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Spatial Fault Size Distribution and Normal Fault System Evolution

Alan P. Morris1, Nathan M. Franklin1, Darrell W. Sims2, and David A. Ferrill1
1 Southwest Research Institute®, San Antonio, TX
2 CNWRA, Southwest Research Institute®, San Antonio, TX

Heightened awareness of reservoir heterogeneities has led to increased interest in the use of seismic data and seismically imaged structures to infer sub-seismic scale deformation, specifically faults and fractures. Field investigations and physical analog model studies by previous workers have shown that the size distribution of large- and small-displacement faults depends on the mechanical thickness of the deforming medium and the evolutionary stage (defined primarily by fault linkage) of the fault system. In this study, we use physical analog modeling to explore the spatial variability and temporal evolution of extensional faults developed in a wet clay layer experiencing distributed extension above a rubber sheet. Using dynamic structured light to generate digital terrain data from the surface of the clay, we show that normal fault systems evolve by: (1) initial rapid fault nucleation; (2) rapid propagation of certain, larger faults, accompanied by (3) continued nucleation of smaller faults in areas of displacement deficit and (4) low fault nucleation rates in strain shadow areas. Continued fault system development leads to (5) propagation and linkage of larger faults through areas of displacement deficit. Depending on the evolutionary stage of the fault system, and the location of the area of interest within the system, small-displacement faults may appear to be negatively correlated (strain-shadow effect), or positively correlated with large-displacement faults. The confidence with which size and spatial distributions of faults in reservoirs can be estimated is improved by considering strain shadow zones, and analyzing fault linkage history.