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From
Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
Bureau Seminar, November 5, 2004 Calibration
of Fracture Development in Fault Damage Zones: Chris Zahm Abstract: Recent advances in elastic dislocation modeling have enabled the prediction of fracture orientation and intensity on the basis of seismically resolvable fault-throw profiles and knowledge of rock properties. Models predict an increase in fracture intensity in a halo around the fault zone (i.e., fault damage zone). Calibration of modeling results is essential to determining the validity of this approach. To date, it has been conducted primarily in siliciclastic rocks. Expansion of the calibration into carbonate rocks necessitated a case study in subtidal shelf deposits of the Lower Glen Rose Formation of central Texas. This outcrop was selected because (1) there is excellent exposure due to road cuts and pipeline cuts, (2) the exposure exists in an area where two seismically resolvable faults form a relay or step-over, and (3) numerous smaller faults and fracture systems are exposed and are kinematically related to the larger faults. A 3-D model of mapped faults
and horizons was built to capture fault geometry and throw. Rock properties
were measured at the outcrop and were used to determine the rock density,
Young’s modulus, Poisson’s ratio, cohesive strength, and coefficient
of internal friction. Elastic dislocation modeling was performed to determine
maximum Coulomb shear stress (MCSS) as a proxy for fracture intensity,
and the orientation of the minimum shear stress (?3) was used to calculate
the orientation of idealized fracture planes. The modeled results were
compared with the outcrop-observed subseismic faults and fracture intensity
to validate modeled results. These findings increase our confidence in
application of this approach in subsurface cases.
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