reservoir or by impermeable material within the fault zone. Fault-zone barriers to fluid flow include shaly smear gouge, cataclastic gouge, mineral deposits, or asphalt or tar impregnation. Longitudinal closure is most commonly formed by a broad positive warp or bowing at the edge of the footwall block or by stratigraphic reservoir terminations. Secondary faults, intersections of primary block faults, and en echelon folds may also provide longitudinal closure.
Prospects can range in importance from secondary extensions of existing highside closures to large traps unrelated to hanging-wall structure. The variety of geometries, relationships that provide transverse and longitudinal closure, and important geologic parameters that determine fault seal are illustrated with examples from oil fields in Sumatra and southern California. These fields can be used as models for the recognition and delineation of prospects in other basins.
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