Fracture Prediction in Fault Damage Zones Using Elastic Dislocation Models: A Carbonate Outcrop Example from Central Texas, U.S.A

Zahm, Chris, Peter H. Hennings, Milt Enderlin, ConocoPhillips Upstream Technology, Houston, TX

Recent advances in elastic dislocation modeling have enabled the prediction of fracture orientation and intensity based on seismically-resolvable fault throw profiles and knowledge of rock properties. Models predict an increase in fracture intensity in a halo around the fault zone. Calibration of modeling results is essential to determine the validity of this approach and has been primarily conducted in siliclastic rocks. Expansion into carbonates is neces­sary so a test case was developed in carbonate rocks (subtidal shelf deposits of the Lower Glen Rose Formation) of central Texas. This outcrop was selected for the following reasons:

(1) excellent exposure due to road cuts and pipeline cuts; (2) the exposure exists in an areawhere 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 3D 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 the maximum Coulomb shear stress (MCSS) as a proxy for fracture intensity, and the orientation of the minimum shear stress (s3) was used to calculate the orientation of idealized fracture planes. The modeled results were compared to the outcrop observed subseismic-faults and fracture intensity to validate the modeled results. These findings increase our confidence in the application of this approach in subsurface cases.