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Fault Populations and Geometric Analyses |
Badley Earth Sciences Ltd, North Beck House, North Beck Lane, Hundleby, Spilsby, Lincolnshire PE23 5NB, UK
Interpretation of the subsurface is still often performed on suites of vertical seismic sections with structures being correlated from line to line to build up a map view at a particular horizon. The advent of 3D seismic surveys and routine use of interpretation software has made it common to produce ‘horizon-based’ interpretations, using auto-tracking techniques. Neither of these approaches guarantees a three-dimensionally consistent interpretation because of ambiguities in lateral and vertical correlation of structures. More reliable descriptions of a faulted surface can be obtained if the three-dimensional properties of a fault network are observed directly. Fault traces picked on vertical sections can be used to model the fault surfaces, which are then viewed in perspective to give an immediate assessment of the plausibility of the interpreted fault geometry. By associating horizon terminations with their respective fault segments this cut-off information can be shown on the perspective view of the fault surface, allowing the interpreter to examine in detail how reservoir layers are disrupted and juxtaposed by the fault network. The horizon cut-off information can also be used to calculate displacement-related values (throw, heave, separation) which may be gridded over the entire picked surface in order to produce displacement maps, also in the perspective view. Isolated single faults have simple displacement patterns with a maximum near the fault centre. Fault arrays show partitioning of displacement between the various splays, with relatively abrupt changes in the displacement at branchlines. Long ‘single faults’ are frequently shown to be segmented into en-echelon arrays rather than being simple continuous structures. Antithetic faults exhibit a downward decrease in displacement towards a tip line near their master fault. In areas of cross-cutting fault trends, displacement patterns can discriminate between different possible kinematic histories. Fault displacement can also be used to calculate other attributes such as fault seal potential, which can also be posted onto the perspective view of the fault surface. These analyses of fault geometry and displacement allow a more objective assessment of subsurface interpretation. They do not rely on particular assumptions about deformation mechanisms, only on the observation that displacement variation on individual fault surfaces is systematic.
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