From Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.

Bureau Seminar, November 5, 2004

Calibration of Fracture Development in Fault Damage Zones:
Outcrop and Subsurface Examples

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.