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A Quantitative Model of the Internal Structure of Fault Zones


Although faults are generally represented as simple planes or surfaces, it is widely recognised that they can comprise several slip-surfaces each carrying a part of the total fault displacement. This complexity in fault zone structure can be mapped at outcrop and in the best quality seismic reflection data. Seismic reflection data are however subject to resolution limits and complex fault zone structure is not readily apparent from seismic data at reservoir depths. Unresolved partitioning of displacement onto two or more slip-surfaces can impact across-fault juxtaposition of reservoir intervals and therefore affect fluid flow in an exploration or production context. The distribution of displacement within a fault zone derives from, and provides a record of, fault zone evolution and published geometric and conceptual models provide qualitative descriptions of this evolution. Here we present the results of analysis of an extensive quantitative dataset of fault zone geometry and displacement partitioning derived from outcrop and seismic mapping. These analyses, combined with conceptual geometrical models, underpin a quantitative geometrical model of fault zone evolution. This quantitative model allows constraints to be placed on questions such as, what is the likelihood that a 20 m thick reservoir is connected across a fault with a throw of 50 m? A key finding of our analysis is that partitioning of displacement onto multiple slip-surfaces occurs at all scales of observation and can be significant in reservoir-scale faults. A second key finding is that there is a statistically significant difference between fault zone structure in different areas. This latter finding provides a basis for attempting to relate fault zone structure to geological controls. We find for example that globally there is a one order of magnitude variation in the frequency of segmentation of faults. The degree of segmentation can be clearly related to the presence or absence of a basement control on fault geometry but no lithological control is recognised. However a lithological control on the strain at which two fault segments become connected is apparent. Although displacement can be partitioned onto multiple slip-surfaces in all settings, across-fault reservoir connectivity can often be ruled out for particular combinations of fault throw, seismic resolution and geological setting.